•NRLF B E SOD PL3NTS FROM THE LIBRARY OF WILLIAM A. SETCHELL,i864-i943 PROFESSOR OF BOTANY BIOLOGY LIBRARY WILLIAM A. SETCHELL WATER PLANTS CAMBRIDGE UNIVERSITY PRESS C. F. CLAY, MANAGER LONDON : FETTER LANE, E.G. 4 LONDON : H. K. LEWIS AND CO., LTD. LONDON : WILLIAM WESLEY AND SON NEW YORK : THE MACMILLAN CO. BOMBAY ) CALCUTTA j-MACMILLAN AND CO., LTD. MADRAS J TORONTO : THE MACMILLAN CO. OF CANADA, LTD. TOKYO :MARUZEN-KABUSHIKI-KAISHA ALL RIGHTS RESERVED Nymphaea lutea, L. The Yellow Waterlily, showing rhizome and submerged leaves from a woodcut in Otto von Brunfels' Herbarum vivae eicones, 1530 (reduced). WILLIAM A. SETCHELJL WATER PLANTS A STUDY OF AQUATIC ANGIOSPERMS BY AGNES ARBER, D.Sc., F.L.S. FELLOW OF NEWNHAM COLLEGE, CAMBRIDGE, AND KEDDEY FLETCHER-WAR R STUDENT OF THE UNIVERSITY OF LONDON WITH A FRONTISPIECE AND ONE HUNDRED AND SEVENTY-ONE TEXT-FIGURES * ' ' V * * » •» ^ •» ,\ CAMBRIDGE AT THE UNIVERSITY PRESS 1920 A7 LN TO THE MEMORY OF E. A. N. A. BIOLOGY LIBRARY PREFACE IT was affirmed a few years ago, by one of the most eminent of living biologists, that it "is no time to discuss the origin of the Mollusca or of Dicotyledons, while we are not even sure how it came to pass that Primula obconica has in twenty-five years produced its abundant new forms almost under our eyes.'* To this statement I venture to demur. I yield to none in my admiration for the results achieved by the analytical methods introduced by Mendel, and I do not doubt the possibility that the direct experimental study of variations and their inheritance may eventually play a large part in bringing the tangled problems of evolution into the full daylight for which we all hope. But this is no reason for condemning those countless uncharted routes which may lead, even if circuitously, to the same goal. Any step towards the solution of the essentially historical problems of Botany — for example those concerned with the origin and development of such morphological groups as the Dicotyledons, or of such biological groups as the Aquatic Angiosperms — must necessarily contribute some mite to our conceptions of the course of evolution. These less direct methods of approaching the central problem of biology may perhaps, at the best, bring only a faint illumination to bear upon it, but in the deep obscurity involving all evolutionary thought at the present time, we cannot afford to despise the feeblest rush-light; even the glimmering of a glow-worm may at least enable us to read the compass, and learn in which direction to expect the dawn. I approached the study of Water Plants with the hope that the consideration of this limited group might impart some degree of precision to my own misty ideas of evolutionary processes. Botanists seem to be universally agreed that the VI PREFACE Aquatic Angiosperms are derived from terrestrial ancestors, and have adopted the water habit at various times subsequent to their first appearance as Flowering Plants. The hydrophytes thus present the great advantage to the student, that they form a group for whose history there is a generally accepted foundation. Throughout the present study I have constantly borne phylogenetic questions in mind, and the first three Parts of this book may be regarded as a clearing of the ground for the more theoretic considerations concerning the evolutionary history of water plants to which the Fourth Part is mainly devoted. In that section of the book, and sporadically in the earlier chapters, I have set down such speculations as have been borne in upon me in the course of a study of water plants with which I have been occupied more or less continuously for the last ten years. The literature relating to Aquatic Angiosperms has now grown to such formidable proportions that I have felt the necessity of trying to provide some clue to the labyrinth. With this end in view I have given a bibliography of the principal sources, which includes a brief indication of the nature and scope of each work, with page numbers showing where it is cited in the text. For the convenience of those seeking informa- tion about any particular plant, I have indexed the families and genera named in the titles enumerated, and in the notes regard- ing the contents of each memoir. I found it impracticable to compile a subject index to the bibliography, but the references under the individual chapters to some extent serve this purpose. It is a pleasure to express my grateful appreciation of the kindness of those botanists who have helped me in various ways during the preparation of this book. I am particularly indebted to Professor A. C. Seward, F.R.S. for valuable sug- gestions and advice; to Dr H. B. Guppy, F.R.S. for reading the pages in Part IV which treat of Distribution; to the Hon. Mrs Huia Onslow (Miss M. Wheldale) for some helpful criticism of the chapters dealing with physiological questions ; to Mr F. W. Lawfield, M.A. for aid in fenland botany; and — PREFACE vii last but not least — to Miss Gulielma Lister, who, many years ago, showed me the winter-buds of the Frogbit in a pool in Epping Forest, and awoke in me the desire to know more of the ways of water plants. I have to thank the Councils of the Linnean Society, and the Cambridge Philosophical Society, and the Editors of The Annals of Botany^ The Journal of Botany^ and The American Naturalist^ for permission to incorporate in this book parts of the text and illustrations of certain of my papers which have appeared in their publications. Of the figures in the present book, about one-third are original; these are indicated by the initials A. A. The sources of the others are acknowledged in the legends, but I must take this opportunity of expressing my obligation to the numerous authors from whose memoirs they are derived. I am indebted to the Clarendon Press for the use of the block for Fig. 127. The photographic reproduction of a number of the illustrations has been carried out by Mr W. Tarns, while some have been re-drawn by Miss Evelyn McLean. I have to thank my sister, Miss Janet Robertson, for the design repro- duced on the cover, which is based upon a wood-cut of the Yellow Waterlily in Lobel's "Kruydtboeck," of 1581. I am much indebted to my father for reading and criticising my manuscript and proofs. To my husband, E. A. Newell Arber, I owed the original impulse to attempt the present study, which arose out of his suggestion that life in Cambridge offered unique oppor- tunities for the observation of river and fenland plants. To his memory I dedicate this book. AGNES ARBER. BALFOUR LABORATORY, CAMBRIDGE. March i, 1920. CONTENTS PART I WATER PLANTS AS A BIOLOGICAL GROUP, WITH A CON- SIDERATION OF CERTAIN TYPICAL LIFE-HISTORIES CHAP. PAGE I. WATER PLANTS AS A BIOLOGICAL GROUP ... 3 (i) Introduction ...... 3 (ii) Biological Classification of Hydrophytes . . 5 II. THE LIFE-HISTORY OF THE ALISMACEAE ... 9 III. THE LIFE-HISTORY OF THE NYMPHAEACEAE AND OF LlMNANTHEMUM ...... 24 IV. THE LIFE-HISTORY OF HYDROCHARIS^ STRA TIOTES, AND OTHER FRESH-WATER HYDROCHARITACEAE . . 42 V. THE LIFE-HISTORY OF THE POTAMOGETONACEAE OF FRESH WATERS ...... 58 VI. THE LIFE-HISTORY OF THE LEMNACEAE AND OF PISTIA 73 VII. THE LIFE-HISTORY OF CERATOPHYLLUM ... 84 VIII. THE LIFE-HISTORY OF THE AQUATIC UTRICULARIAS AND OF ALDROVANDIA . . . . . . 91 IX. THE LIFE-HISTORY OF THE TRISTICHACEAE AND PODOSTEMACEAE . . . . . .112 X. THE LIFE-HISTORY OF THE MARINE ANGIOSPERMS . 123 PART II THE VEGETATIVE AND REPRODUCTIVE ORGANS OF WATER PLANTS, CONSIDERED GENERALLY XL LEAF TYPES AND HETEROPHYLLY IN AQUATICS . . 139 (i) Types of Leaf in Water Plants . . 1 39 (ii) The Facts of Heterophylly under Natural Conditions . . . . . .143 (iii) The Interpretation of Heterophylly . . 155 XII. THE ANATOMY OF SUBMERGED LEAVES . . .163 XIII. THE MORPHOLOGY AND VASCULAR ANATOMY OF AQUATIC STEMS . . . . . .172 XIV. THE AERATING SYSTEM IN THE TISSUES OF HYDRO- PHYTES . ..... 183 CONTENTS 204 210 227 239 CHAP. PAGE XV. LAND FORMS OF WATER PLANTS, AND THE EFFECT OF WATER UPON LAND PLANTS . XVI. THE ROOTS OF WATER PLANTS XVII. THE VEGETATIVE REPRODUCTION AND WINTERING OF WATER PLANTS ..... XVIII. THE FLOWERS OF WATER PLANTS AND THEIR RELA- TION TO THE ENVIRONMENT XIX. THE FRUITS, SEEDS AND SEEDLINGS OF WATER PLANTS ........ PART III THE PHYSIOLOGICAL CONDITIONS OF PLANT LIFE IN WATER XX. GASEOUS EXCHANGE IN WATER PLANTS . . 253 XXI. ABSORPTION OF WATER AND TRANSPIRATION CURRENT IN HYDROPHYTES .... 260 XXII. THE INFLUENCE OF CERTAIN PHYSICAL FACTORS IN THE LIFE OF WATER PLANTS . . .273 XXIII. THE ECOLOGY OF WATER PLANTS . . . 285 PART IV THE STUDY OF WATER PLANTS FROM THE PHYLOGENETIC AND EVOLUTIONARY STANDPOINTS XXIV. THE DISPERSAL AND GEOGRAPHICAL DISTRIBUTION OF WATER PLANTS ..... 295 XXV. THE AFFINITIES OF WATER PLANTS AND THEIR SYSTEMATIC DISTRIBUTION AMONG THE ANGIO- SPERMS ....... 308 (i) The Affinities of Certain Aquatic Angio- sperms . . . . . 308 (ii) Theoretical Considerations . . . 317 XXVI. THE THEORY OF THE AQUATIC ORIGIN OF MONO- COTYLEDONS ...... 322 XXVII. WATER PLANTS AND THE THEORY OF NATURAL SELECTION, WITH SPECIAL REFERENCE TO THE PODOSTEMACEAE ..... XXVIII. WATER PLANTS AND THE 'LAW OF Loss' IN EVOLUTION 327 336 BIBLIOGRAPHY INDEX TO BIBLIOGRAPHY INDEX 349 415 422 LIST OF ILLUSTRATIONS FIG. PAGE Nymphaea lutea, L. [Otto von Brunfels, Herbarum vivae eicones, 1530] ........ Frontispiece 1. Sagittaria sagittifolia, L. Inflorescence. [A. A.] 10 2. Sagittaria sagittifolia, L. Infructescence. [A. A.] 10 3. " Gramen bulbosum aquaticum" [Gaspard Bauhin, 1620] . . II 4. Sagittaria sagittifolia, L. Tuber and submerged leaves. [A. A.] . 13 5. Sagittaria sagittifolia, L. Leaves. [A. A.] 14 6. Sagittaria sagittifolia, L. Plant with stolons and tubers. [A. A.] . 16 7. Sagittaria sagittifolia, L. Base of plant with old tuber and young stolons. [A. A.] 18 8. Sagittaria sagittifolia, L. Diaphragm of petiole. [Blanc, M. le (1912)] ..... 19 9. Ecbinodorusranunculoides^L.^Engtlm. Land and water forms. [A. A.] 21 10. Nymphaea lutea, L. Rhizome. [A. A.] 25 11. Castalia alba, Greene. Rhizome. [A. A.] 26 12. Nymphaea lutea, L. Rhizome with submerged leaves. [A. A.] . 27 13. Castalia alba, Greene. Seedlings. [Massart, J. (1910)] . . 28 14. Cabomba. Shoot with floating and dissected submerged leaves. [Goebel, K. (1891-1893)] . . 29 15. Castalia alba, Greene. Peduncle and flower-bud. [A. A.] . . 31 16. Victoria regia, Lindl. Seedling. [A. A.] 33 17. Nymphaea lutea, L. Fruit. [A. A.] 34 1 8. Nymphaea lutea, L. Seedlings. [A. A.] ..... 35 19. Castalia Lotus, Tratt. Germination of tuber. [Barber, C. A. (1889)] 37 20. Brasenia. Mucilage hairs. [Goebel, K. (1891-1893)] ... 38 21. Map of existing and extinct distribution of Nelumbo. [Berry, E. W. (I91?)] • 39 22. Limnanthemum nymphoides, Hoifmgg. and Link. [A. A.] . . 41 23. Limnanthemum nymphoides, Hoffmgg. and Link. Rhizome. [Wagner, R. (1895)] . 41 24. Hydrocharis Morsus-ranae, L. Buds. [A. A.] .... 43 25. Hydrocharis Morsus-ranae, L. Leaf anatomy. [A. A.] ... 44 26. Hydrocharis Morsus-ranae, L. Stomate. [A. A.] .... 45 27. Hydrocharis Morsus-ranae, L. T. S. submerged leaf. [A. A.] . 45 28. Hydrocharis Morsus-ranae, L. Midrib and inverted bundle from leaf. [A. A.] 46 29. Hydrocharis Morsus-ranae, L. Plant with turions. [A. A.] . . 47 xii LIST OF ILLUSTRATIONS FIG. PAGE 30. Hydrocbaris Morsus-ranae, L. Turion plantlet. [A. A.] . . 49 31. Stratiotes aloides, L. Stem bisected. [Arber, A. (1914)] . . 49 32. Stratiotes aloides, L. Habit drawing. [Nolte, E. F. (1825)] . . 53 33. Stratiotes abides, L. Female flower. [A. A.] .... 54 34. Elodea canadensis, Michx. Wintering shoot. [Raunkiaer, C. (1896)] 55 35. Elodea ioensis, Wylie. Male flowers. [Wylie, R. B. (1912)] . . 56 36. Potamogeton perfoliatus, L. Winter shoots. [A. A.] 59 37. Potamogeton. Branch system. [Sauvageau, C. (1894)] ... 60 38. Potamogeton zosterifolius, Schum. Vascular strands and bast bundles of leaf. [Raunkiaer, C. (1903)] 6l 39. Potamogeton pulcber, Tuckerm., P. natans, L. and P. crispus, L.; — stem-stele. [Chrysler, M. A. (1907)] 62 40. Potamogeton crispus, L., P. lucens, L., P. pusillus, L., P. pectinatus, L.;— stem-stele. [Schenck, H. (1886)] 64 41. Potamogeton natans, L., P. densus, L., P. pectinatus, L.; — root anatomy. [Schenck, H. (1886)] 65 42. Potamogeton crispus, L. Germinating turion. [A. A.] ... 67 43. Potamogeton crispus, L. Germinated turion at advanced stage. [Sauvageau, C. (1894)] 68 44. Potamogeton rufescens, Schrad. T. S. turion. [Gliick, H. (1906)] . 69 45. Zannicbellia polycarpa, Nolte. Flowers. [A. A.] .... 70 46. Potamogeton perfoliatus, L. Fruit wall. [A. A.] .... 72 47. Spirodela polyrrbiza, Schleid. Inflorescence. [Hegelmaier, F. (1871)] 74 48. Lemnagibba, L. [Hegelmaier, F. (1868)] 76 49. Lemna trtsulca, L. [Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.)] . . . -79 50. Lemna trisulca, L. Flowering shoot. [Hegelmaier, F. (1868)] . 79 51. Lemna trisulca, L. T. S. bundle from stalk of frond. [Schenck, H. (1886)] - 79 52. Lemna trisulca, L. Germination. [Hegelmaier, F. (1868)] . . 81 53. Pistia Stratiotes, L. Leaf apex. [Minden, M. von (1899)] . . 82 54. Ceratopbyllum demersum, L. Flowers. [A. A.] .... 85 55. Ceratophyllum demersum, L. Seedling. [Guppy, H. B. (I8941)] . 86 56. Ceratophyllum demersum, L. Stem-stele. [Schenck, H. (1886)] . 87 57. Ceratopbyllum demersum, L. Rhizoid. [Gliick, H. (1906)] . . 89 58. Ceratopbyllum demersum, L. Leaves of water shoot and rhizoid. [Gliick, H. (1906)] . .89 59. Utricularia neglecta, Lehm. Leaf with bladders. [Gliick, H. (1906)] 92 60. Utricularia flexuosa, Vahl. Section through bladder. [Goebel, K. (1891-1893)] 92 61. Utricularia Bremii, Heer. Glands from bladder. [Meierhofer, H. 93 LIST OF ILLUSTRATIONS xiii FIG. PAGE 62. Utricularia Bremii, Heer. Part of leaf with bladder. [Meierhofer, H. (!902)] - 95 63. Utricularia minor, L., with earth-shoot. [Gliick, H. (1906)] . . 96 64. Utricularia minor, L. Leaves of water- and earth-shoots. [Gliick, H. (1906)] . ... 96 65. Utricularia vulgaris, L., with air-shoot. [Goebel, K. (1891-1893)] 98 66. Utricularia neglecta, Lehm. Rhizoids. [Gliick, H. (1906)] . . 99 67. Utricularia vulgaris, L. Germinating seed. [Kamienski, F. (1877)] 100 68. Utricularia exoleta, R.Br. Germinating seed. [Goebel, K. (1891)] 100 69. Utricularia minor, L. Foliage leaf and turion leaf. [Gliick, H. (1906)] . . 102 70. Utricularia vulgaris, L. Leaf with adventitious shoots. [Goebel, K. (1904)] . . 104 71. Utricularia vulgaris, L. Inflorescence axis with adventitious shoots. [Luetzelburg, P. von (1910)] 105 72. Utricularia vulgaris, L. Apical development of shoot. [Pringsheim, N. (1869)] 106 73. Utricularia vulgaris, L. Developing leaf. [Meierhofer, H. (1902)] 107 74. Utricularia minor, L. Anatomy of leaf. [Schenck, H. (1886)] . 108 75. Aldrovandia vesiculosa, L. Leaves. [Caspary, R. (1859)] • • IIX 76. Hydrobryum olivaceum, (Gardn.) Tul. [Warming, E. (i8832)] . 115 77. Dicraea elongata, (Gardn.) Tul. [Warming, E. (i 88 32)] . . 115 78. Dicraea stylosa, Wight. Seedling. [Willis, J. C. (1902)] . . 115 79. Dicraea stylosa, Wight. [Warming, E. (i8832)] .... 116 80. Dicraea stylosa, Wight. Anatomy of thallus. [Willis, J. C. (1902)] . 118 81. Oenone multibranchiata, Matt. [Matthiesen, F. (1908)] . . 119 82. Podostemon Barberi, Willis. Cleistogamic flower. [Willis, J.C. (1902)] 121 83. Cymodocea aequorea, Kon. [Bornet, E. (1864)] .... 124 84. Cymodocea aequorea, Kon. [Sauvageau, C. (I89I1)] . . .125 85. Zostera marina, L. Anatomy of leaf. [Sauvageau, C. (I89I1)] . 128 86. Zostera marina, L. Median bundle of leaf. [Sauvageau, C. (I89I1)] 128 87. Halopbila ovalis, (R. Br.) Hook. fil. [Balfour, I. B. (1879)] . .130 88. Halodule uninervis, Boiss. [Sauvageau, C. (I89I1)] . . .132 89. Posidonia Caulini, Kon. Anatomy of leaf . [Sauvageau, C. (1891 x)] 132 90. Sagittaria sagittifolia, L. Young plant with ribbon leaves. [A. A.] 141 91. Aponogetonfenestralis, (Poir.) Hook. f. Perforated leaf. [Sergueeff, M. (1907)] . . 142 92. Ranunculus Purschii, Rich. Water leaf and land leaf. [Goebel, K. (1891-1893)] . . 144 93. Ranunculus hederaceus, L. [A. A.] . . . . . .145 94. Callitricbe verna, L. Heterophylly. [A. A.] .... 147 95. Hippuris vulgaris, L. Water leaves and air leaves. [Gliick, H. (191 1)] 147 xiv LIST OF ILLUSTRATIONS FIG. PAGE 96. Hippuris vulgaris, L. A case of air leaves followed by water leaves. [A. A.] ... .148 97. Sium latifolium, L. Heterophylly. [A. A.] 149 98. Sium latifolium, L. Submerged leaf. [A. A.] . . . .150 99. Polygonum amphibium^L. Water and land forms. [Massart, J. (1910)] 152 100. Polygonum amphibium, L. Epidermis of water and land leaves. [Massart, J. (1910)] 152 101. Alisma Plantago, L. Seedlings. [A. A.] 153 102. Alisma Plantago, L. Water form. [A. A.] 153 103. Potamogeton natans, L. Effect of transferring land plant to water. [Goebel, K. (1891-1893)] . 154 104. Potamogeton fluitans, Roth. Effect of poor nutrition in water upon a land plant. [Esenbeck, E. (1914)] 158 105. Potamogeton natans, L. Effect of growth as a cutting. [Esenbeck, E. (1914)] . . 159 106. Elodea canadensis, Michx. Leaf anatomy. [Schenck, H. (1886)] . 165 107. Submerged stomates of Callapalustris, L. and Potamogeton natans, L. [Porsch, O. (1905)] 167 108. Potamogeton densus, L. Leaf apex. [Sauvageau, C. (I89I1)] . . 167 109. Myriopbyllum spicatum, L. Leaf anatomy. [Schenck, H. (1886)] . 168 no. Myriopbyllum verticillatum, L. Trichomes. [Perrot, iL (1900)] . 170 in. Callitricbe verna, L. Leaf anatomy of land and water forms. [Schenck, H. (1886)] . 170 112. Hippuris vulgaris, L. Rhizome. [Irmisch, T. (1854)] . . . 173 113. Ranunculus trichopbyllus, Chaix. Stem anatomy. [A. A.] . . 176 114. Callitriche stagnalis, Scop. Stem stele of land and water forms. [Schenck, H. (1886)] ... . . . 176 115. Hippuris vulgaris, L. Relation of cauline and leaf trace xylem. [A. A.] 178 116. Myriopbyllum spicatum, L. Stem anatomy. [Vochting, H. (1872)] 179 117. Myriophyllum spicatum, L. Details of stem anatomy. [Vochting, H. (1872)] . "... 179 n8. Potamogeton natans , L. Diaphragm of stem. [Blanc, M. le (1912)] . 184 119. Hippuris vulgaris, L. Development of stem diaphragms. [A. A.] . 184 1 20. Hippuris vulgaris, L. Origin of cortical lacunae in stem. [Barratt, K. (I9l6)] - 185 121. Stratiotes aloides, L. Origin of cortical lacunae in root. [Arber, A. (1914)] . . 186 122. Jussiaea peruviana, L. Aerenchyma. [Schenck, H. (1889)] . . 190 123. Neptunia oleracea, Lour. Floating shoot. [Rosanoff, S. (1871)] . 191 124. Nesaea verticillata, H. B. & K. Floating tissue. [Schrenk, J. (1889)] 193 125. Potamogeton natans, L. Land form. [A. A.] .... 196 LIST OF ILLUSTRATIONS xv FIG. PAGE 126. Ranunculus aquatilis, L. Water and land seedlings. [Askenasy, E. (1870)] . .196 127. Hottonia palustris, L. Land and water forms. [Prankerd, T. L. (1911)] . . . . 197 128. Littorella lacustris, L. Water and land forms. [Gluck, H. (1911)] . 198 129. Caltha palustris, L. Submerged and air leaves. [Gluck, H. (1911)] 199 130. Cirsium anglicum, D. C. Land and water forms. [Gluck, H. (1911)] 199 131. Water forms of Cuscuta alba, J. & C. Presl, Echinodorus ranuncu- loides (L.), Engelm. and Trifolium resupinatum, L. [Gluck, H. (1911)] . .... 199 132. Hydrocotyle vulgaris, L. Water shoot. [A. A.] .... 201 133. Cardamine pratensis, L. Anatomy of aerial and submerged plants. [Schenck, H. (1884)] 202 134. Ranunculus Flammula, L. Floating leaved form and land form. [Gluck, H. (1911)] . . 203 135. Ranunculus Flammula, L. Submerged form. [Gluck, H. (1911)] . 203 136. Hydrilla verticillata, Presl. Tendril roots. [Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.)] 205 137. Zannichellia palustris, L. and Potamogeton densus, L. Tendril roots. [Hochreutiner, G. (1896)] 206 138. Callitriche stagnalis, Scop. Root stele. [Schenck, H. (i 886)] . 209 139. Vallisneria spiralis, L. Root anatomy. [Schenck, H. (1886)] . 209 140. Naias major, All. and N. minor, All. Root anatomy. [Sauvageau, C. (1889!)]. ..... ... 209 141. Cardamine pratensis, L. Leaves bearing adventitious plantlets. [A. A.] . . . .2,7 142. Littorella lacustris, L. [A. A.] 218 143. Utricularia intermedia, Hayne. Turion leaf and foliage leaf. [Goebel, K. (1891-1893)] 220 144. Myriophyllum verticillatum, L. Habit drawing with inflorescence and turions. [A. A.] 221 145. Myriophyllum verticittatum, L. Germinating turion. [A. A.] . . 222 146. Myriophyllum verticillatum, L. Land form with turions. [Gluck, H. (1906)] . . ... 223 147. Echinodorus ranuncukides, (L.) Engelm. var. repens f. terrestris. [Gliick, H. (1905)] . 224 148. Caldesia parnassifolia, (Bassi) Parl. With turions. [Gliick, H. (1905)] 225 149. Caldesia parnassifolia, (Bassi) Parl. With turions. [Gliick, H. (1905)] 225 150. Utricularia inflata, Walt. Floating organs. [Goebel, K. (1891- 1893)] ... . . .229 151. Hippuris vulgaris, L. Habit drawing. [A. A.] . . . .231 152. Peplis Portula, L. Flowers. [A. A.] 232 153. Heter anther a dubia, (Jacq.) MacM. Cleistogamic flower. [Wylie, 234 xvi LIST OF ILLUSTRATIONS FIG. PAGE 154. Callitriche verna, L. Flowering shoot. [A. A.] . . . 237 155. Pontederia rotundifolia, L. Flowering shoot. [Hauman-Merck, L. (191 31)] . 240 156. Limnanthemum nymphoides, Hoffmgg. and Link. Fruit and seed. [A. A.] . . 240 157. Limnanthemum nympbotdes, Hoffmgg. and Link. Fruit wall. [A. A.] 242 158. Elatine bexandra, D. C. Germination of seed. [Klebs, G. (1884)] 245 159. Zannicbellia polycarpa, Nolte. Fruit. [Raunkiaer, C. (1896)] . 246 160. TrapanatanSjL,. Seed and germination. [Goebel, K. (1891-1893)] 247 161. Zoster a marina, L. Fruit. [Raunkiaer, C. (1896)] .... 248 162. Transpiration experiment. [Sauvageau, C. (I89I1)] . . . 262 163. Callitriche autumnalis, L. Leaf apex. [Borodin, J. (1870)] . . 268 164. Hydrocleis nymphoides, Buchen. Apical cavity of leaf. [Sauvageau, C. (1893)] ... 270 165. Section across White Moss Loch. [Matthews, J. R. (1914)] . . 288 1 66. Ruppia bracbypus, J. Gay. Fruit. [Raunkiaer, C. (1896)] . .319 167. Potamogeton lucens, L. Range of leaf form. [Raunkiaer, C. (1896)] 339 1 68. Potamogeton natans, L. Range of leaf form. [Raunkiaer, C. (1896)] 339 169. " Lamina " of Pontederia cordata, L. and Eicbbornia speciosa, Kunth. [Arber,A. (1918)] . 341 170. Leaf anatomy of Pontederiaceae. [Arber, A. (1918)] . . . 342 171. Leaf anatomy of Sagittaria. [Arber, A. (1918)] .... 345 PART I WATER PLANTS AS A BIOLOGICAL GROUP, WITH A CONSIDERATION OF CERTAIN TYPICAL LIFE-HISTORIES A. W. P. "If... an inquiry into the Nature of Vegetation may be of good Import; It will be requisite to see, first of all, What may offer it self to be enquired of; or to understand, what our Scope is: That so doing, we may take our aim the better in making, and having made, in applying our Observations thereunto." Nehemiah Grew, The Anatomy of Plants, 1682. [ 3 ] CHAPTER I WATER PLANTS AS A BIOLOGICAL GROUP (i) INTRODUCTION WE are living at the present day in what may be described botanically as the Epoch of Angiosperms, or Flowering Plants. The members of this group now represent the dominant type of vegetation and are distributed over nearly all the land surfaces of the globe. The vast majority are typically terrestrial, carrying on their existence with their flowers and leafy shoots in the air, but with their roots embedded in soil of varying degrees of moisture, from which they derive their water supply. This water supply is one of the prime necessities of their life, and in their relation thereto, the plasticity of their organisation is notably exhibited. At one end of the scale there are plants which can withstand long periods of drought and are capable of flou- rishing under desert conditions in which the water supply is minimal. At the other extreme we meet with hydrophytes — plants which have exchanged terrestrial for aquatic life. Those which have embraced this change most thoroughly, live with their leafy shoots completely submerged, and have, in some cases, ceased to take root in the substratum, so that all their vegetative life is passed floating freely in the water — which is to them what atmosphere and soil are to terrestrial plants. The ultimate term in the acceptance of aquatic conditions is reached in certain hydrophytes with submerged flowers, in which even the pollination is aquatic — water replacing air as the medium through which the pollen grain is transferred to the stigma. These fundamental changes in habit are necessarily associated with marked divergences from the structure and life-history of land plants. The result has been that the aquatic flowering plants have come to form a distinct assemblage, varying widely 4 j INTRODUCTION [CH. among themselves, but characterised, broadly speaking, by a number of features associated with their peculiar mode of life. It is the biological group thus formed which we propose to study in the present book. There is good reason to assume that the Angiosperms were originally a terrestrial group and hence that the aquatic Flower- ing Plants existing at the present day can trace back their pedi- gree to terrestrial ancestors. If this be the case, we may interpret the various gradations existing within the hydrophytic group as illustrating a series of stages leading from ordinary terrestrial life to the completest adoption of an aquatic career. At one end of the series we have plants which are normally terrestrial, but which are able to endure occasional submergence, while at the other end we have those wholly aquatic species whose organisa- tion is so closely related to water life that they have lost all capacity for a terrestrial existence. Between these extremes there is an assemblage of forms, bewildering in number and variety. In order to clear one's ideas, it is necessary to make some attempt to classify hydrophytes according to the degree to which they have become committed to water life. It must be realised, however, that, though such a scheme is convenient and helpful in * pigeon-holeing ' the known facts about aquatics, little stress ought to be laid upon it, except as illustrating the striking variety of form and structure met with among these plants. A classification of aquatics on biological lines is highly artificial, and, since it sometimes places in juxtaposition plants which are quite remote in natural affinity, it has only an indirect bearing on questions of phylogeny. The classification of aquatics which forms the second part of the present chapter, is based upon a scheme put forward by Schenck1 more than thirty years ago, which in its main outlines has never been superseded. But the wider knowledge of the group, which has been acquired since that date, has resulted, as is so often the case, in the blurring of the sharp lines of demar- cation between the individual bionomic classes recognised at an 1 Schenck, H. (1885). i] BIOLOGICAL CLASSIFICATION 5 earlier stage. The present writer has freely modified Schenck's scheme, and has carried the sub-division to a further point. The various types met with amongst aquatics are arranged in a linear series for the sake of simplicity ; but this plan is obviously open to the same criticisms as all other linear systems, whether bio- logical or phylogenetic. The following classification is outlined with the utmost brevity, and aims merely at supplying a key to the biological forms encountered. The life-histories of typical plants illustrating the characters of the more important sub- divisions will be considered in some detail in Chapters n— x; but the order in which the life-histories are grouped in these chapters has been determined mainly by reasons of natural affinity, and thus bears no close relation to the following scheme. (2) BIOLOGICAL CLASSIFICATION OF HYDROPHYTES I. Plants rooted in the soil. A. Plants which are essentially terrestrial, but which are capable of living as submerged water plants, though without marked adaptation of the leaves to aquatic life. E.g., Achillea ptarmica, L. (Sneezewort). Cuscuta alba^ J. and C. Presl (Dodder). Glechoma hederacea^ L. (Ground Ivy). B. Plants which are sometimes terrestrial, but sometimes produce submerged leaves differing markedly from the air type. The air leaves are associated with the flowering stage. E.g., Certain Umbelliferae, such as Slum latifolium^ L. (Water Parsnip). C. Plants which produce three types of leaf, (a) submerged, (ft) floating and (c) aerial, according to the conditions — internal or environmental. (i) Plants in which the aerial type of leaf is generally associated with the flowering stage. E.g., Many Alismaceae, such as Sagittaria sagitti- folia, L. (Arrowhead). 6 BIOLOGICAL CLASSIFICATION [CH. (ii) Plants in which the floating type of leaf is generally associated with the flowering stage. E.g., Nymphaea lutea^ L. (Yellow Waterlily). Castalia albay Greene (White Waterlily). Various Batrachian Ranunculi (Water Butter- cups). Callitriche verna, L. (Water Starwort). Potamogeton natans^ L. (Pondweed). D. Plants which may, in certain cases, occur as land forms, but are normally submerged and are characterised by a creep- ing axis bearing long, branching, leafy shoots with no floating leaves, or by a plexus of leafy, rooting shoots without a creeping rhizome. (i) Leafy aerial shoots produced at the flowering period. E.g., Myriophyllum verticillatum, L. (Water Mil- foil). Hippuris vulgaris, L. (Mare's-tail). (ii) Inflorescence raised out of the water, but no aerial foliage leaves except in the land forms. E.g., J^yriophyllum (except M. verticillatum) (Water Milfoil). Hottonia palustris, L. (Water Violet). Many Potamogetons (Pondweeds). (iii) Inflorescence submerged, but essential organs raised to the surface. E.g., Elodea canadensis^ Michx. (Water Thyme). (iv) Inflorescence entirely submerged and pollination hydrophilous. E.g., Naias. Zannichellia (Horned Pondweed). Zostera (Grass- wrack). Callitriche autumnalis^ L. (Water Starwort). Halophila. i] BIOLOGICAL CLASSIFICATION 7 E. Plants which in some cases may occur as land forms, but which are very commonly submerged, and are characterised by an abbreviated axis from which linear leaves arise. (i) Inflorescence raised above the water or borne on a land plant. E.g., Lobelia Dortmanna, L. (Water Lobelia). Littorella lacustris, L. Sagittaria teres, Wats. (ii) Inflorescence sometimes raised above water or some- times submerged. E.g., Subularia aquatica, L. (Awlwort). F. Plants which are entirely submerged as regards the vege- tative organs and which have a thallus (morphologically either of root or shoot nature) attached to the substratum. The flowers are aerial. Tristichaceae and Podostemaceae. II. Plants which are not rooted in the soil, but live unattached in the water. (A transition between I and II is found in Stratiotes abides, L. (Water Soldier), which is rooted during part of the year but floats freely during another part. There are also a number of rooted plants, such as Hottonia -palustris and Elodea canadensis, which are capable of living unattached for considerable periods.) A. Plants with floating leaves or leaf-like shoots. Flowers raised into the air. (i) Roots not penetrating the soil. E.g., Hydrocharis Morsus-ranae, L. (Frogbit). Sfirodela polyrrhiza, Schleid., 1 (Duck- Lemna minor, L. and L. gibba, L.J weeds). (ii) Rootless. Wolffia (Rootless Duckweed). BIOLOGICAL CLASSIFICATION [CH. i B. Plants entirely or partially submerged. (i) Rooted, but roots not penetrating the soil. Floating shoots, formed at flowering time, which raise the flowers into the air. Lemna trtsulca^ L. (Ivy-leaved Duckweed), (ii) Rootless. (a) Inflorescence raised out of the water. Aldrovandia. Utricularia (Bladderwort). (£) Flowers submerged; hydrophilous pollination. Cer atop hy Hum (Horn wort). [ 9 ] CHAPTER II THE LIFE-HISTORY OF THE ALISMACEAE THE Alismaceae1 are perhaps the most typically amphi- bious of all water plants and they vary in appearance according to their environment in a thoroughly protean fashion. The Arrowhead, Sagittaria sagittifolia, L., may be chosen for description as a characteristic member of the family. Seen in ditches and backwaters in the late summer, its fine sagittate leaves and bold inflorescences2 (Fig. i, p. 10) make it one of the most striking of our water plants. It is apparently insect polli- nated, but the records on the subject seem to be confined to the statement that, in the Low Countries, certain species of Fly have been observed to visit the flowers3. The present writer has once noticed a Water-snail crawling over a female flower and engaged in eating the perianth; it is conceivable that these animals may play an occasional part in pollination. The large fruits, whose hassock-shaped receptacles are completely clothed with com- pressed, winged achenes, give the plant a highly individual character (Fig. 2, p. 10). In complete contrast to the flowering form, is the guise which the Arrowhead assumes in deep and rapidly-flowing water. As long ago as 1596* a tuber, bearing strap-shaped leaves, was described by Gaspard Bauhin under the name of "Gramen bulbosum," while in 1 62O5 he published a figure of it, 1 For a systematic review of the Alismaceae see Buchenau, F. (I9O31), and, for a general study of their life-history, Gliick, H. (1905); Gliick's work has been largely drawn upon in the present chapter. 2 On the detailed structure of the reproductive organs see Schaffner, J. H. (1897). 3 MacLeod, J. (1893). 4 Bauhin, G. (1596). 5 Ibid. (1620). 10 ALISMACEAE [CH FIG. i. Sagittaria sagittifolia, L. Top of inflorescence, August 17. JQi?- 6* = whorl of male flowers; $ = whorl of female flowers with withered perianths, (fnat. size.) [A. A.] FIG. 2. Sagittaria sagittifolia, L. Top of infructescence, September 8, 1917. (f nat. size.) A, Longitudinal section of fruit. [A. A.] n] "GRAMEN BULBOSUM AQUATICUM ' n which is here reproduced (Fig. 3). A century later, Loeselius1 recognised these strap-shaped leaves as belonging to the Arrow- head; under the name of "Sagittaria aquatica foliis variis," FIG. 3. Sagittaria sagittifolia, L. An illustration given by Gaspard Bauhin in the Prodromes Theatri Botanici, 1620, under the name of " Gramen bulbosum aquati- cum/' but which in reality represents a germinated tuber of the Arrowhead, bearing ribbon-leaves. he figured a plant bearing both ribbon-leaves and leaves of sagittate shape. The ribbon-leaved, deep-water form has been distinguished as f. vallisneriifolia. An opportunity of examining 1 Loeselius, J. (1703). 12 ALISMACEAE [CH. the plant in its submerged state sometimes occurs when weeds are being cleared out of a river. The semi-transparent leaves — which have been regarded by some authors as purely petiolar1, while others consider them to represent the entire leaf in a rudimentary form2 — often grow to great lengths; the present writer has measured one as long as 6 ft. 9 in.3 from the river Cam. As many as twenty ribbon-leaves are said to be some- times borne by a single plant in very deep water4. The streaming ribbon-leaves of Sagittaria and other submerged plants with the same type of foliage, have a singular beauty when seen forming, as it were, a meadow beneath the surface of the water, moving in the current in a way that recalls a field of wheat swayed by the wind. The ribbon-leaved form of Sagittaria sagittifolia is generally sterile, but the appearance of flowers at this stage is not un- known5. In moderately shallow water, transitions between the aquatic and aerial types of leaf may be observed. The first- formed leaves are band-shaped and submerged, while later ones begin to spread at the apex so as to form a distinct lamina. Some of these transitional leaf-blades, which are of lanceolate to ovate form, float on the water. In another species, Sagittaria natans*, these floating leaves represent the mature type of leaf and are associated with the inflorescence, but, in the Arrowhead itself, yet a third kind of leaf is produced. The abbreviated axis gives off, in succession to the leaves with floating blades, others whose petioles rise into the air and whose laminae become more and more sagittate at the base, until the typical arrowhead form is achieved. The band-shaped leaves, though characteristic of the plant which is wholly or partially submerged, are not con- fined to it. The first leaves produced by a germinating seed or tuber are ribbon-like, whether the plantlet develops in air or water. At the end of May, the present writer has found young 1 Candolle, A. P. de (1827). 2 Goebel, K. (1880). 3 A length of two metres (6 ft. 6 in.) has been recorded by Costantin, J. (1886). 4 /to. (1886). 5 Kirschleger, F. (1856). 6 Wachter, W. n] THE ARROWHEAD 13 plants growing from tubers, among the drift at the edge of a river, with a varying number of ribbon-like leaves, succeeded in some cases by one or two of slightly spathulate form (Fig. 4). Fig. 5, p. 14 represents a young plant found in July which shows a series of leaf stages between the early band-like form and the FIG. 4. Sagittaria sagittifolia, L. Plant with soft submerged leaves growing from a tuber, t; from river drift at the edge of the Cam near Waterbeach, May 31, 1911. (fnat. size.) [A. A.] mature * arrowhead ' type. The significance of this heterophylly and its relation to the environment will be discussed in Chapter xi . .Sagittaria^ like the other Alismaceae, is characterised by the presence of mucilage-secreting trichomes, in the form of scales, in the axils of the leaves. In a paper published a few years ago, ALISMACEAE [CH, FIG. 5. Sagittaria sagitti folia, L. Young plant, July 16, 1910, showing transitions from ribbon-shaped to arrowhead type of leaf. (Reduced.) [A. A.] n] THE ARROWHEAD 15 two American writers1, in describing the seedling of Sagittaria variability allude to the occurrence of a cellular plate just within the cotyledonary sheath. They refer to this as " a vestigial structure " and interpret it as probably representing a second cotyledon. It appears, however, to the present writer that it is much more reasonable, judging from the figure and description given, to suppose that this scale is merely one of the " squamulae intravaginales," whose existence in the seedlings of Sagittaria was placed on record by Fauth2. These structures, which are so common among water plants, belong to the category of hairs; they contain no vascular tissue and cannot be homolo- gised with a foliar organ such as the cotyledon. Plants of the Arrowhead, carefully dug up in the late summer, are found to show preparations for the winter's rest and for next season's growth3. From among the bases of the crowded leaves arising on the short main axis, a number of white stolons protrude (j, Fig. 6, p. 1 6), distinguished from the roots by their greater thickness. They each bear one or more scale-leaves and terminate in a bud (/). The present writer measured a stolon on July 1 6, 1910, which had reached a length of 25 cms.4. Later on, the two internodes below the terminal bud swell up and form a tuber which may be 5 cms. long. As many as ten stolons may arise from the base of a single plant, so that, where Sagittaria grows freely, a very large quantity of tubers are produced. One author5 records that he collected two to three litres of tubers on digging up soil whose superficial area was one square metre. By a downward curve of the stolons, these reproductive bodies are carried some depth into the mud, where they pass the winter. The mature tubers are coloured blue by anthocyanin, which 1 Coulter, J. M. and Land, W. J. G. (1914). 2 Fauth, A. (1903). 3 Nolte, E. F. (1825), Walter, F. (1842) and M (inter, J. (1845). 4 The stolons seem to develop earlier in terrestrial plants than in plants growing in water. The present writer has found that vigorous plants growing in water may show only quite short stolons in the middle of August. 5 Klinge, J. (i 88 1). i6 ALISMACEAE [CH, FIG. 6. Sagittaria sagittifolia, L. Plant dug up July 16, 1910, with five stolons (s) growing from its base among roots, and terminating in young tubers (t). (J nat. size.) [A. A.] n] THE ARROWHEAD 17 occurs in the epidermis. The blue tint seems very constant; it is recorded by European writers and is shown in the coloured illustrations to that splendid Japanese flora, "Honzo Zufu1." The store of reserve material, laid up in the tuber for the succeeding year's growth, makes the Arrowhead a potential food plant. In Germany the tubers are sometimes employed to feed pigs under the name of "Bruch-Eicheln2." They are used in Japan3, while in China the plant is actually grown for the sake of its tubers, which, in cultivation, reach the size of a clenched fist4. The tubers of the related Sagittaria variabilis, sometimes called "Swan's Potatoes5," are said to be eaten by the American Indians under the name of" Wapatoo6." By winter time, the decay of the stolons sets the tubers free from the parent plant, which does not itself survive until the next season. In the spring, the apical region of the tuber grows out into an elongated axis bearing scale leaves, and carrying the terminal bud up to the surface of the mud, where it pro- duces a new plant. Food is absorbed from the parent tuber for some time; it is possible to find a plant still attached to the tuber from which it arose (Fig. 7, p. 1 8) and already itself producing the stolons (j/2) which will develop into the tubers of the next generation. At this stage the parent tuber (/) has given up its stores of food material and is in a dry, spongy, exhausted state. The conditions which influence tuber formation will be dis- cussed in Chapter xvn, when the wintering of water plants comes under consideration. The Arrowhead is reproduced by seed as well as vegetatively. The tubers suffice for colonisation of a limited area, but the seeds serve to distribute the species over greater distances. The mericarps, which each enclose a single seed, are flattened and air-containing; they are suitable for dispersal either by wind or water. Their specific gravity is still further lowered by the presence of an oil in the secretory ducts of the pericarp. The 1 Anon. (1828). 2 Walter, F. (i 842). 3 Anon. (1895). 4 Qsbeck, P. (1771). 5 Paillieux, A. and Bois, D. (1888). 6 Buchenau, F. (1882). A. W. P. 1 8 ALISMACEAE [CH. surface of the mericarps is non-wettable and they often float for long periods, sometimes until frost produces waterlogging of the fruit wall. After the decay of the latter, the embryo is still protected by the cuticularised testa1. The petioles of Sagittaria sagittifolia contain lacunae crossed at intervals by diaphragms (D in Fig. 8). A peculiarity, which has been recorded in connexion with the life-history, is that not st FIG. 7. Sagittaria sagittifolia, L. Base of plant dug up July 16, 1910, showing remains of old stolon (stj from plant of previous year bearing tuber (t) with scale leaves (sc) ; the plant of the current year has also produced a stolon (stz) which will give rise to a tuber later in the autumn. At this stage the old tuber is dry and spongy in texture, having contributed all its reserves to the plant which has sprung from it. (§ nat size.) [A. A.] only roots but also stolons may sometimes break through the diaphragms of the leaf-sheath of living leaves and penetrate as far as 10 cms., running in the petiole parallel to its long axis2. It would be interesting to know whether any significance is to 1 Fauth, A. (1903). 2 Klinge, J. (1881). ii] THE WATER PLANTAIN 19 be attached to this observation, which, as its author points out, suggests a case of auto-parasitism. After the Arrowhead, probably the best known British member of the Alismaceae is the Water Plantain, Alisma Plantago^ L.1 Ac- cording to modern views2, this Linnean species in- cludes two plants which are each worthy of spe- cific rank — Alisma Plan- tago^ (L.) Michalet, and A. graminifolium, Ehrh. The former is more suited to land life, while the latter is typically a water plant. A. PlantagO) (L.) Mich, generally lives in FIG. 8. Sagittaria sagUtifolia, L. Diaphragm (D) , n 1 • of petiole in transverse section. T = wall of shallow water, where air la^una. L = lacuna [BlanCj M le (igi2)^ leaves form the chief as- similatory organs. These are preceded, however, by band-shaped primary leaves (Fig. 101 #, p. 153) and then generally some swimming leaves (Fig. 102, p. 1 53), so that the Water Plantain, like the Arrowhead, produces three distinct types of leaf. In dark situations the swimming leaves may be replaced by sub- merged leaves differing from the ordinary submerged band-leaves in possessing a distinct lamina. This species only flowers suc- cessfully in relatively shallow water in which air leaves can be produced; in moderately deep water, in which submerged and swimming leaves occur, a reduced inflorescence is occasionally formed, but, in very deep water, where all the leaves are band- shaped and submerged, flowers are always absent. Alisma graminifolium^ on the other hand, has its optimum growth in 1 On the details of fertilisation, etc., in this species see Schaffner, J. H. (1896). 2Gluck, H. (1905). 2 — 2 20 ALISMACEAE [CH. deeper water than A. Plantago and is capable of flowering at a stage when it bears band-shaped leaves alone. It was figured in this condition by Loeselius1, more than two hundred years ago, under the name of "Plantago aquatica." It never, either in the seedling or adult form, produces floating leaves. It grows and flowers best in 50 to 70 cms. of water; at a greater depth (2 to 4 metres) flowering is inhibited. In spite of the marked tendency of this species towards a strictly aquatic life, a land form can be obtained in cultivation ; this proves to be identical with the plant, sometimes found wild, which has been called Alisma arcuatum^ Mich. Another closely related genus is represented by the pretty little Echinodorus ranunculoides, whose different forms can be observed, among many other water plants, at Wicken Fen near Cambridge — an untouched fragment of fenland, which has re- tained many of its primitive features. Fig. 9 C shows the young aquatic form, with both narrow submerged leaves and leaves with floating blades. An entirely submerged form has been described, which may flower under water at a depth of three feet2. Fig. 9 B shows the luxuriance which the mature plant may attain, when it grows in water, but raises its leaves and flowers into the air, while Fig. 9 A indicates the general dwarf- ing of the land form. Fig. 147, p. 224, shows the transitions which sometimes occur in this species between inflorescences and entirely vegetative rosettes. The related genus Elisma, with its single species, E. natans^ (L.) Buchenau, is chiefly in- teresting on account of a similar intimate relationship between the inflorescence and the vegetative shoot. The bracts of the inflorescence are in whorls of three ; flowers typically arise in the axils of two of the bracts, while a leafy shoot is developed in the axil of the third. The inflorescences are thus partly repro- ductive and partly vegetative; there are also certain purely vegetative off-shoots, which may be interpreted, in a morpho- logical sense, as inflorescences which have become wholly sterile. 1 Loeselius, J. (1703). 2 West, G. (1910). THE LESSER WATER PLANTAIN 21 FIG. 9. Echinodorus ranunculoides, (L.) Engelm. A, plant from a dried up fen, August 5, 1911 ; B, plant growing in water in a ditch, with aerial leaves only, and very long petioles and flower stalks, June 27, 1914; C, plant with submerged and floating leaves only, from a shallow pool, June 27, 1914. (Reduced.) [A. A.] 22 ALISMACEAE [CH. Another case, in which the replacement of the inflorescence by vegetative structures has been carried much further, is that viCaldesia-parnasslfolia^ (Bassi) Parl., a plant which is somewhat widely distributed in Southern Europe, but does not reach Britain. When it grows in water 30 to 60 cms. deep, the ' in- florescences ' often bear, instead of flowers, vegetative buds about 2 cms. long, which are able to reproduce the plant (Figs. 148 and 149, p. 225). Sometimes these 'turions' as they are called, and also flowers, may occur in the same whorl. Gllick, to whose work on the Alismaceae we owe so much, regards these buds as flower rudiments, which, in consequence of submerged life, have developed in a degenerate vegetative form. This species seems to be losing its power of sexual reproduction, for, even when it bears flowers, they commonly fail to set fertile seed. It affords a good instance of a tendency, common among water plants, to substitute vegetative for sexual reproduction; this characteristic will be discussed more fully in Chapter xvn. The range of leaf-form met with amongst the Alismaceae — not only in passing from species to species, but also in the same individual under different conditions — prompts one to ask which of these divergent types are fundamental and which are derived. Gllick's study of the family has led him to the conclu- sion that the ribbon form of leaf is primitive, and, on this assumption, he suggests the following scheme, as representing successive phyletic stages which may have occurred in the evolution of the leaves ; he admits, however, that the series may conceivably be read in the reverse order. This seriation merely illustrates possible progressive steps and, obviously, does not represent the actual phylogeny of the genera, since examples of Stage I, the most primitive leaf type, and Stage VI, the most highly evolved, are to be found within the limits of the one genus Sagittaria. Stage I. Band leaves alone developed, e.g. Sagittaria feres, Watson. Stage Ha. Band leaves extremely important and associated with the n] LEAVES OF ALISMACEAE 23 flower, but leaves with lanceolate blades also occurring, e.g. Alisma grami- nifolium, Ehrh. Stage II b. Band leaves of considerable importance, but the flowering stage generally associated with aerial leaves with lanceolate blades^ e.g. Echinodorus ranunculoides^ (L.) Engelm. Stage III. Band leaves still important and sometimes associated with the flower, but floating leaves also produced, with a broadly elliptical lamina, sharply marked off from the petiole, e.g. Elisma natans, (L.) Buchenau. Stage IV a. Band-shaped leaves produced, as well as floating leaves and air leaves with a slightly cordate base, e.g. Damasonium stellatum, (Rich.) Pers. Stage IV b. Similar to Stage 1 Va, but the band leaves of less importance, e.g. Alisma Plantago, (L.) Mich. Stage V. Similar to Stage IV, but the base of the lamina definitely cor- date, giving a Nymphaeaceae-like leaf. Band leaves extremely reduced, e.g. Caldesia parnassifoliay (Bassi) Parl. Stage VI. Air leaves of sagittate form. In the transition from the band leaves to the mature leaves analogies can be found for all the preceding types, e.g. Sagittaria sagittifolia, L. CHAPTER III THE LIFE-HISTORY OF THE NYMPHAEACEAE AND OF LIMNANTHEMUM ^ I ^HE Nymphaeaceae, like the Alismaceae dealt with in JL the last chapter, are a typically aquatic family, but, in the Nymphaeaceae, the water habit has become even more firmly established than in the Alismaceae, land forms being relatively rare. The dominant type of leaf has a floating blade, whereas, although this form of leaf occurs among the Alisma- ceae, it occupies as a rule a minor place. The rhizome again, which is seldom a conspicuous organ in the Alismaceae, as- sumes considerable importance in the case of some Nymph- aeaceae, although the family includes also a number of annuals. Our British Waterlilies perenniate by means of rhizomes; these are rich in starch and in the case of some foreign species are used for food1. That of the Yellow Waterlily is epigeal2, with the result that small specimens are occasionally torn from their moorings and found among river drift. The hypogeal rhizomes of the White Waterlily, on the other hand, can seldom be obtained unless they are actually dragged up with a boat- hook out of the mud. The rhizome of Nymphaea lutea, L.3 is a very striking object (Fig. 10 A). It is slightly flattened and of a greenish colour on the upper surface, but pallid and yellowish below. It is decorated with the scars of the leaves (/.j.) of previous years — punctuated by the vascular strands which supplied them — and also with the scars of the peduncles (/>.J.), which can be distinguished by their rounded form. With each leaf-base, three roots are usually associated ; at r^ these roots can 1 Paillieux, A. and Bois, D. (1888). 2Royer,C. (1881-1883). 3 = Nuphar luteumy Sibth. et Sm. CH. in] WATERLILY RHIZOMES 25 be seen as rudiments and at rz as scars, while numerous groups of three mature roots are also shown (e.g. r2). Fig. 10 B repre- sents such a group in further detail. The root system is very elaborate, since the adventitious roots bear branches (Fig. io5) which themselves branch again (Fig. 10 C). At the apex arises the rosette of leaves and flower stalks belonging to the current year, and lateral buds may also be produced (Fig. 10 A^ /.£.). The rhizome may be as thick as a man's arm. FIG. 10. Nymphaea lutea, L. June 30, 1913. A, Part of old rhizome, bases, only, of leaves indicated and all root-branches omitted ; p.s. = scar of peduncle; l.s. = leaf- scar; rlt group of three rudimentary roots arising from a leaf base; r2, correspond- ing group of three roots fully developed; rs, scars of three dead roots; ap, apical region of rhizome; l.b., lateral branch bearing leaves of submerged type only. B and C, roots in greater detail, placed horizontally to save space. B, three young roots from a single leaf-base to show laterals ; C, part of an old root to show branch- ing of laterals. (J nat. size.) [A. A.I Castalia alba 1, Greene, has a shorter rhizome with the leaves crowded in the apical region (Fig. 1 1 A, p. 26). When the older leaves and flower stalks have been removed to expose the apical bud, the most remarkable feature revealed is the occurrence of large membranous stipules, one of which accompanies each young leaf adaxially; each appears to represent a fused pair (j/. in Fig. 1 1 #, C, D). A larger number of roots is associated with each leaf than in the case of the Yellow Waterlily. These i = Nymphaea alba, L, 26 NYMPHAEACEAE [CH. roots may be seen in Fig. n A, and their rudiments (r.) in Fig. 1 1 B and D. In Nymphaea lute a stipules are absent but the FIG. ii. Caslalia alba, Greene. Apical part of rhizome pulled up from bottom of water, June 30, 1913. In A, the leaves and flowers have mostly been cut away to show the young flower buds, the young leaves and the stipules which protect them. In B, C, D, three views are given of a young leaf and its stipule (or pair of stipules united on the adaxial side) st. In B and D the rudiments of the roots, r., are seen at the leaf-base. ($ nat. size.) [A. A.] petioles are winged, and the stipules seem to be replaced by a silky fringe of hairs1. 1 IrmischjT. (1853). in] THE YELLOW WATERLILY 27 Sometimes, if a young rhizome ofNymphaea lute a be brought up from the bottom of the water, it will be found to bear leaves differing widely from the familiar floating type1. They are wholly submerged, relatively short-stalked, translucent, sinu- ous, and of a delicate, flaccid texture recalling the fronds of Ufoa (Fig. 12). In a wood-cut in the famous Herbarum vtvae eicones FIG. 12. Nymphaea lutea, L. Leafy rhizome found floating on Cam near Water- beach, May 17, 1911. Leaves all of submerged type, flaccid, translucent and some- what sinuous at the margin. Rhizome shows leaf-scars, and root-scars in rows of two or three on leaf-bases on under side. (£ nat. size.) [A. A.] of Otto von Brunfels (i 530) — reproduced in the Frontispiece of the present book— some of the outer leaves with short petioles undoubtedly belong to this type, though no description of the submerged leaves of the Waterlilies occurs in botanical litera- ture until a hundred years later2. They were re-discovered — 1Royer, C. (1881-1883), Arcangeli, G. (1890), Brand, F. (1894). 2 Bauhin, G. (1623). See also Desmoulins, C. (1849). 28 NYMPHAEACEAE [CH. like so many matters well known to the ancients — in the nine- teenth century1. These submerged leaves, which are stomateless, are charac- teristically produced in the winter and spring2, and are usually succeeded, in the course of the season, by floating leaves; in FIG. 13. Castalia alba, Greene. Seedlings of various ages illustrating the effect of sowing at different levels on or in the mud (M) at the bottom of the water (W) ; accommodation takes place in length of first internode, acicular first leaf, and petiole of second leaf with ovate lamina. [Massart, J. (1910).] deeper, rapidly flowing water, however, foliage of the submerged type may be exclusively produced for years, even when the plant is so old as to have a massive rhizome3. If the water does 1 The submerged leaves of Nuphar minima^ Smith, were described by Spenner, F. C. L. (1827). 2 Costantin, J. (1886). 3 Goebel, K. (1891-1893). m] THE WATER-SHIELD 29 not freeze, the submerged leaves may vegetate throughout the winter. In 1911, the present writer observed a number of plants of the Yellow Waterlily flowering without having pro- duced any but submerged leaves. Possibly this was associated with the peculiarly brilliant sunshine of that summer, which may have supplied the submerged leaves with unwontedly intense light for assimilatory purposes. Castalia alba produces submerged leaves less freely than Nymphaea lutea and they are said to be incapable of surviving the winter; the first leaves of the seedling are of this type (Fig. 13). FIG. 14. Cdbomba aquatica. Habit drawing to show entire floating leaves and dissected submerged leaves. [Goebel, K. (1891-1893).] The American Water-shield, Cabomba, which is placed in a different tribe from Castalia and Nymphaea, and might, indeed, almost be assigned to a different family, produces submerged leaves of a very distinct type1 (Fig. 14). They are finely dissected and comparable with the submerged leaves of various Batrachian Ranunculi. 1 Goebel, K. (1891-1893) and Raciborski, M. (18942). 30 NYMPHAEACEAE [CH. The floating leaves of the British Waterlilies are typical of swimming leaves in general. The lamina is coriaceous and non- wettable. No leaf which attains to any size can float success- fully unless it be of a strong, leathery texture, since the motion of the water exposes it to tearing, and in heavy rain it is liable to be much more severely battered than an air leaf, which can yield freely in a medium so elastic as the atmosphere1. The normal stomates are borne upon the upper surface of the floating leaves, where they are in contact with the air, but water stomates have been observed on the lower surface in two Ameri- can species of Nympkaea2. These water pores occur in direct communication with the finest ramifications of the tracheal system. The floating leaves are differentiated from the sub- merged leaves at a very early stage, stomates being developed while the leaf is still in the bud3. Floating leaves of an orbicular or peltate form4, more or less recalling those of the Nymphaea- ceae, occur both among Monocotyledons and Dicotyledons and appear to be well adjusted to their particular type of habitat. It is clear, in the first place, that a leaf with an entire outline is less easily wetted and submerged than one which is sub- divided. It is obvious, also, that the centre of gravity of a floating leaf which approximates to the circular form, lies at its central point, and that this is therefore the most mechanically economical position for petiolar support5. In a peltate leaf, such as that of Victoria regia^ this position is approximately achieved, while, in the orbicular Waterlily leaf with a deep sinus at the base, some approach is made to the same condition. All the floating leaves belonging to any associated group of plants, unlike a corresponding series of air leaves, have, without exception, to expand their laminae in one horizontal plane. The competition among the leaves for space is shown by the way in which every available square inch of water surface is 1 Schenck,H. (1885). 2 Schrenk, J. (1888). 3 Costantin, J. (1886). 4 For a mathematical demonstration of the physical advantages accru- ing to a floating leaf from a circular form, see Hiern, W. P. (1872). *Jahn,E.(i897). in] FLOATING LEAVES 31 covered in any spot where Waterlilies abound. In the case of Nymphaea and Castalia^ a place in the sun is secured through the pliability of the petioles, which vary in length according to the depth of the water, but do not rigidly determine the position of the lamina. The variation in length of the peduncles goes hand in hand with that of the petioles. The present writer has measured a peduncle of Castalia alba over six feet in length, and such length is by no means uncommon; Fig. 15 shows the proportion borne by peduncle to flower in this case, the peduncle being represented coiled in order to include its entire length in the sketch. It is rather curious that in the gigantic Victoria regia this great elongation of the peti- oles and peduncles does not occur; the plant flourishes in the shallows and has been recorded in the Amazon region in water only two feet deep1. Another result of the length and flexibility of the leaf-stalk in the Waterlilies is that the lamina can re- spond freely to the move- ment of the water and is hence less liable to be sub- merged. This response is also shown in many other FIG. 15. Castalia alba, Greene. Peduncle and flower-bud to show great proportionate length of peduncle. The peduncle, which was more than 6 feet long, is represented coiled in order to include its whole length in the «U^*.o ,Trk:~U ~~~ * t. A m order to include its wnoie lengtn in me plants Which are rooted at diagram. (Reduced.) May 30, 1911. [A. A.] the bottom of the water and bear floating leaves. Potamogeton natans* is a good example. Here the axis from which the leaves arise, instead of being a solid rhizome lying in or on the mud, as in the Waterlilies, 1 Spruce, R. (1908). a Jahn, E. (1897). 32 NYMPHAEACEAE [CH. takes the form of a slender stem occupying a slanting position in the water. The petioles arise obliquely from the flexible axis, to which they have a very pliable attachment. If the stem be pushed to and fro in the water, the leaves follow all its move- ments while yet retaining their position on the surface. In the case of such a hydrophyte as Hydrocharis, on the other hand, in which not only the leaves but the rosette as a whole floats freely, the entire plant responds to every movement of the water. In spite, however, of a form and structure suited, up to a certain point, to their environment, floating leaves still remain liable to serious risks of wetting and submergence; this is proved by the fact that plants bearing such leaves are quite unable to colonise windy and exposed surfaces where the water is liable to be rough1. In the summer, in addition to the floating leaves of Castalia alba, others may be seen which rise well above the water surface and are typical air leaves in appearance. The White Waterlily is even able, on occasion, to develop a terrestrial form which can vegetate for an entire summer without submergence2. The leaves of this land form are described as generally being short-stalked, with their lower surfaces almost on the ground. Eighteen centimetres is the greatest diameter recorded: the margins are inrolled towards the upper side. We showed that in the Alismaceae it is possible to arrange the species in a series beginning with those in which the leaves are extremely simple and concluding with those in which they are highly differentiated, such as Sagittaria sagittifolia. We also pointed out that in the Arrowhead the successive juvenile leaves epitomised the series — recalling the various mature forms of leaf characteristic of the less highly differentiated species. In both respects the Nymphaeoideae run strictly parallel with the Alis- maceae. Victoria regia may be regarded as occupying the same position among the Nymphaeoideae as Sagittaria sagittifolia 1 See pp. 288, 289. 2 Bachmann, H. (1896). A land form of Nuphar pumilum (Nymphaea pumila^ Hoffm.) was obtained experimentally by Mer, £ (I8821). Ill] SEEDLINGS OF VICTORIA REGIA 33 among the Alismaceae. The leaf-succession in the Giant Waterlily of the Amazons was long ago recorded1, but the full appreciation of its significance we owe to Gwynne-Vaughan2, who contributed greatly to our knowledge of the Nym- phaeaceae. He pointed out that the successive leaves of the Victoria regia seedling show a progressive change from the acicular primor- dial leaf to the peltate form of the mature leaf. The fol- io wing account of the series is derived from his work : The first leaf is acicular and without a blade (/a in Fig. 1 6). The second leaf is elon- gate lanceolate, sometimes with two hastate lobes, and resembles the adult leaves of Barclay a (/2 in Fig. 1 6). The third leaf \s elongate- hastate to deltoid-hastate, recalling the sagittate leaves of Castalia pygmaea, Salisb., etc. At the base of the lamina, just above the insertion of the petiole, FIG. 1 6. Victoriaregia.Undl. Seedling, showing there is a little pocket or a£c;jlar first; leaf '/• and ha*tate sec°nd and r. ^ third leaves 12 and ls. (According to Gwynne- pOUch On the adaxial Side, Vaughan, D. T. (1897), the second leaf is more whirh ar^Mrcf^ K/» fru-m^rl commonly lanceolate.) st., stipules of second Which appears tO Deformed leaf which protect the third. (Nat. size.) [A. A.] by the fusion of the auricles at their bases. The fourth leaf is the first swimming leaf, and is distinctly 1 Trecul, A. (1854). 2 Gwynne-Vaughan, D. T. (1897). A.W. P. 34 NYMPHAEACEAE [CH. peltate, by the further fusion of the two auricles. It recalls the adult leaf of many Castalias, e.g. Castalia Lotus, Tratt. It is the first leaf to bear spines. The succeeding leaves become more and more orbicular in outline, as the auricles become fused along a successively greater part of their length. As in the case of Sagittaria sagitti- folia, the leaf of the mature plant passes, in its youth, through stages parallel to those permanently retained by the embryonic leaves. The flowers of the Nymphaeaceae do not show any obvious relationship to their aquatic life, except perhaps in the case of Euryale ferox1, which is described as exhibiting submerged cleistogamy. The enormous flowers of Victoria regia^ the Giant Waterlily, apparently attract night-flying insects, but no critical observations seem to have been made in the native haunts of the plant. In captivity, each flower partially opens one evening, closes next morning and opens completely on the next evening. It remains open until the hotter hours of the suc- ceeding day, when it finally closes2. When the flowers open they exhale a strong scent, and much heat is also evolved; the tempera- ture of the flower may rise to 10° C. above that of the surrounding air. The heat and perfume are developed mainly in the carmine-red, sigma-shaped outgrowths at FlG I?> ^ymphaealutea> the apices of the carpels, apparently at the L- Fruit showing Per- c i 11-11 • Q sistent calyx. August n, expense of the starch which they contain3. I9IO (jnat.size.) [A.A.j The flower sinks after pollination, and the fruit ripens in the water about six weeks after flowering4. The fruits and seedlings5 of our British Waterlilies are of considerable interest, although the young plants do not display 1 Goebel, K. (1891-1893). 2 Seidel, C. F. (1869). 3 Knoch, E. (1899). * Seidel, C. F. (1869). 5 For very early and good figures of the primordial leaves of the White and the Yellow Waterlily see Tittmann, J. A. (1821). m] FRUITS OF YELLOW WATERLILY 35 such an extensive series of leaf-forms as the seedling of Victoria regia. The green bottle-shaped fruits which succeed the yellow flowers ofNymphaea lutea (Fig. 17) are usually found floating just at the surface of the water. Water-fowl are occasionally seen pecking at them1. In order to follow the dehiscence and germination, the present writer brought some fruits collected on October i, 1914, into the laboratory, and kept them in an aquarium. In the course of the first few days the pericarp began to disintegrate. The green fruit- wall burst irregularly at the base and the torn ' segments gradually curled right up round the stigmatic disc, disclosing the seed-containing loculi. These, which were snow-white, owing to the presence of air in their walls, soon became detached from the fruit, and for a time floated on the water, either singly or in groups ; but, in a couple of days or so, they ill J , ' * FIG. 1 8. Nymphaea lutea, L. .4, seedling had become water-logged and April 23, 1915 (x 2). B, seedling May 28, B had Sunk to the bottom of the laroirootnairs;c,c=cotyledons;^.=plu- bellT-jar2. It has been shown3 mule; llt 12, 13, first, second and third that these detached loculi are leavesofplumule^=seed;o=operculum. clothed with thin-walled cells which secrete much mucilage outwards. The cells have at first 1Guppy, H. B. (1893). 2 On dehiscence of Castalia alba see p. 302. 3Raciborski, M. (18942). 3—2 36 NYMPHAEACEAE [CH. a rich starch content, which diminishes as the mucilage is developed — suggesting that the mucilage is formed at the expense of the starch. By November 24, 1914, the loculi under observation had mostly decayed completely, and the seeds were set free. They remained dormant throughout the winter, but by April 23, 1915, a large number had germinated and there was a forest of slender, grass-like, first leaves arising from the bottom of the bell-jar. Seedlings at this stage are shown in Fig. 1 8 A^p* 35 ; the seed-coat opens by means of an operculum (o) to emit the radicle. During the summer these seedlings developed a number of submerged leaves with lanceolate blades (Fig. 18 5), which increased in number until, on September 18, some of the plants had as many as seven such leaves. In spite of the unna- tural conditions under which they were living, many of the little plants survived the winter and, by the spring of 1 9 1 6, they had developed distinct but miniature rhizomes marked with leaf- scars. The leaves were still of the submerged type only. But the most interesting event of this second spring was the germina- tion of a very large number of seeds which had remained dor- mant for eighteen months. This delay in the sprouting of the seeds is not unusual in water plants (see p. 243). Unfortunately the frost of the very severe winter 1916-1917 destroyed the aquarium, and these observations came to an abrupt end. Vegetative reproduction, though not so universal among Waterlilies as in some other groups of aquatics, is by no means rare. In certain cases tubers are formed as part of the ordinary course of development of the species, while in Castalia Lotus1- the flowers may, under the abnormal conditions due to cultivation, be replaced by tubers which can reproduce the plant (Fig. 19). Like the seedlings, these young plants deve- loped from a germinating tuber have a simple type of first leaf ft). The anatomy of the Nymphaeaceae has been investigated by 1 Barber, C. A. (1889). m] POLYSTELY 37 Gwynne-Vaughan1. The rhizomes contain an indescribable con- fusion of bundles, which he suggests may have been derived from a simpler structure previously existent in a stem with longer internodes; the adoption of a rhizomic habit, associated with telescoping of the internodes, might well lead to this extreme complexity. The most interesting anatomical feature of the family, however, is the occurrence of polystely. In the rhizome of Victoria regia " all the root-bearing bundles belonging to the same leaf-base are grouped together so as to form a structure having the appearance of a defi- nite and distinct stele," in which about twenty bundles form a ring. However the most typi- . 1 . FIG. 19. Castalia Lotus, Tratt. (Nymphaea Lotus, Cal polystely OCCUrS, not L.) var. monstrosa. Germination in spring of a in the rhizomes them- tuber which has developed in place of a flower ; /t, simple first leal (Reduced.) [Barber, C. A. (1889).] selves, but in the elon- gated tuber-bearing stolons, which certain species of Castalia produce as lateral branches. In the stolons of Castalia flava, for instance, the bundles are arranged in four or five widely separated groups or steles, each enclosed in an endodermis and surrounding a protoxylem canal. In Cabomba, on the other hand, it is the rhizome in which polystely occurs, though in the simplest possible form; two steles occur throughout, each consisting of a single pair of bundles. The significance of polystely in aquatic plants will be considered in Chapter xm. The Nymphaeaceae have a remarkably well-developed aerating system in their leaf- and flower-stalks. The long peduncles of Waterlily flowers are said to have been sold in the bazaars at Cairo as tobacco pipes: the base of the flower, which was 1 Gwynne-Vaughan, D. T. (1897); see also Trecul, A. (1845) and (1854), Wigand, A. (1871), Blenk, P. (1884), Strasburger, E. (1884), etc. 38 NYMPHAEACEAE [CH. destroyed, formed a hollow for the lighted tobacco, and the smoke passed through the air-spaces in the stalk1. The mucilage which coats the young organs in most of the Nymphaeaceae will be considered later2. It is secreted by glandular hairs (Fig. 20). Oiir ~Rrif-ic;h Waterlilips FlG' 2O' Brasenia Schreberi, J. F. Gmel. Part sn wate ines of transverse section of yojmg leaf to show belong tO the Central tribe thesecretoryhairs,w. h. .surrounded by alayer r « r .-, XT , . of clear mucilage, m. [Goebel, K. (1891-93).] of the family — Nymphaeoi- deae — of which Euryale and Victoria also form part. Two other tribes are recognised — the Cabomboideae and the Nelum- bonoideae — which differ markedly from the Nymphaeoideae. The Cabomboideae are in many respects relatively simple ; they have free carpels, and Cabomba also has a less complex type of anatomy than the rest of the family. Brasenia Schreberi, which belongs to this tribe, is notable for the enormous development of surface mucilage (Fig. 20) 2. The Nelumbonoideae include the Sacred Lotus, Nelumbo Nelumbo and one other living species belonging to the same genus. In Cretaceous and Tertiary times the genus had, how- ever, a cosmopolitan range (Fig. 2 1)3. This tribe, and the Water- lilies proper, differ so much that they have been described as having nothing in common except the number of cotyledons, the polypetalous flowers, the numerous stamens, and the medium in which they live4. The acyclic arrangement of the petals and stamens might also be mentioned as constituting a similarity to some of the Nymphaeoideae. The exalbuminous seeds5 and the carpels sunk in the curious obconical receptacle, are indeed difficult to reconcile with the characters of the other Water- lilies. Gwynne-Vaughan6 pointed out that Nelumbo shows an 1 Raffeneau-Delile, A. (1841). 2 See pp. 271, 272. 3 Berry, E. W. (1917). 4 Trecul, A. (1854). 6 Wettstein, R. von (1888). 6 Gwynne-Vaughan, D. T. (1897). in] NELUMBO AND LIMNANTHEMUM 39 almost complete absence, both in leaf and stem, of these fea- tures that may be regarded as primitive for the family. Nelumbo may possibly be interpreted as the most highly differentiated of the Waterlilies, and part of its peculiarities may perhaps be due to the fact that it is rather a marsh plant than a true aquatic. Possibly it is a genus descended from aquatic ancestors, which has reverted in some degree towards a terrestrial life1. Another genus which, though extremely distant from the Waterlilies in its systematic position, yet in its life-history resembles them in some degree, may be mentioned at this FIG. 21. Sketch map showing the existing and geologic distribution of Nelumbo. The obliquely lined areas represent the range of the two existing species, while the Cretaceous and Tertiary records which occur outside these areas are marked by solid black circles. [Berry, E. W. (1917).] point. This is Limnanthemum (Villarsia\ a member of the Gentianaceae, which is represented in Britain by the beautiful Z. nymphoides with its fringed yellow flowers. Like Castalia and Nymphaea it has a rhizome at the bottom of the water while its leaves float at the surface (Fig. 22, p. 41). The length of the inter- nodes of the rhizome varies with the time of year2 (Fig. 23, p. 41). In the autumn, the leaves are closely packed and the adventitious roots hold the axis with its abbreviated internodes close to the ground. In the spring, elongated internodes are 1 Dollo, L. (1912). 2 Wagner, R. (1895). 40 LIMNANTHEMUM [CH. produced and the axis ends in a cymose inflorescence with a ter- minal flower. The shoot morphology is somewhat puzzling, and remained obscure until it was elucidated by Goebel1 who studied L. indicum and other species from this point of view. In plants of Limnanthemum^ examined at the flowering season, it is found that a long stalk given off from the rhizome appears to bear both a lamina and flowers, or, in other words, that the flowers seem to arise laterally from a leaf-stalk. In reality this long stalk is however the axis of the inflorescence, and only the short segment of leaf-stalk above the inflorescence is actually the petiole. This petiole has a short, sheathing base, which in youth surrounds the inflorescence. In development, the foliage leaf pushes the growing point to one side and comes to occupy the terminal position. Goebel considers that this peculiar mode of growth confers a definite biological advantage. The breadth of the leaf-surface resting on the water gives the inflorescence the necessary support, while the elongated inflorescence axis forms a substitute for both the elongated petiole and peduncle of the Waterlilies. The flower is raised well above the surface of the associated leaf and thus rendered conspicuous to insects. The products of assimilation find their way by the shortest route to the ripening fruit, whereas in Castalia and Nymphaea they have to descend many feet to the bottom of the water and then rise again a similar distance to the flower, because there is no connexion between lamina and flower, except via the rhizome. But, as Goebel suggests, such an arrangement as that met with in Limnanthemum would have less value in the case of the Waterlilies, because the Nymphaeaceae store so much food in their rhizomes that the ripening fruit is not dependent upon the products of contemporaneous assimilation. It would be utterly unsafe, however, to suppose that the morphological differences between the Waterlilies and Limnanthemum are to be explained on such simple adaptational lines, though it is obvious, from the success which both families achieve, that their re- spective types of construction must be well suited to aquatic life. 1 Goebel, K. (1891) and (1891-1893). Ill] LIMNANTHEMUM FIG. 22. Limnanthemum nymphoides, Hoffmgg. and Link, showing rhizome and leaf -scars. River Ouse. May 30, 1911. (Reduced.) [A. A.] FIG. 23. Limnanthemum nymphoides, Hoffmgg. and Link. Rhizome with long and short internodes; T, terminal flower. (Reduced.) [Wagner, R. (1895).] CHAPTER IV THE LIFE-HISTORY OF HTDROCHARIS, STR4TIOTES, AND OTHER FRESH-WATER HYDROCHARITACEAE A BIOLOGICAL classification of water plants, such as £\. that outlined in Chapter i, has little in common with any phyletic scheme. The incompatibility between biological and phylogenetic systems is particularly well illustrated in the Hydrocharitaceae, which include — besides some marine ge- nera— both marsh or shallow-water plants with air leaves, sub- merged plants and floating plants. As an example of the latter we may choose the Frogbit, Hydrocharis Morsus-ranae, the only British plant with typical floating leaves which swims freely in the water. Other members of the genus however, e.g., H. asiatica1 and H. parnassifo/ia2, have air leaves. In the case of H. Morsus- ranae it is possible to produce a land form artificially3, and this form has also been recorded on one occasion in nature4. In places where the Frogbit flourishes, the surfaces of the ditches and dykes which it inhabits are often completely covered by its leaves, which resemble a miniature edition of those of the White Waterlily. These leaves are produced in rosettes from a tiny, abbreviated stem, which gives rise during the summer to numerous lateral stolons, each ending in a rosette similar to the parent, and repeating the production of stolons da capo. At the base of each rosette, a number of roots of a greenish colour are produced. They hang down into the water, but do not enter the substratum except occasionally in the shallows5. These roots bear, along the greater part of their length, a very large number of unusually long root-hairs, which are well known as 1 Solereder, H. (1913). 2 Solereder, H. (1914). 3 Mer, E. (I8821). 4 Gluck, H. (1906). 5 Goebel, K. (1891-1893). CH. iv] THE FROGBIT 43 favourable material for observing the rotation of protoplasm. The roots, with their thick mat of root-hairs, get much tangled together, and the countless stolons growing in every direction are similarly enlaced, with the result that Hydrocharis forms a thick carpet which can scarcely be submerged even by rough movements of the water. Detritus collects between the root- FIG. 24. Hydrocharis Morsus-ranae, L. A, dissection of a summer bud, just open- ing; (i)-(vi) show the result of removing successive members. B (i), a bud of which one leaf has unfolded ; B (ii) shows the result of removing the outer scale leaves and the stipules of the first foliage leaf; ax± and ax2, stolons terminating in first and second bud; flf fz, fs, successive foliage leaves; stl, st2, stipules belonging to /! and/2; s and s, outer scale leaves; rx and rz, roots belonging to first and second bud. [A. A.] hairs and may serve as a source of food. This colonial mode of growth offers serious resistance to the intrusion of other water plants. The bud, in which each stolon terminates, is enclosed in two delicate, membranous scales (s and s Fig. 24 A (i)). These are 44 HYDROCHARITACEAE [CH. interpreted as paired axillary stipules, whose leaf-blade is generally rudimentary1. They are succeeded by a foliage leaf, with its blade tightly inrolled (/j), whose stipules (st-^ enclose the next foliage leaf (/2). The young stolon of the next genera- tion (*#2) is also present in the bud. Fig. 24 B (i) shows a bud at a later stage in which the first foliage leaf is fully expanded and the root has grown to a considerable length. oc. FIG. 25. Hydrocharis Morsus-ranae, L. A, T.S. leaf; B, tangential section through leaf at level of arrow in A ; C, upper epidermis with open stomates ( x 78 circa) ; st. = stomate; ac. = air cavity; d^ = diaphragm in section; dz = diaphragm in surface view; /= fibres; vb. = vascular bundle; xy. = xylem; p h. = phloem; ue. = upper epidermis; le. = lower epidermis; c = thin layer of cuticle on upper surface ; p = palisade parenchyma. [A. A.] The structure of the lamina of Hydrocharis may be described in some detail as an example of the anatomy of a floating leaf (Fig. 25). The upper surface is clothed with an epidermis whose cells contain a few chlorophyll grains. The outer wall is sculptured internally, and bears a delicate layer of cuticle externally. The stomates, which are confined to the upper 1 Glttck, H. (1901). iv] THE FROGBIT 45 surface of the leaf, have slightly prominent, external, cuticular ridges (Fig. 26); it is probable that here, as in Trianea and in certain other plants with floating leaves, the closure of the stomates is brought about by the approximation of these ridges, rather than by the bulging of the ventral walls1. Haberlandt has suggested that this form of stomate is adapted to diminish the risk of capillary occlusion of the aperture by water. The palisade parenchyma, which lies beneath the upper epidermis, is, in normal leaves of Hydrocharis^ extremely well differentiated (Fig. 25). On one occasion, however, in the latter part of May, the present writer found a number of plants which were entirely submerged, the winter buds having FIG. 26. Hydrocharis Morsus-ranae, L. FIG. 27. Hydrocharis Morsus-ranae, L. T.S. upper epidermis passing through a T.S. leaf of young plant growing entirely stomate. (x3i8.) [A. A.] submerged at the bottom of a ditch, May 17, 1911. (x78 circa.} [A. A.] apparently been caught in an algal tangle at the bottom of a ditch, so that they were unable to reach the surface, but un- folded beneath the water. The green colour of these leaves was unusually pale, and a section of one of them revealed the fact that the palisade region was poorly differentiated, the cells being scarcely elongated (Fig. 27); it was, in fact, a typical ' shade leaf.' The spongy mesophyll was developed normally. In Hydrocharis this tissue is not distributed in the irregular fashion with which we are familiar in land plants, but it takes the form of plates of cells disposed in a polygonal mesh-work over the lower epidermis, which itself contains a small amount of chlorophyll (Fig. 25 $). Attention has been drawn by 1 Haberlandt, G. (1914). 46 HYDROCHARITACEAE [CH. Solereder1 to an anatomical peculiarity of the laminae, the occurrence, namely, of small inversely orientated bundles in the mesophyll (Fig. 28)2. Hydrocharis is generally described as dioecious, but further observations are needed on this point. A botanist who examined the species in Sweden records that, if the male and female * plants ' are removed from the water without breaking the intermediate stolons, they are found in reality to be shoots xy- FIG. 28. Hydrocharis Morsus-ranae, L. Midrib (m.) and adjacent inverted bundle (i.b.) from transverse section of leaf. xy. = xylem ; ph. = phloem ; l.b. = lateral branch of midrib ; px. = protoxylem. ( x 198 circa.} [A. A.] belonging to the same complicated vegetative system, and not separate individuals3. Though the flowers of the Frogbit are not uncommon, seed is hardly ever set in this country. The ripened seed vessels are to 1 Solereder, H. (1913). 2 £f. StratioteS) p. 52. See also pp. 337-345. 3Lindberg,S. O. (1873). iv] THE FROGBIT 47 be found, however, in Continental stations; dehiscence is said to be brought about through the pressure of a slimy mucilaginous mass produced from the testas1. As in so many water plants, vegetative reproduction is the chief method of continuance of the species; it occurs by means of winter buds or * turions,' which in the late summer begin to replace the ordinary buds (Fig. 29). The turions differ from the leaf-buds, which FIG. 29. Hydrocharis Morsus-ranae, L. Part of plant, October i, 1910, showing turions, marked solid black. (Reduced.) [A. A.] precede them throughout the spring and early summer, in the fact that the two first scale leaves do not unfold, but firmly enwrap the succeeding leaves, while the roots, instead of being developed at once, remain within the axis as rudiments. The iQoebel, K. (1891-1893). 48 HYDROCHARITACEAE [CH. cells of the short, thick stem are packed with large, compound starch grains. The stolons bearing winter buds are readily distinguishable, since they incline downwards in the water, whereas those bearing the summer buds are horizontal or turn slightly upwards. By the early autumn (e.g. October i), the turions are ripe and a slight touch detaches them at the absciss layer, which traverses the stolon close to the base of the bud. They sink through the water, owing to the starch with which they are laden, and, since the centre of gravity lies in the solid, basal region, the morphological apex always remains uppermost. If a handful of turions be dropped into a tumbler of water, it is very pretty to see them all balanced erect, only the tiny segment of the stolon, between the absciss layer and the base of the turion, resting on the bottom and forming, as it were, an almost microscopic pedestal. They recall the little tumbling toys made for children, which are so weighted that no treat- ment, however rough, can prevent their coming to equilibrium in the vertical position. The turions, which are protected externally by a layer of muci- lage, pass the winter in the mud at the bottom of the water. It has been demonstrated experimentally that they can remain dormant for at least two years without losing their power of germination. The dormancy has been shown to be due to lack of light1 and can be induced if the buds are not buried but are merely dark- ened. The increased sunshine of spring or early summer is the signal for renewed development. The present writer has found that these turions will readily survive the winter at the bottom of an ordinary rain-water tub. It was noticed in one season that, whereas no plantlets were visible in the tub on May 10, by May 1 5 about seven had risen to the top and were unfolding. This occurred after a long period of warm weather. The de- velopment of the little plants coincided remarkably in point of time ; on May 1 6 they were practically all at the same stage (Fig. 30). In each case the three outer scales had turned back 1 Terras, J. A. (1900). See also p. 280. iv] THE WATER SOLDIER 49 so that their tips were below the base of the bud and four or five foliage leaves had unfolded. The two first of these leaves had tiny laminae; no roots were yet developed. The rare land form of Hydrocharis Morsus-ranae produces turions earlier in the year than the water form; they are gene- rally subterranean l. Stratiotes aloides, another British member of the Hydro- charitaceae, resembles Hydrocharis very closely in its flower, but is quite unique in vegetative structure. One of its names, " Water Aloe," vividly suggests the character of its appearance. -St. FIG. 30. Hydrocharis Morsus-ranae, L. FIG. 31. Stratiotes aloides, L. Semi- Young plant developed from a turion, diagrammatic sketch of stem, as it showing the stage reached on May 16, appears in August, bisected longitudi- 1911. (Nat. size.) [A. A.] nally (v.c.= vascular region of stem; c=stem cortex; l.t.= leaf- trace; /=leaf ; st. = young stolon ; s = squamula intrava- ginalis ; i= adventitious root) . (Slightly enlarged.) [Arber, A. (1914).] From an abbreviated, almost tuberous stem (Fig. 31) arise a very large number of long, linear leaves, serrated at the edge so sharply as to demand a caution in handling which justifies the plant's generic name and also its commonest English title — " Water Soldier." The leaves may be nearly two feet long. Though the plants of Stratiotes live submerged for the greater part of the year, the present writer has noticed, in cultivating them among other aquatics, that their aloe-like form has the effect of keeping the water surface above them clear of swimming plants. 1 Gluck, H. (1906). A. W. P. 50 HYDROCHARITACEAE [CH. From the lower part of the stem of the Water Soldier, nu- merous green, unbranched roots hang down into the water. These roots may attain great lengths. On August n, 1910, the present writer measured three roots, each over 40 inches long, growing from the base of one big plant, while on June 30, 1913, seven roots belonging to a single plant, were found to attain an average of nearly 33 inches in length. The rate of growth of these roots is singularly rapid ; an elongation of over 2 inches in 24 hours was several times recorded in the case of plants growing under somewhat uncongenial conditions in a London garden1. There is no doubt that, at stages when the Water Soldier is floating freely, these long roots balance it in an erect position ; if they are destroyed it is found that the plant can no longer maintain its equilibrium. The classic account of the life-history of Stratiotes aloides is that by Nolte2 which was published nearly a century ago. He describes the young plants as rising to the surface in the spring, sinking at fruiting time and rising again, after the seed has been shed, before finally sinking for the winter. The process appears, however, to be much less regular than would be gathered from Nolte's description3 and no later observer seems to have wit- nessed the rising of the Water Soldier twice during the year. The plants certainly show a gradual rise in the spring and summer, while they sink again in the autumn, but the move- ments probably vary with the depth and composition of the water, and they may be influenced by the achievement or failure of fertilisation. The actual mechanism of the rising and sinking process has now been ascertained4. Stratiotes is apt to frequent water rich in lime5 and the sinking in autumn is brought about by the deposition of calcium carbonate upon the surface of the leaves, until a point is reached at which the specific gravity of the plant becomes higher than that of the surrounding 1 Arber, A. (1914). 2 Nolte, E. F. (1825). 3 Geldart, A. M. (1906) and Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.). 4 Montesantos, N. (1913). 5 Davie, R. C. (1913). iv] THE WATER SOLDIER 51 water1. It has been shown experimentally2 that, if the chalky deposit be carefully removed from the surface of a plant which is stationed at the bottom of the water, it immediately rises to the top. In nature, the rising of the plant in spring is brought about by the relative lightness of the young leaves, due to the absence of a surface layer of calcium carbonate. As these young leaves become more and more numerous in proportion to the old leaves with their heavy deposit, the specific gravity of the plant becomes less and less, until at last it is lighter than water and floats up to the surface. The incrustation of the leaves of Stratiotes is by no means unique; it has long been known that aquatic plants living in 1 hard ' water are liable to become covered with a chalky coat. The generally recognised explanation is that, since calcium car- bonate is scarcely soluble except in water containing carbonic acid, the abstraction of carbon dioxide, by the green organs of aquatics, leaves the chalk as a deposit on their surfaces. This theory is due to Pringsheim3, who demonstrated the truth of his view by a series of very delicate experiments, in which he actually observed microscopically the deposition of crystals of calcium carbonate upon the surface of moss leaves, algal fila- ments, etc., immersed in water containing carbon dioxide and calcium carbonate in solution. Owing to the curious mode of life of Stratiotes^ its youngest leaves are usually entirely submerged, but when mature, they may be submerged for part of the year but raised above the surface for another part. It was formerly supposed that the distribution of the stomates on the leaves could be directly traced to the action of the environment. For instance, it has been stated4 that, in a single leaf which was partly submerged 1 In justice to Nolte, it ought to be mentioned that he anticipated the discovery that the rising and sinking of the plant was due to differences in specific gravity between the old and young leaves, but he made the mistake of supposing that the greater weight of the old leaves was due to waterlogging. 2 Montesantos, N. (1913). 3 Pringsheim, N. (1888). * Costantin, J. (i8853) and (1886). 4—2 52 HYDROCHARITACEAE [CH. and partly aerial, the exposed region bore stomates, while the submerged part had none. Recent work has shown that this is altogether too simple an account of the position. It has been demonstrated1, for instance, that leaves which are entirely, or almost entirely submerged, may nevertheless have stomates throughout their entire length. On the other hand, in the case of a plant which was growing at the bottom of the water, and of which the outer leaves were partly aerial, it was found that these outer leaves bore no stomates whatever, but a transition to stomate-bearing leaves was observed among the younger leaves; the youngest, which were also the deepest in the water, bore the most numerous stomates. The interpretation sug- gested by the writer to whom we owe these observations, is that the leaf with stomates is the higher form, which can only be developed in favourable surroundings, while the stomate-free leaves are primary leaves, occurring typically under conditions of poor nutrition. We shall return to this subject later on, in considering heterophylly in general2. Besides the epidermis, the other tissues of the leaf show certain interesting features. The vascular skeleton consists of five, or more, strong longitudinal veins united by transverse connexions. Spirally thickened tracheids occur in the bundles even in the submerged leaves. In the transverse section of the rather thick lamina, besides the main row of normally orientated bundles, there are two rows of small bundles, one row lying near the under side and normally orientated, and the other towards the upper surface and inversely orientated3. The occur- rence of these inverted bundles in the leaves of the Hydrochari- taceae is significant in connexion with the 'phyllode theory' of the Monocotyledonous leaf4. In the axil of each leaf of the Water Soldier are found the mucilage-secreting scales (sguamulae intrav agin ales) character- istic of the Helobieae5. 1 Montesantos, N. (1913). 2 See pp. 156-160. 3 Solereder, H. (1913). 4 Arber, A. (1918); see also p. 46. 5 Nolte, E. F. (1825) and Irmisch, T. (18582). iv] THE WATER SOLDIER 53 If vigorous plants of Stratiotes be examined in the late sum- mer, they will be found to have produced numerous lateral stolons terminating in buds1 (Fig. 32). These buds do not, like those of Hydrocharis, pass the winter in a closed con- dition, but open at once, and may be described as winter-buds FIG. 32. Stratiotes aloides, L. Plant after flowering in August, bearing five plant- lets at the ends of stolons. (Reduced.) [Modified from Nolte, E. F. (1825).] which germinate while attached to the parent plant. There is, in fact, no interruption in the vegetative life, since the daughter shoots, as soon as they become free from the parent axis in autumn or winter, begin to form new winter-buds themselves. In North Germany, the Water Soldier was described in 1860 1 Gluck, H. (1906). 54 HYDROCHARITACEAE [CH. as so abundant as to be a troublesome weed, the plantlets sur- viving the hardest winter1. In the great majority of localities the continued existence of Stratiotes depends absolutely upon bud-formation, since the plant is dioecious, and only in a small part of its range is it found with both male and female flowers. In England only the female plant is usually met with (Fig. 33). There are some records of the occurrence of hermaphrodite flowers2, but ripe fa--' A B C FIG. 33. Stratiotes aloides, L. A, unopened female flower emerging from two bracts (6). B, female flower with bracts and perianth removed to show ovary (o), stigmas (st.) of which there are six, each bifurcated to base, and staminodes (sta.). C, unfertilised fruit (o) emerging obliquely from the bracts. [A. A.] seed does not seem to be formed in this country at the present day, though fruits with seeds are known from Pliocene and Pleistocene deposits3. The geographical distribution of the sexes is rather curious. According to Nolte4, in the northerly part of the range of the species only female plants occur, while at the southern extremity the plants are either predominantly or entirely male. In an intermediate area both sexes occur. In addition to the Frogbit and the Water Soldier, Hydrilla 1 Klinsmann,F.(i86o). 2 Geldart, A.M. (1906). 3 Reid, €.(1893). 4 Nolte, E. F. (1825); see also Caspary, R. (1875). iv] THE CANADIAN WATERWEED 55 verticillata, another member of the Hydrocharitaceae, has re- cently been recorded from one station in Britain, though it is typically a plant of warm climates1. But a fourth genus, Elodea, represented by the Canadian Waterweed, a submerged plant, which was apparently introduced into this country about i8432, has become very much more common than any other member of the family. In nearly all the localities in Britain, only the female plant is found, though the male has been recorded as occurring near Edinburgh3. The reproduc- tion of Elodea canadensis, which is amazingly rapid, is thus entirely vegetative; the snapping of the slender, brittle stems sets free fragments which live independently, while special winter- FIG. 34. Elodea shoots may also be produced (Fig. 34). The %£££•*££ small leaves, which are arranged in whorls [Raunkiaer, c. r i 11 1 • i i • • (1896).] or three, are only two cells thick and it is to their extreme delicacy that the plant probably owes its incapacity to produce a land form 4. The pollination mechanism of the genus Elodea is of some significance, owing to the different phases met with in different species. Most of the species have inconspicuous flowers. The male flowers either become detached and rise separately to the surface of the water, e.g. E. canadensis 5, or they are carried up by the growth of their thread-like stalks, e.g. E. ioensis* (Fig. 35, p. 56). The stigmas reach the surface owing to the elongation of the floral tube which in E. canadensis may reach a length of 30 cms.7. In an Argentine species, E. callitrichoides*, in 1 Bennett, A. (1914). 2 Marshall, W.(i852)and(i 857), Caspary,R. (18582) and SiddallJ. D. (1885). See pp. 2 1 0-2 1 3 for a further account of the spread of this plant in the British Isles. 3 Douglas, D. (1880). 4 Schenck, H. (1885). 5 Wylie, R. B. (1904). 6 Wylie, R. B. (1912), also E. canadensis according to Douglas, D. (1880). 7 Wylie, R. B. (1904). 8 Hauman-Merck, L. (i9i32). 56 HYDROCHARITACEAE [CH. which the pollination has been described in detail, the sub- merged male buds are found to be each occupied by a bubble of gas, probably carbon dioxide. Directly the flower reaches the FIG. 35. Elodea ioensis, Wylie. i, open staminate flower attached to plant. 2, mature staminate flower enclosed within the spathe. 3, staminate flower emerging from the spathe. 4, detached and empty staminate flower floating on the water with elongated axis trailing. [Wylie, R. B. (1912).] surface by the elongation of its filiform axis1, it opens suddenly and at the same moment the pollen sacs dehisce explosively. It thus comes about that abundant pollen floats on the surface of 1 This axis is mentioned by Caspary, R. (i8582) with the incorrect de- scription " tubus calicis filiformis." iv] POLLINATION AND LEAF FORM 57 the water, and surrounds the stigmas of the female flower. It has been suggested that perhaps the pollen may be attracted to the receptive surfaces by currents due to some secretion from the stigmas. It has been shown in E. canadensis that the spines on the outer coat of the pollen-grain hold back the surface-film from contact with the body of the spore and thus imprison enough air to keep it afloat1. A somewhat different method, in which water also plays a part, is found in Fallisneria^ while in Hydromy stria the pollination is sometimes effected by wind and sometimes by water3. In Elodea dens a 4, the large white flowers contain nectar, and insect pollination occurs; this genus thus shows transitions between the entomophilous members of the family, such as Hydrockaris, and the hydrophilous and anemophilous genera. Among vegetative characters, perhaps the most notable feature of the Hydrocharitaceae is the great variation in the form and mode of life of the leaf in the different genera. To illustrate this we may briefly enumerate the leaf characters of a few genera selected entirely from the fresh-water members of the family. Hydrocharis. In certain species, heart-shaped floating leaves alone. Strattotes. Stiff, serrated, linear leaves, sometimes entirely submerged, sometimes partially aerial. Boottia. Lower leaves short-stalked and submerged; upper leaves long- stalked and often aerial. Ottelia. Leaves differentiated into submerged leaves, with a narrow blade, and stalked leaves with broader blades, which may be submerged, floating or aerial. Vallisneria. Leaves entirely submerged, ribbon-like, growing in rosettes. Hydrilla and Elodea. Leaves entirely submerged, short and linear, growing on elongated axes. Three genera of the Hydrocharitaceae, Enhalus^ Halophila and Thalassia^ live in salt water; these we shall consider in Chapter x. 1 Wylie, R. B. (1904). 2 See p. 235. 3 Hauman, L. (1915). 4 Hauman-Merck, L. (i9i32). CHAPTER V THE LIFE-HISTORY OF THE POTAMOGETONA- CEAE OF FRESH WATERS1 POTAMOGETON, the central genus of the Potamoge- tonaceae, includes the very numerous Pondweeds, so common in temperate waters, and is the richest in species of all our native aquatic genera. The Pondweeds are an exceedingly difficult group from the point of view of the student of system- atic botany, as the numerous species can, in many cases, only be discriminated as the result of much experience. A character which increases the difficulty of identifying them is the capa- city for variation in form shown by one and the same individual. The present writer took a typical shoot of Potamogeton perfoliatus from the Cam in July, and kept it floating in a rain-water tub. By October I most of the large perfoliate leaves had decayed and those on the new shoots were so much narrower and less perfoliate as to make it difficult to believe that they belonged to the same species (Fig. 36). This power of variation in leaf-form within one individual is a well-known feature of P. perfoliatus. It has been recorded that an isolated plant in a newly-dug clay-pit, observed during several years, changed so much in the shape, colour and texture of the leaves as to give rise to the idea that all the British forms of the species which have been described, may possibly be mere states and not variations2. The most obvious difference between the Potamogetons and the water plants hitherto considered, lies in the extreme com- plexity of the shoot systems of the Pondweeds. The rhizomes 1 The marine Potamogetonaceae are considered in Chapter x. 2 Fryer, A., Bennett, A. and Evans, A. H. (1898-1915). This account of the British Potamogetons is of the first importance. CH. v] THE PONDWEEDS 59 form mats at the bottom of the water, retaining the soil in their meshes and thus consolidating it, while, from these rhizomes, a forest of leafy shoots rises into the water1. An examination of the individual axes shows the branch system to be sympo- dial 2. The shoots are of two kinds; the first is horizontal, more or less buried in the soil, colourless and scale-bearing, while the second is erect, floating to some degree, and producing perfect leaves. Fig. 37, p. 60, illustrates the general scheme of branching. The creeping stem is a sympodium formed by the union, end to end, of the two first internodes of successive generations (I, II, FIG. 36. Potamogeton perfoliatus, L. Detached floating shoot, October i, 1910, showing how much the plant at this time of year may depart from the perfoliate leaf type. Several ''winter shoots" have developed. (^ nat. size.) [A. A.] Ill, etc.), the succeeding internodes constituting the erect stem. In one season a great many of these rhizome units may be formed. The first scale leaf of each erect shoot (#, a', a", a'") bears a reserve bud on its axil, which may give rise to another segment of rhizome, again repeating the entire process, so that the whole ramification becomes extremely complicated. In Fig. 37, IF, I IF, represents a reserve shoot, arising in the axil of c, the third scale leaf of Shoot I. By the decay of the older parts of the rhizomes fresh individuals become separated, and even the 1 Graebner, G. in Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.). 2 Irmisch, T. (i8583) and Sauvageau, C. (1894). 6o POTAMOGETONACEAE [CH. erect shoots, if detached from the parent, can form new plants. The leafy shoots branch relatively sparsely in the large-leaved forms, but more freely in those with small leaves. FIG. 37. General branch system of a typical Potamogeton. I, II, III, . . .the different shoot-generations; a, b, c: a', b', c',. . .the three first scale-leaves borne by each shoot-generation; II', III' is a reserve shoot arising in the axil of leaf c belonging to shoot I. [Adapted from Sauvageau, C. (1894).] LEAVES OF THE PONDWEEDS 61 The various species of Potamogeton show transitions between plants with floating leaves, capable of producing a land form, and plants with submerged leaves, living entirely beneath the water-surface, except that they raise their flowers slightly into the air. Potamogeton natans may be taken as a type of the Pond- weeds with floating leaves; these consist of a sheathing base with stipules, a long petiole and an elliptical to lanceolate blade, leathery in texture. The early leaves on each shoot, which do not reach the water-surface, are phyllodic and represent only the petioles of the perfect leaves. Intermediate leaf-forms also occur, with small, spoon-like expansions of the apex1. The relation between the narrow submerged leaves and the broad floating leaves is identical with that subsisting between the two corresponding leaf-types in Sagittaria. The land form of Potamogeton natans is shown in Fig. 125, p. 196. Another species of Potamogeton^ P. pulcher, Tuckerm., of N. America, produces not only broad floating leaves but broad submerged leaves, while others, such as P. heterophyllus, Schreb., have ovate or oblong floating leaves, but their submerged leaves are of a narrower type. The more completely aquatic species ... form submerged leaves alone, with ** A ** •' , r i it'- i i i T-. i FIG. 38. Potamogeton zosteri- lammae of variable breadth. Examples foiius, Schum. Upper part of this group are P. lucensy P.perfoliatus and P. crispus. In these and related species the blade is exceedingly thin, often with only one plate of cells be- tween the two epidermal layers, but it is supported by fibrous strands running the length of the leaf (s in Fig. 38). The lamina is often crisped or undulated at the margin in a 1 Schenck, H. (1885); see also Fig. 168, p. 339. of leaf; mn, snlt sn.2, tn, vas- cular bundles; s, bast bun- dles; rs, bast bundle along margin, (x 12 circa.) [Raun- kiaer, C. (1903).] 62 POTAMOGETONACEAE [CH. graceful way. A similar undulation is characteristic of Apono- geton ulvaceus. Baker1. A curious feature of the leaves of various species, e.g. P. lucens and P. praelongus, is their shining oily surface2, which is due to the presence, in the epidermal cells, of large oil drops secreted by special colourless plastids. The non-wettable, slippery surface thus produced may be, it is suggested, a protection against water animals and micro-para- sites. It has also been supposed that the oil may hinder diffu- sion and hence prevent the soluble products of assimilation from being washed out of the leaf. But it seems to the present writer more probable that the oil is a mere by-product of the plant's metabolism; there is no valid reason for making the assumption that it performs any special function in the life- history. FIG. 39. Diagrammatic T.S. of stem stele of three species of Potamogeton to show reduction and fusion of vascular strands. tlf 7\, *lf traces of next higher leaf; *a. T2> *2> traces of second higher leaf; remaining strands cauline. A, P. pulcher, Tuckerm. B, P. natans, L. C, P. crispus, L. [Chrysler, M. A. (1907).] Such species as Potamogeton trichoides and P. pectinatus have very narrow submerged leaves which are linear in form and tender and translucent in texture. The species belonging to Potamogeton and the allied genus Zannichellia can be arranged, according to the anatomy of their stems and roots, in a reduction series, beginning with the types with floating leaves, whose axes show a complicated internal structure, and ending with entirely submerged, narrow-leaved species, in which the anatomy is reduced to a state of extreme simplicity3. But it is uncertain whether this sequence completely 1 Krause, K. and Engler, A. (1906). 2 Lundstrom, A. N. (1888). 3 Schenck, H. (1886) and Raunkiaer, C. (1903). vj STEM ANATOMY OF PONDWEEDS 63 represents the evolutionary history, since it is possible that certain forms with floating leaves may have had a submerged ancestry. The species whose central cylinder diverges least from a normal terrestrial type, seems to be Potamogeton pulcher^ (Fig. 39 A). Here a section across an internode of the leafy shoot reveals, within the central cylinder, three distinct bun- dles (/! , jTj_ and /x) which are the traces of the leaf immediately above, and three more (/2 , T2 and /2) which entered at a still higher node. In addition there are several bundles which are purely cauline. The type represented by our native P. natans (Fig. 39 E] differs from that of P. pulcher in the fact that the traces belonging to each leaf do not so fully retain their inde- pendence in the central cylinder. P. perfoliatus belongs to the type of P. natans. In P. crispus (Figs. 39 C and 40 A^ p. 64) the stele is more condensed, the bundles being collected into three groups. In very slender stems of this species, the two passages in each group representing the xylem may fuse so that the distinctness of the bundles is maintained by the phloems alone. P. lucens2 (Fig. 40 R) has a median and two lateral bundle- groups, but these are more reduced — the median group con- sisting of one xylem passage and two phloem regions, and the laterals, of one xylem passage, and one patch of phloem. In this species the tendency to concentric arrangement begins to make itself felt. In P. pusillus (Fig. 40 C) the lateral bundles are entirely fused with the median, as far as the xylem is con- cerned, but the phloems still remain distinct. In P. pectinatus (Fig. 40 Z)) the ultimate term in the reduction series is reached : a ring of phloem surrounds a single xylem passage. Zannichellia closely resembles P. pectinatus \ ephemeral xylem vessels have been detected in the apical region of the stem3. In the case of the related genus Althenia^^ vessels are also retained in this region and in the nodes. 1 Chrysler, M. A. (1907). 2 On this and other species, Sauvageau, C. (1894) should be consulted. His account diverges in some points from that of Schenck. 3 Schleiden, M. J. (1837). * Prillieux, E. (1864). POTAMOGETONACEAE [CH. end C D FIG. 40. Reduction series in central cylinder of stem in Potamogeton. A, P. crispus, L. (cf. diagrammatic Fig. 39, C) (x 160) ;'B, P. lucens, L., in which fusion of the strands has gone further, so that each of the three bundle groups has one xylem only; mp = conjunctive tissue (xi3o); C, P. pusillus, L., in which the xylems of all the individual bundles form a single central passage ( x 290) ; D, P. pectinatus, L., completely concentric structure in which all trace of the component bundles is lost; end = endodermis (x29o). [Schenck, H. (1886).] v] ANATOMY OF THE PONDWEEDS 65 The tendency to condensation and simplification of the stem stele, which is so well illustrated among the Potamogetons *, is, as we shall see in Chapter xm, a characteristic of many aquatics. The stem of the Pondweeds is, however, peculiar in that the bundles are not confined to the central cylinder. In some spe- cies there is a complicated system of cortical strands, occurring at the intersection of the diaphragms separating the lacunae. These cortical bundles communicate with one another and with the axial strand by means of anastomoses at the nodes. A B FIG. 41. Structure of central cylinder of root in Potamogeton. A, P. natans, L. gef, vessel; s, sieve tube with companion cell; p, pericycle; cj, conjunctive tissue (x 470). B, P. densus, L. Similar to P. natans, but vessels and endodermis thin- walled; sieve tubes shaded (x 470). C, P. pectinatus, L., xylem reduced to single vessel (x 470). [Schenck, H. (1886).] We have so far been considering the anatomy of the leafy shoot alone. It should be noted that the structure of the hori- zontal rhizome and of the inflorescence axis are often markedly different. For instance, in the creeping stem of P. pulcher, the central cylinder takes the form of " a truly dicotyledonous look- ing ring of collateral bundles," while the flowering axis of P. natans also has its vascular strands arranged in a regular ring2. 1 Sanio, C. (1865) first recognised that the apparently simple axial strand of certain Potamogetons was really the reduced representative of a whole system of bundles. 2 Raunkiaer, C. (1903) and Chrysler, M. A. (1907). A. W. P. z 66 POTAMOGETONACEAE [CH. A similar reduction series to that met with in the central cylinder of the stem can be traced in the root1. In Potamogeton natans (Fig. 41 A^ p. 65) the root is pentarch and the walls of all the elements, except the sieve tubes, are thickened. P. densus (Fig. 41 B) has the same type of structure, but the cell- walls remain thin. In P. fectinatus (Fig. 41 C) the five protoxylem elements are absent, and the xylem is represented merely by a single central vessel with delicate, spiral thickening2. The structure of the root of Zannichellia is similar, but the axial vessel is unthickened. The Potamogetons tide over the winter in various ways. In P. fectinatus^ the Fennel Pondweed, common in fresh and brackish waters, the leafy shoots give rise to tubers in the autumn. These tubers are usually formed by the swelling of the two basal internodes of that part of the axis which would otherwise become erect and leafy. Each tuber is enclosed in a scale leaf and terminates in a bud ; it contains starch and, as it is easily detached, it forms a means of vegetative multiplication. Other species are reproduced by special buds, or turions3, in which the leaves, rather than the axis, play the chief part. A group of submerged Pondweeds with linear leaves, of which P. pusillus and P. trichoides are examples, is characterised by winter-buds enclosed in scales corresponding morphologically to axillary stipules accompanied by rudimentary laminae. In this group of species there is no rhizome, branching sympodially in the mud, the only part corresponding to such a rhizome being the elongated axis of the turion; the branched leafy shoots play the chief role in the axial development. The whole vegetative body in these species dies off in the autumn and the turions alone remain. These buds are formed in great numbers, and iSchenck, H. (1886). 2 Sauvageau,C. (18892) describes the roots ofP.pectinatus as having, in general, a less degraded type of structure than that attributed to them by Schenck, H. (1886). 3 Gliick, H. (1906) deals comprehensively with the turions of the genus. v] WINTER-BUDS OF THE PONDWEEDS 67 often many thousands lie on the soil at the bottom of the water. They germinate without rising to the surface. The formation of winter-buds in this group of Pondweeds, as indeed in aquatics in general, is encouraged by unfavourable conditions1. For instance, if the environment is otherwise satisfactory, but the depth of the water is excessive, causing the plant to exhaust itself in the production of long axes, turion formation may occur unusually early in the year. Potamogeton crzspus2 is related, in its wintering habits, to the group just dealt with, but its turions are singular in certain respects. The word * bud ' seems in this case to be a misnomer, as the thick, toothed leaves of the turion do not enfold one B FIG. 42. Potamogeton crispus, L. Germinating turion. A, a turion from bottom of water, March 16, 1912, with one lateral branch. B, the same turion, April n, 1912, when it had developed a number of lateral branches and a root. (Nat, size.) [A. A.] another, but stand out at a wide angle from the axis. They are of unusual consistency, being hard and horny. The turions may be from 10 to 50 mm. long and bear three to seven leaves. As their discoverer, Clos, pointed out more than sixty years ago, their mode of germination is quite peculiar, since there is no elongation of the axis, and further development is due entirely to the production of axillary branches. The process of germi- nation can be followed in Fig. 42 and Fig. 43, p. 68. Figs. 1 See pp. 222-224. 2 Clos, D. (1856), Treviranus, L. C. (1857), Hildebrand, F. (1861), Coster, B. F. (1875) and Gluck, H. (1906). 5—2 68 POTAMOGETONACEAE [CH, FIG. 43. Potamogeton crispus, L. Advanced stage in the germination of a turion (reduced). The first shoot, A, produced from the turion, T, has given rise to three lateral sympodia, B, C, D. The first and second shoot- generations of B have given rise to two reserve shoots, a and b. [Adapted from Sauvageau, C. (1894).] v] WINTER-BUDS OF THE PONDWEEDS 69 42 A and B were drawn from a bud which had passed the winter at the bottom of a rain-water tub in the present writer's garden. The turions of this species seem to be primarily repro- ductive bodies, and to be only secondarily concerned with tiding over the winter, for large numbers germinate without a resting period. Not only the rhizomes, but certain of the leafy shoots, are capable of lasting over the cold season, if they are not actually frozen. The special winter branches differ some- what from the summer shoots in having leaves without a crisped margin, and they have hence been sometimes mis- taken for a distinct species. A second group of Pond- weeds is characterised by winter-buds whose enclosing scales consist merely of axil- lary stipules, the correspond- ing blades having wholly disappeared. Fig. 44 repre- sents a transverse section of a turion of Potamogeton rufescens, which conforms to this type. In this species the winter-buds are formed chiefly on the underground rhizome, while in P.fluitans, Roth — a species closely re- FIG. 44 lated to P. natans — they occur in this situation only. Potamogeton perfoliatus, L. Potamogeton rufescens, Schrad. T.S. through a turion. 'A and B, outer scale leaves equivalent to stipules; I-IV, foliage leaves, whose stipules are marked 1-4 and put in in solid black. Squamulae intravaginales are omitted. (Enlarged.) [After Gliick, H. (1906), Wasser- und Sumpfgewachse, Bd. n, p. 160, Fig. 23.] forms winter-buds which are not deciduous but unfold in situ (Fig. 36, p. 59). In flower structure1, as well as in anatomy, a reduction series iSchenck, H. (1885). 70 POTAMOGETONACEAE [CH. can be traced in the Potamogetonaceae. This series ranges from forms such as Potamogeton natans^ with an erect spike of numerous flowers, through various intermediate types, to the related genus Ruppia, in which the pollen_flqatsi_ajid the 2^f /stigmas are raised to the surface to receive it, and ultimately to Zannichellia and various marine members of the family, in which the pollination is entirely submerged. Even within the genus Potamogeton itself, there are a number of gradations in A B FIG. 45. Zannichellia polycarpa, Nolte. A, shoot (nat. size) with flowers (/). B, flowers (enlarged); st, stamen; g, gynaeceum; m.c., membranous cup. May 25, 1912. [A. A.] the direction of submerged life. The flowers possess, typically, four stamens, and four free carpels. They appear, at first glance, to possess also four perianth members, but more careful exami- nation reveals that these are, in reality, leaf-like outgrowths from the staminal connective1. The spike of P. natans is sup- ported above the water by the two floating leaves immediately below it. These are always opposite (cf. Fig. 37, p. 60), although otherwise the leaves are alternate. In some species, e.g. P. pec- tinatus, the spikes, instead of being stiff and erect, are thin and flexible, and float horizontally on the water. In these forms 1 Information as to the morphology and development of the flower and fruit will be found in Hegelmaier, F. (1870), Schumann, K. (1892), etc. v] THE HORNED PONDWEED 71 the flowers are distant, and when mature they are lifted, one by one, a little above the water-surface. In other cases the inflo- rescences are much reduced — only four flowers being developed in Potamogeton pusillus — while in P. trichoides the individual flowers are modified, the number of carpels being reduced to one. In Zannichelliapolycarpa the flowers areunisexual (Fig. 45), a male and female flower (or inflorescence) being found together in one leaf-axil; the male flowers are generally reduced to a single stamen (sf Fig. 45 B\ while several carpels with funnel- shaped stigmas (g) are grouped together, and enclosed in a membranous cup (m.c.*). This cup has been interpreted as a spathe enclosing a group of female flowers, each reduced to one carpel. The filament is at first very short, but elongates so as to rise above the pistils at anthesis. The anther dehisces and the pollen grains fall into the open mouths of the cornucopia- shaped stigma, and slide down the stylar canal, whose diameter is almost double that of the pollen grains. The descent of the pollen grains through the water is due to the fact that when they become ripe they are weighted with starch grains1. Owing to the air spaces in the pericarp wall, the achenes of some of the Potamogetons float for a time, before becoming waterlogged and sinking. The air-containing tissue of the peri- carp in P.perfoliatus, and the cuticularised epidermal layer (0.£.), are shown in Fig. 46, p. 72. The fruits of the Pondweeds, after becoming to all appear- ance ripe, often rest for a considerable period before germina- tion2, except in the case of P. densus, in which the achenes sprout a few days after they fall. But this species is rather remote from the rest of the genus in other respects, such as the opposite arrangement of the leaves, and the absence of the ligule. Sauvageau3 has shown by experiment that in P. crispus it is the hard integument which delays germination; when the embryo is laid bare by the removal of part of the seed coat, sprouting rapidly occurs. The same author observed that when 1 Roze, E. (1887). 2 The delayed germination of aquatics in general is considered in Chapter xix, p. 243. 3 Sauvageau, C. (1894). 72 POTAMOGETONACEAE [CH. v fifty fruits of P. natans, which had been gathered in September, 1889, were kept in water at the temperature of the laboratory, none germinated in 1 8 90 or 1891, six germinated in 1892, and thirty in 1893, i.e. after lying dormant for three years and a half. FIG. 46. Potamogeton perfoliatus, L. Transverse section of fruit wall to show air spaces in the outer region of the wall, and also the thick outer cell-wall of outer epidermis (o.e.). The cross-hatching indicates the non-cuticularised part of the wall: only the outermost surface layer, shown in black, is converted into cuticle (c). Chlorophyll grains in epidermis. (x26o.) [A. A.] The most striking feature of the Potamogetonaceae, as a family, seems to be the remarkable reduction series shown by the vegetative and reproductive organs — the degree of reduc- tion serving in general as a gauge for the degree of completeness with which the aquatic life has been adopted. [73] CHAPTER VI THE LIFE-HISTORY OF THE LEMNACEAE1 AND OF PISTIA EACH of the families with which we have been con- cerned in the preceding chapters, has shown very great variation in vegetative structure associated with the differing degrees in which its members have adopted the aquatic habit. In the Lemnaceae, which we propose now to discuss, we have, on the other hand, a remarkably sophisticated and uniform group of plants, all of which pass their life floating at or near the surface of the water; the members of the family show, throughout their structure, a high degree of similarity to one another, and a marked difference from other aquatics. The Duckweeds have a very wide range, and occur almost as generally in the Tropics as in the northern countries where we know them so well2. In the Lemnaceae the modification of the vegetative body has been carried so far that the usual distinction between stem and leaf is no longer obviously maintained. The Duckweeds are not unique in this disregard of morphological categories — two other groups of water plants, the Utricularias and the Podostemaceae, carry this infringement of botanical conven- tions to an even more extreme point. The little green fronds of the Duckweeds produce similar fronds of the second order, and also inflorescences of an ex- tremely reduced type (Fig. 47, p. 74 and Fig. 50, p. 79) from pockets occurring on either side in the basal region. The nature of the fronds has been very variously interpreted. Hegelmaier 3, 1 Hegelmaier, F. (1868) is still the classic monograph of this group. See also Schleiden, M. J. (1839) and Hegelmaier, F. (1871) and (1885). 2Kurz, S. (1867). 3 Hegelmaier, F. (1868). For another view see Dutailly, G. (1878). 74 LEMNACEAE [CH. in his monograph of the Lemnaceae, treats them as stem organs which are modified to perform the work of leaves. Engler1, on the other hand, follows van Horen2 in in- terpreting the distal end of the frond as foliar, while the proximal end is axial. Yet a third view is that of Goebel 3 who expresses the opinion that the leaf-like organs of the Lemnaceae are actually leaves, pure and simple. He explains the origin of the lateral shoots of each generation from the base of the preceding one, by assuming that the base FIG. 47. spirodeiapoiyr- of each leaf has the power of functioning rhi*a> Schlf d- *nfl°r- r o escence. stt and s/2, as a growing point. Undoubtedly Engler's male flowers reduced i • i • i i i to stamens; c., female view — which is based upon a comprehen- flower reduced to a sive study of the Araceae, and a critical gynaeceum;s£.,spathe; / r ~. i i -r /., lateral shoot. [Hegel- exammation of Ptstta and the Lemnaceae — maier, F. (1871).] may be accepted as the best founded. The present writer has recently carried Engler's comparison further, and has shown that the buds in the case of Pistia arise in minute pockets closely recalling those of the Duckweeds4. The three genera into which the family is divided — Spiro- dela, Lemna and Wolffia — are all represented in Britain. Spirodela polyrrhiza, Schleid.5, is the largest member of the Lemnaceae; when it is growing vigorously its fronds attain to about J- of an inch both in length and breadth. Several roots with conspicuous root-caps hang from the underside of each frond. They are somewhat heavier than water and their tips are the heaviest part. It has been suggested that one of the functions of these roots may be to ensure the equilibrium of the plant 6. Spirodela forms special shoots which outlast the winter. 1 Engler, A. (1877). 2 Horen, F. van (1869). 3 Goebel, K. (1891-1893). 4 Arber, A. (i9i94). 5 For a description of the very rare flowers of this species see Hegel- maier, F. (1871). 6 Gasparini, quoted by Hegelmaier, F. (1868); Ludwig, F. in Kirch- ner, O. von, Loew, E. and Schroter, C. (1908, etc.). vi] WINTER-BUDS OF GREATER DUCKWEED 75 Such turions are of great importance throughout the family, since the flowers are rare and relatively little seed is set. The winter-fronds of Spirodela are smaller than the summer ones and almost kidney-shaped. The air spaces in the tissues are reduced, and the cells are packed with starch, with the result that the fronds are heavier than water. The roots remain un- developed. These winter-buds become detached from the parent frond in the autumn and sink to the bottom of the water. In the spring, a lateral frond begins to grow out; in so doing it absorbs the starch from the parent, and on this account, and also by development of air spaces, the whole body becomes lighter and rises to the surface1. The present writer has found that the rising of the winter-buds can be induced, as early as January, as a result of a few days in a warm room, even in a dim light. The time of year at which the turions begin to be formed is variable, and depends on external conditions. It has been shown by van Horen2 that in shady places they develop very late or even fail altogether, whereas they occur early in bright sun- light, especially if the water is stagnant. Guppy 3, who has made a special study of the habits of the Lemnaceae, mentions that on one occasion he found a large number of plants of Spirodela polyrrhiza in a ditch, producing winter-buds, at the beginning of July, to an extent he had never seen before or since; the conditions were precisely those indicated by the previous ob- server as being favourable to the early occurrence of this phase — namely almost stagnant water which was brilliantly insolated. During the few weeks preceding the observation of the winter- buds, Guppy records that the surface was frequently heated in the day time to 80° Fahr. (nearly 27° C.). It is difficult to understand why conditions so favourable for vegetative growth should initiate turion formation, since in most water plants their production is induced by a state of poor nutrition. Pos- sibly the explanation may lie in the great size of the winter-bud of the Lemnaceae in relation to the entire vegetative body of the 1 Hegelmaier, F. (1868). 2 Horen, F. van (1869). ^ Guppy, H. B. (1894*). LEMNACEAE [CH. parent, when compared with the small proportion that the turions of other aquatics bear to the plant producing them. To synthesize enough starch to fill the cells of the winter-bud may be a considerable tax on the parent frond, and may only be possible under conditions peculiarly favourable for photo- synthesis. The commonest British Duckweed is Lemna minor, L.1, which seems to be in some ways the least specialised, among our native species, for its particular mode of life. No definite turions are formed, and the plants are to be found swimming at the surface of the water at almost all seasons. When frozen, the older fronds become water-logged more readily than the younger ones, and they sink to the bottom, dragging down the young laterals with them. Another species, Lemna gibba, L.2, is notable for having the under- side of the frond modified as a spongy aerenchyma — the gibbous form so produced giving the species its name (Fig. 48). The degree of development of the air tissue varies with the external conditions; the fronds are most conspicuously gibbous in running water where the insolation is moderate3. At certain periods of the life- FIG. 48. Lemna gibba, L., with fruit,/. [Hegelmaier, F. (1868).] 1 On the flowering of Lemna minor see Brongniart, A. (1833) and Kalberlah, A. (1895); on the gametophytes and fertilisation, Caldwell O. W. (1899). 2 On the flowers and seed of Lemna gibba see Micheli, P. A. (1729) and Brongniart, A. (1833)5 on the germination, Wilson, W. (1830). 3 Horen, F. van (1869). vi] THE GIBBOUS DUCKWEED 77 history, flat fronds are however produced and we owe to Guppy1 the elucidation of the part played by the two types of shoot. He observed one hot summer, when Lemna gibba flowered profusely in July, that, during August, the gibbous plants gave rise to numerous thin, flat fronds of a dark green hue. These were the turions, and their appearance was accompanied by the death of a large number of the gibbous mother-plants, a result which this author attributes to exhaustion after flowering. Many of the gibbous plants, however, survived and continued to bud off winter-fronds except during the severest weather. Early in Feb- ruary the budding recommenced, but the gibbous character was not displayed until the weather became warmer. This author thinks that for the development of the gibbosity the plants re- quire an average daily maximum temperature at the surface of the water, not much, if at all, under 70° Fahr. (21° C.). After cool summers when Lemna gibba does not flower, no flat winter-buds are formed, but the gibbous fronds survive until the next spring. One of the reasons for the relative rarity of L. gibba, as compared with L. minor, is probably that, as Guppy has shown, it requires a higher temperature than that needed by the Lesser Duckweed, both for initiation of budding in spring and for flowering. Under suitable conditions, however, it shows a wonderful vigour of vegetative growth. It has been recorded, for instance, that an area of water of about half an acre, which was edged on a certain date in June by a border of this plant a few feet wide, nineteen days later was thickly covered with the fronds over almost its entire surface2. The surface-living Duckweeds can survive for a time if stranded on the mud by the lowering of the water in which they grow, and in cultivation it has been found possible to establish land forms which can fulfil the whole cycle of normal vegetative development3. For instance, Lemna minor has been grown for as long as twenty months on wet mud, where it throve and budded at all seasons of the year. Two plants set apart in 1 Guppy, H. B. (18942). 2 Hegelmaier, F. (1868). 3 Guppy, H. B. (18942). 78 LEMNACEAE [CH. October had increased under these conditions to fifty in the course of a year. Spirodela polyrrhiza can also be cultivated on mud from the winter-buds through the summer phase to the winter-buds again. The genus Lemna contains another British species which is more deeply committed to the water life than either L. minor or L. gibba. This is L. trisuka^ L., the Ivy-leaved Duckweed, a submerged plant, floating beneath the surface level1. The fronds of L. /ra#/£# are longer than those of the other Duckweeds and this elongation may be connected with the tempering of the light due to its passage through a layer of water. Its shoots form very decorative, symmetrical patterns, owing to the cir- cumstance that branches of many different generations remain attached to one another (Fig. 49). This fact is probably to be associated with the relatively sheltered habitat of the Ivy- leaved Duckweed, as compared with Lemna minor •, L. gibba> etc.2. These floating species are exposed to all the surface move- ments of the water — a fact which must encourage detachment. That it is the difference between floating and submerged life that determines the question of the fronds becoming isolated or remaining attached, is confirmed by the fact that the partially surface-floating, fertile fronds of L. trisulca (Fig. 50) tend more to separation. In these fertile fronds the basal part, which bears the inflorescence, floats on the surface, but the apical region dips down into the water3. The sterile fronds and the submerged part of the fertile fronds agree in having no sto- mates, whereas the floating part of the fertile frond bears stomates and approaches more closely in structure to the fronds of Lemna minor than do the submerged sterile shoots. The very simple vascular strands are dorsiventral with xylem above and phloem below; one vessel and one sieve tube form a character- istic combination4 (Fig. 51). 1 Clavaud, A. (1876) puts forward a theory concerning the cause of submergence in this species which seems to be quite unfounded. 2 Schenck, H. (1885). 3 Hoffmann, J. F. (1840). * Schenck, H. (1886). VI] THE IVY-LEAVED DUCKWEED 79 FIG. 49. Lemna trisulca, L. Habit drawing. (Slightly enlarged.) [Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.).] FIG. 50. Lemna trisulca, L. Flowering shoot. (Enlarged.) [Hegelmaier, F. (1868).] FIG. 51. Lemna trisulca, L. T.S. bundle from stalk of frond. One vessel (gef) and one sieve tube (s) with two companion cells. (x475.) [Schenck, H. (1886).] 8o LEMNACEAE [CH. Wolffia^ the third and last genus of the Lemnaceae, enjoys the distinction of including the most minute of all flowering plants. The tiny, simple fronds are devoid of roots. The species which occurs in England, Wolffia Michelii, Schleid., has fronds which in no dimension exceed 1-5 mm., while W* brasiliensisy Wedd., is described as being only one-half to two-thirds of this size. Its discoverer, Weddell1, records that about twelve flower- ing individuals of this tiny species could be accommodated upon a single frond of Lemna minor. He noticed this little Wolffia growing in the neighbourhood of that most gigantic of aquatics, Victoria regia^ the Waterlily of the Amazons, and their propin- quity drew from him the exclamation, "Singuliere bizarrerie de la nature d'avoir seme ensemble ces deux vegetaux! " Our native species winters at the bottom of the water, its minute fronds being just sufficiently weighted with starch grains to induce sinking. The flowers of the Lemnaceae are reduced to the simplest possible terms. Spirodela polyrrhiza2' (Fig. 47, p. 74), for instance, has an inflorescence consisting merely of a spathe (j/>.) enclosing two male flowers each represented by a stamen only (j/j and j/2) and a female flower simply formed of a gynae- ceum (£.) with one or two ovules. Lemna minor^, and probably other members of the family, appear to be pollinated by insects. The essential organs are raised above the water level, but they are short and stiff, while the pollen is scanty, so anemophily seems improbable. Small beetles and aquatic insects have been observed crawling about among the flowering fronds, which are markedly protandrous. The seeds of the Lemnaceae, in the relatively rare cases in which they are produced, may germinate as soon as they are ripe in the summer — sometimes even while attached to the parent plant — but in other cases they may rest through the winter and defer germination until the spring4. Fig. 52 illustrates the seedling stage of Lemna trisulca. 1 Weddell, H. A. (1849). 2 Hegelmaier, F. (1871). *Ludwig, F. (1881). ' 4 Hegelmaier, F. (1868). VI] DUCKWEED SEEDLINGS 81 The extreme reduction and specialisation, which charac- terise the Lemnaceae, are united with great vigour and vitality. We have already alluded (p. 77) to a special case of the rapid power of vegetative reproduction shown by Lemna gibba, and the same capacity characterises other members of the family. Another remarkable trait of the Duckweeds is their power of FIG. 52. Lemna trisulca, L. Germination. A , germinating seed with operculum (o) just coming away. B, seedling seen from the side. C, seedling further developed, seen from above, ch = chalaza, c = cotyledon, pi = plumule, / = lateral shoot from plumule, 2 / = secondary lateral shoot, r = radicle. (Enlarged.) [Hegel- maier, F. (1868).] living and flourishing in water which is so full of organic im- purities that no other Phanerogams can survive in it. If introduced into water with a bad smell, they will purify it until it is a fit habitation for small animals1. 1 Ludwig, F. in Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.); see also p. 287. A.W. P. 6 82 PISTIA [CH. The Lemnaceae are generally regarded as related to the Aroids, so it may be well to conclude this chapter by a further reference to Pistia Stratiotes, L.1, the River Lettuce of FIG. 53. Pistia Stratiotes, L. A, radial longitudinal section of leaf apex showing groove into which the water pores open and the space beneath them into which tracheids emerge. B, surface view of water pore. [Minden, M. von (1899).] the Tropics — the member of the Araceae most nearly allied to the Duckweeds. This plant has a floating rosette of leaves, and multiplies by runners from which fresh rosettes arise. The lower side of each sessile leaf bears a swelling, which may reach the size of a pigeon's egg. This swelling consists of spongy air-containing tissues, and serves as a float. The upper i On Pistia see Koch, K. (1852), Hofmeister, W. (1858), Engler, A. (1877) and Arber A. vi] THE RIVER LETTUCE 83 and lower leaf-surfaces are covered with minute depressed hairs, which prevent the leaves from being wetted1. Fig. 53 shows the apical opening, so often found in aquatics, through which water is eliminated from the leaf2. Like the Lemnaceae, Pistia represents a type which is singularly successful in the matter of vegetative growth. Its reproduction is so rapid that it sometimes chokes water-channels and proves a serious hin- drance to navigation3. 1 Ito, T. (1899). 2 Minden, M. von (1899). See also p. 267. 3 This subject is dealt with more fully in Chapter xvn, p. 213. 6—2 84 CHAPTER VII THE LIFE-HISTORY OF CERATOPHTLLUM EACH of those aquatic families whose life-histories we have hitherto considered, contains a considerable num- ber of species, representing, in the case of the Lemnaceae, three genera, while, in the case of the other groups discussed, the number is much higher, as many as fourteen genera being in- cluded, for instance, in the Hydrocharitaceae. The family Ceratophyllaceae, the subject of the present chapter, offers a marked contrast on this point, since it includes only a single genus, containing three species, or, on other interpretations, one alone1. Ceratophyllum, the Horn wort, is extremely isolated in its structure and habits, so much so that there has been, at various times, the widest diversity of opinion as to the posi- tion which should be assigned to the family; the plant, from its taxonomic wanderings, has been opprobriously styled " a vegetable vagabond." The question of its affinities will be discussed in Chapter xxv. In the genus Ceratophyllum the aquatic habit seems to have reached its ultimate expression. The plant not only lives entirely submerged throughout its vegetative life, but even its stigmas do not reach the surface, and the pollen is conveyed to them by the water2. The Hornwort is monoecious, the male flowers con- sisting of a group of stamens enclosed in a perianth of about a dozen members (p in Fig. 54 5). These stamens, when the flower is mature, become detached — the terminal expansion of the connective acting as a float3 — and rise to the surface of the water. They then dehisce and the pollen, having a specific gravity very slightly higher than that of water, sinks gently, iSchleiden, M. J. (1837). 2 Delpino, F. and Ascherson, P. (1871). 3 Ludwig, F. (i'88i). CH. vn] THE HORNWORT 85 and thus comes into contact with the stigmas1. This water- carriage of the pollen is the more striking, since the great majority of aquatic plants show a strong tendency to retain the aerial pollination mechanism of their terrestrial ancestors. As regards vegetative structure, the most notable feature of the Hornwort is the entire absence of roots. The radicle '-St. C B FIG. 54. Ceratophyllum demersum, L. A, node bearing two male flowers (<£) (En- larged) ; a branch (b) and all the leaves but two (I) have been cut across. B, a single male flower on a larger scale; p, perianth of about 12 members; st, stamens. On the left, a stamen is in the act of being squeezed out. C, $ flower; a, showing perianth, style and stigmas; b, with perianth removed showing ovary. The stigma varies from being single to being sometimes much more deeply bifid than in C. [A. A.] never develops beyond a rudimentary stage and no adventitious roots are produced. Fig. 55, p. 86, shows a seedling2 with its re- duced radicle (r). The seed germinates at the bottom of the water, the plantlet rising to the surface when it is about three inches long. The leaves of the first pair (/) are linear and decussate. 1 Willdenow, C. L. (1806), Dutailly, G. (1892), Roze, E. (1892), Strasburger, E. (1902). 2 Guppy, H. B. 86 CERATOPHYLLUM [CH. r..-- FIG. 55. Ceratophyllum de- mersum, L. Seedling one week old. (Enlarged.) c= co- tyledon; /= member of first pair of leaves which decus- sate with the cotyledons; r= rudimentary radicle which never elongates. [Guppy, H. B. (I8Q41).] The forked leaves characteristic of the mature plant (/ in Fig. 54 A) p. 85) are not formed im- mediately; they are preceded by a juve- nile type which is simple and linear. It is not until the fourth node above the cotyledonary node that every mem- ber of the whorl attains the characteristic form. Each of the slender axes of the mature plant, with its whorls of forked leaves (B in Fig. 57, p. 89), often occupies a more or less vertical position in the water and quite deserves the description given many years ago by a German writer1: "A Christmas tree for tiny water nixies." The Hornwort sometimes flourishes at a considerable depth; in Iowa it has been recorded to grow with marked success beneath nearly thirty feet of water2. The stem structure of Ceratophyllum may be taken to repre- sent one of the ultimate terms in the reduction series met with among Dicotyledonous water plants (Fig. 56). The fully- developed internode has a central axial passage which has arisen through the resorption of a small group of narrow-lumened thin-walled procambial cells3. There is complete absence of lignification. The water content of the plant is very high, representing 88 per cent, of the total weight4, but as the young parts are cuticularised to a degree unusual in submerged plants, the texture of the shoots is less fragile than one might expect, and collapse does not occur so rapidly in a dry atmosphere as in the case of many hydrophytes. The curious mucilage-containing hairs borne by the leaves, stamens, etc., have been much dis- cussed5. They seem to differ from the common mucilage hairs i Schleiden, M. J. (1837). 2 Wylie, R. B. (1912). 3 Sanio, C. (1865). 4 Schleiden, M. J. (1837). 5 Goppert, H. R. (1848), Borodin, J. (1870), Strasburger, E. (1902). vii] THE HORNWORT 87 of water plants in not excreting any slime, and their special function — if they possess one — remains a mystery. It is characteristic of the Hornwort to occur sometimes in such great abundance that it drives out nearly all other com- petitors. It has been described, in the case of a certain Scottish loch, as so luxuriant that a boat could only be rowed through it with difficulty1. The present writer has seen it at Roslyn Pits, near Ely, at the beginning of October, in such quantity that the effect, on looking down into the water, was that of gazing into a pure forest of C er atop hy Hum. The axis at this season of the year FIG. 56. Ceratophyllum demersum, L. Vascular cylinder of stem in T.S. Small xylem space in the centre; xylem parenchyma thickened; phloem zone well developed with large sieve tubes, (x 130.) [Schenck, H. (1886).] is extremely brittle, snapping asunder at the slightest touch and thus giving rise to countless detached fragments capable of reproducing the plant. The apical regions of the shoots are more crowded with leaves and more deeply green than the rest of the plant, but are scarcely specialised enough to be called winter-buds2. During the cold season the stems remain at the iWest, G. (1910). 2 The existence of these winter shoots was noted by Royer, C. (1881- 1883); that the plant may vegetate throughout the winter was recorded by Irmisch, T. (1853). 88 CERATOPHYLLUM [CH. bottom of the water, weighted down with a "living freight of aquatic molluscs, insects and annelids1." The young shoots formed in the spring, since they have not had time to become so ballasted, rise erect in the water. The stems of the previous year gradually decay away, and by the flowering time, in June or July, they have practically disappeared. The fact that the Hornwort, which has no surface layer of mucilage, becomes, to so remarkable an extent, an asylum for aquatic animals, may possibly be taken to afford some negative evidence for the theory that the mucilaginous coat, which is almost universal in hydrophytes, may have some value in preventing small foreign organisms attaching themselves to the plant's surface. In addition to the normal leafy shoots, a second type of branch is produced, which appears in some degree to take the place of the absent roots (Fig. 57)2. These shoots, which are described as * rhizoid-branches,' are whitish in colour and bear leaves with extremely fine and delicate segments. Fig. 58 shows the contrast between a rhizoid-leaf (A) and a water-leaf (5). The rhizoid-shoots penetrate into the mud, where they pre- sumably serve as anchors and absorbing organs. Although Cer atop hy Hum is not uncommon in northern lati- tudes, there are certain indications that its birth-place may have been in some more genial climate. Guppy3 has shown, for instance, that a very high temperature is required for the matu- ration of the fruit. He noticed that in the drought of the hot summer of 1893, the ovaries ripened well in a shallow pond where the temperature of the water always rose above 80° Fahr. (27° C.) in the afternoons, and occasionally as high as 95°, Fahr. (35° C.), while in the neighbouring waters, which were not so much overheated, no fruits were produced. Curiously enough, even in Fiji the fruit is only matured in the superheated waters of shallow pools, tanks and ditches4. Conversely, the vegetative organs cannot endure freezing, even for a period so brief as to be quite harmless to many other aquatics; 1 Guppy, H. B. (1894!). 2 Glttck, H. (1906). 3 Guppy, H. B. (18941). 4 Guppy, H. B. (1906). vn] RHIZOIDS OF THE HORNWORT 89 Guppy found that the shoots were mostly killed by five or six days inclusion in ice. B FIG. 58. Ceratophyllumdemersum, L, A, single leaf of a rhizoid. B, single leaf of a water shoot (Enlarged.) [After Gliick, H. (1906), Wasser-und Sumpfgewachse, Bd. n, Figs. 27 A and B, p. 195.] FIG. 57. Ceratophyllumdemersum, L. Part of an axis, A , which is lying on the soil and bears a normal leaf- whorl, B, and a rhizoid penetrating the soil. The lowest leaf -whorl of the rhizoid, U, has transition leaves. (Slightly enlarged.) [After Gliick, H. (1906), Wasser-und Sumpfgewachse, Bd, n, PI. VI, Fig. 76.] The various peculiarities of structure and habit to which we have referred in the preceding pages, are not the only 90 CERATOPHYLLUM [CH. vn singularities exhibited by Ceratophyllum. In 1877 a French observer, Rodier1, recorded the existence of certain spontaneous movements which characterise the shoots of this plant. He noted that the shoot moved in one direction for six hours, and then returned for another six — then moved for four hours in the opposite direction, and in another four hours returned again to its original position. Darwin 2 drew attention to certain obscu- rities in Rodier's description, but no more recent work appears to have been done on the subject; the movements of Cerato- •phyllum might repay further investigation3. 1 Rodier, E. (iS;;1) and 2 Darwin, C. and F. (1880). 3 See also p. 281. CHAPTER VIII THE LIFE-HISTORY OF THE AQUATIC UTRICULARIAS AND OF ALDROFANDIA OF all our native aquatics, the Bladderworts (Utricularia) diverge most in their vegetative characters from ordi- nary terrestrial plants. When not in flower, they live wholly submerged. Roots are entirely absent and the plant consists of an elongated branching axis producing delicate, finely-divided leaves on which small utricles are borne. This is not, however, the only type of vegetative body represented in the genus. Outside Europe there are a number of terrestrial species in which entire leaves of a simple type are produced in addition to bladder-bearing organs. The family to which the genus be- longs— Lentibulariaceae — consists chiefly of aquatic and marsh plants; it is probable that the water Utricularias, with which alone we are concerned in this chapter, are the descendants of marsh forms, which, in the course of evolution, have become more and more completely involved in aquatic life1. It is im- possible to draw a sharp line within the genus between the land and water types; the terrestrial species sometimes produce water forms, and the aquatic species can, to a limited extent, take to life on land. Even among our native Bladderworts, we find that, though Utricularia vulgaris cannot live except as a submerged plant, U. minor and U. intermedia are able, on rare occasions, to produce land forms2, which are so far adapted to aerial life as to develop stomates — but in this condition they do not flower. The land form of £7. minor is said to grow as a close moss-like turf. The little utricles borne by the leaves (Fig. 59, p. 92), which give the Bladderworts their unique appearance, and to which 1Goebel,K. (1891-1893). 2 Gliick, H. (1906) and Luetzelburg, P. von (1910). 92 UTRICULARIA [CH. they owe both their Latin and their English names, are hollow structures with a small apical aperture, closed by a flap serving as FIG. 59. Uhicularia neglecta, Lehm. A single trifid leaf with bladders. (Slightly reduced.) [Adapted from Gliick, H. (1906), Wasser- und Sumpfgewachse, Bd. n, PI. II, Fig. 15 6.] FIG. 60. Utricularia flexuosa, Vahl. Longitudinal section through a bladder. (Enlarged.) Kl. = valve. [Goebel, K. (1891-1893).] a valve. Fig. 60 represents a section of the utricle of U.flexuosa^ a species which plays a part in India corresponding to that of vm] THE BLADDERS OF THE BLADDERWORTS 93 U. vulgaris in Europe. Darwin1 describes the valve of Utricu- laria neglecta as attached on all sides to the bladder, excepting by its posterior margin, which is free and forms one edge of the slit-like orifice. This margin is sharp, thin and smooth, and rests on the edge of a rim or collar which projects into the interior of the bladder. The collar obstructs any outward move- ment, with the result that the valve can only open inwards. The function of the bladders was for a long time in dis- pute. Certain ingenious but mistaken theorisers regarded the little four-armed hairs (Fig. 61), which occur within the bladders, as root-hairs, and supposed that the bladders existed in order to pro- tect these delicate organs from the direct FlG- 6l- c . Heer. Glands from the in- action or light and the depredations or tenor of a bladder. [Meier- Crustacea2! On a more plausible view, hofer' H- r and flora of Flagellates, Rotifers, Bacteria and Algae accumulate in their neighbourhood, without ap- parently being responsible for their actual initiation ; probably 1 Henslow, G. (1893), 2 Krause, K. and Engler, A. (1906). 3 Sergueeff, M. (1907). FIG. 91. Perforated leaf of -4£ono- geton fenestralis, Hook. i.-Ouvi- xi] THE LACE-PLANT OF MADAGASCAR 143 they merely make use of the debris of those cells which are sacrificed to form the perforations. That the fenestration in Ouvirandra may be of some value in connexion with aeration was suggested by Goebel's statement that the tissue of the leaf is poor in intercellular spaces1. Mlle Sergueeff, however, shows that Goebel is in error on this point, since numerous lacunae occur in the mesophyll, and she con- cludes that the main function of fenestration is not aeration, but the reduction of resistance to current. In this connexion it may be recalled that all the imperforate, submerged leaves found among the Aponogetons are thin and ribbon-shaped, thus yielding easily to the movement of the water (e.g. A. angus- tifolius. Ait.)2. It may also be significant that A. (Ouvirandra) fenestraliS) in its Madagascan home, though it sometimes grows in stagnant water, is also capable of living in torrents. Hans- girg3 had previously suggested that some forms of submerged leaf might be compared with those of such * anemophytes ' among terrestrial plants as Palms, Bananas, etc., in which the slitting, sub-division and perforation of the leaves are interpreted by some authors as modifications designed to avoid tearing by the wind. But the view that would regard all types of submerged leaf as definite adaptations to water life, probably needs con- siderable revision. We do not propose to criticise it at this point, since it is included in the broader question of the relation of leaf form to environment, which is better considered in connexion with heterophylly4. (2) THE FACTS OF HETEROPHYLLY UNDER NATURAL CONDITIONS5 The occurrence of two or more different types of leaf upon one individual, which is so frequently characteristic of water plants, has long attracted the interest of botanists. 1 Goebel, K. (1891-1893). 2 Krause,K.andEngler, A.(i9o6) 3 Hansgirg, A. (1903). 4 See Section (3) of this Chapter. 5 Arber, A. (i9i93) has been largely incorporated in Sections (2) and (3) of this Chapter. i44 HETEROPHYLLY [CH. Lyte's Herball(i 578) contains a vivid description of hetero- phylly in the Water Buttercup — a free translation of that given in Dodoens' Histoire des Plantes of 1 557. Since this description is also noteworthy for its insistence on the influence of external conditions upon the form of the leaves, it may be cited here. " Amongst the fleeting [floating] herbes, there is also a cer- tayne herbe whiche some call water Lyverworte, at the rootes whereof hang very many hearie strings like rootes, the which doth oftentimes change his uppermost leaves according to the places where as it groweth. That whiche groweth within the water, carrieth, upon slender stalkes, his leaves very small cut, much like the leaves of the common Cammomill, but before they be under the water, and growing above about the toppe of the stalkes, it beareth small rounde leaves, somewhat dented, or unevenly cut about. That kind which groweth out of the water in the borders of diches, hath none other but the small jagged leaves. That whiche groweth adjoyning to the water, and is sometimes drenched or over- whelmed with water, hath also at the top of the stalkes, small rounde leaves, but much more dented than the round leaves of that whiche groweth alwayes in the water." The water and land leaves of Ranunculus Purschii are illus- trated in Fig. 92 A and B. The heterophylly of the Water Buttercups has been subjected to a great deal of critical investi- gation. It has been shown that, in the case of Ranunculus aqua- tilis^ L.1, it is impossible to say at an early stage whether a leaf rudiment will produce the floating or submerged form. Up 1 Askenasy, E. (1870). See also Rossmann, J. (1854). B FIG. 92. Ranunculus Purschii, Rich. A , water leaf (£ nat. size) and B, leaf of the land form (reduced). [Goebel, K. (1891-1893)-] xi] THE WATER BUTTERCUPS 145 to a certain point they develop alike and are both deeply sub- divided ; then the water leaf ceases to change in shape and the segments merely increase in size, while the floating leaf gradu- ally assumes its typical, relatively entire form. In general, the type of leaf produced by the plant can be changed at will by altering the conditions. If a plant that has begun to grow on dry land, be submerged, the new leaves produced by further growth are of the submerged type. The existing leaves, though they cannot alter their form, may, in the basal region which is still capable of growth, develop transitional features as regards the epidermis. Among species related to Ranunculus heterophyllus. Fries, in which floating as well as submerged leaves are usually present, we find some, such as R. fluitans^ Lamk., in which the floating leaves are rare, while in R. circinatuS) Sibth. they are un- known. On the other hand, R. hederaceus, L. (Fig. 93), which generally grows in shallow ponds and ditches, possesses lobed reni- form leaves only, and none that are finely divided and belong to the submerged type. Heterophylly is not confined to the Batrachian Ranunculi, but is widespread in the genus. Ranunculus Flammula, the Lesser Spearwort, though generally terrestrial, may live as a water plant1, in which case it can develop both submerged and floating leaves. The submerged leaves are not, however, subdivided as in the case of Ranunculus heterophyllus. Heterophylly has also 1 Bailey, C. (i 8 87), West, G. (1910), Gluck,H. (191 1); references will be found in West, G. (1910) to the earlier writers who observed this form. FIG. 93. Ranunculus hederaceus, L. An example of a Batrachian Ranun- culus with undivided leaves, (f nat. size.) Shallow pool, Ware Undercliff, March 27, 1912. The gynaeceum, G, is bending down to ripen under water. [A. A.] A. W. P. 146 HETEROPHYLLY [CH. been recorded in R. sceleratus1^ R. Lingua2 and other species. In R. sceleratus the present writer has observed that, in aerial and in floating leaves, stomates occur on both surfaces, but in the case of the floating leaf, the stomates were found to be less numerous on the lower surface than in a leaf growing in air. The heterophylly of the Nymphaeaceae has been discussed in Chapter in3, so it is now only necessary to recall that aerial leaves, floating leaves and submerged leaves may occur, the latter belonging either to the t/A^-like type of Nymphaea and Castalia, or the dissected type of Cabomba. Leaving the Ranales, it may be worth while to pass rapidly in review the more pronounced cases of heterophylly met with in the remaining families of Angiosperms. In Callitriche verna the submerged leaves are not very different, superficially, from the floating leaves, but are distinguished by their narrower and more elongated form (Fig- 94)- Hippuris vulgaris furnishes a particularly well-marked in- stance of heterophylly. In May, when its flowering shoots rise out of the water, there is the sharpest contrast between the close whorls of rigid, short, aerial leaves (B-D in Fig. 95) and the submerged whorls, with their long, flaccid leaves, visible beneath the water surface (A in Fig. 95; see also Fig. 151, p. 231). Goebel records that he once found Hippuris growing entirely submerged at a depth of 3 metres, with leaves 7 cms. or more long4. Towards July, when the plant is at its period of maximum activity, the new shoots formed under water, even at a depth of 50 cms., are reported to be of the aerial type and to bear stomates5. This statement is of importance in connexion with the problem of the significance of heterophylly, which will 1 Ascherson, P. (1873), and Karsten, G. (i 2 Roper, F. C. S. (1885). 3 See pp. 27-29, and Figs. 12 and 14. 4 Goebel, K. (1891-1893). 5 Costantin, J. (1886). xi] SUBMERGED LEAVES 147 be discussed later in the present chapter. When winter comes on, the thin, submerged, stomateless type of leaf is again produced. Fig. 96, p. 148, represents a rather curious case, in which a shoot had reverted to submerged leaves (a) after bearing aerial leaves (c). It had apparently been beaten down into the water by heavy rains, and this involuntary return to submerged life had induced the production of the submerged type of leaf in the apical region. FIG. 94. Callitriche verna, L. Shoot from a ditch near the Cam, May 17, 1911, to show the difference between the submerged and floating leaves. The leaves down to, and including, the pair marked a, a were floating. (Reduced.) [A. A.] FIG 95. Hippuris vulgaris, L. Leaf whorls, (f nat. size.) A, water leaves; B-D, air leaves of land form. B and C have fruits in the leaf axils. [After Gluck, H. (1911), Wasser- und Sumpfgewachse, Bd. in, Figs. 23 a-d, p. 250.] Among the Umbelliferae, a differentiation between water leaves and aerial leaves is not at all uncommon. There are several instances even among our native plants. Slum latifolium is a very striking case. At the end of May, at Roslyn Pits, Ely, the present writer has seen a quantity of this plant, in a non- flowering condition, bearing three types of leaf — all three some- times occurring on a single individual (Fig. 97, p. 149). These were — firstly, submerged leaves, either simply-pinnate but deeply incised (Fig. 98, p. 150), or compound-pinnate with 148 HETEROPHYLLY [CH, FIG. 96. Hippuns vulgaris, L. A shoot which was found lying horizontally in the water, August 17, 1917. It had produced air leaves (c), but presumably in very heavy rains, which had terminated a fortnight earlier, it had been beaten down and had produced (6) transition leaves and (a) water leaves. An axillary shoot (ax) bore water leaves. (£ nat. size.) Fig. 96 should be compared with Fig. 151, p. 231, which shows the normal relations of the two leaf types. [A. A.] xi] THE WATER PARSNIP 149 linear segments (a in Fig. 97): secondly, compound-pinnate air leaves, with each leaflet of the second degree toothed and lobed FIG. 97. Sium latifolium, L. Plant from Roslyn Pits, May 30, 1911, showing three types of leaf : a, submerged compound-pinnate leaf with linear segments ; b, erect air-leaf, compound-pinnate; c and d, erect air leaves, simply-pinnate. (Reduced.) [A. A.] (b in Fig. 97); and thirdly, air leaves, once pinnate, with the leaflets toothed but not lobed (c and din Fig. 97). Some small plants were found bearing the submerged type of leaf alone. 150 HETEROPHYLLY [CH. Where the three types were borne together, the simply-pinnate leaves were the latest to be produced, and the submerged leaves the earliest, while the compound- pinnate air leaves were intermediate. Oenanthe Phellandrium, Lamk. v&r.fluviatilis, Colem.1 is very com- mon in the Cam near Cambridge. It has graceful, finely cut, pinnate leaves with obcuneate segments, and the plant is generally entirely submerged; a shoot as long as 235 cms. has been recorded. Its identity is liable to be puzzling at first sight, since its aerial axes are comparatively rarely to be found. On one occasion, however, on which the present writer found the plant bearing both submerged and aerial leaves, Oenanthe Phellan- drium (proper) was noticed, at the same time, growing magnificently in a neighbouring ditch. It had a very stout, lacunate stem, bearing numerous aerial leaves and also a relatively small number of sub- merged leaves with capillary seg- ments; the abundant lateral roots were lacunate. A comparison of the two plants suggested that Oenanthe Phellandrium var. fluviatilis is a mutation which has taken more whole-heartedly to water life than the type form of the species. Polygonum amphibium is an example of a hydrophyte which 1 It is a matter of opinion whether this plant should be regarded as a distinct species or as a variety. See Coleman, W. H. (1844). FIG. 98. Sium latifolium, L. Sub- merged leaf from a plant found at Wicken Fen, June 27, 1914. Less finely divided than leaf a in Fig. 97. (Reduced.) [A. A.] xi] SYMPETALAE AND MONOCOTYLEDONS 151 can produce either air leaves or water leaves with the utmost facility. The floating leaves and air leaves differ in internal anatomy and in the characters of the epidermis, and also show obvious external differences (Figs. 99 and 100, p. 152); the floating leaves are shiny, leathery and absolutely glabrous, while the air leaves are wrinkled and covered with hairs1. The lateral branches from a shoot with floating leaves, or even the end of the branch itself, may rise into the air and develop the characteristics of the land form2. Certain Scrophulariaceae are heterophyllous, such as Ambulia (Limnophila) hottonoides and Hydrotriche hottoniaefolia. In these cases the submerged leaves are finely divided. Among the Pedaliaceae, Trapella* has deltoid-rotundate floating leaves and linear-oblong submerged leaves, while Limnosipanea Spruceana, of the Rubiaceae, also shows a distinction between water and air leaves4. Bidens Beckii* is an example of a Composite showing heterophylly. The heterophylly of the Alismaceae and Hydrocharitaceae need not be reconsidered now, since it has been dealt with in Chapters n and iv6. Two additional figures may, however, be included here, to illustrate the effect of transferring to water a small terrestrial seedling of Alisma Plantago found growing wild (Fig. 101, p. 1 53). After between two and three months, it had developed into the typical water plant shown in Fig. 102, p. 1 53. There are many other cases of heterophylly among the Monocotyledons. Certain Potamogetons, e.g. P. fluitans, have air leaves, floating leaves and narrow submerged leaves7. Potamogeton natans is also a particularly good example; the narrow submerged leaves may attain a length of 50 cms. in running water5. The result of planting a land form of P. natans in water has been recorded5. The aerial leaves soon died, and 1 Costantin, J. (1886). 2 Schmidt, E. M. Inaug.-Diss. Bonn, 1879, quoted by Schenck, H. (1885). 3 Oliver, F. W. (1888). 4 Hansgirg, A. (1903). 5 Goebel, K. (1891-1893). 6 See pp. 9-14, 19-23, 51-52, 57> and Figs- 3-6> 9- 7 Esenbeck, E. (1914). 152 HETEROPHYLLY [CH. B FIG. 99. Polygonum amphibium, L. A , branch of aquatic plant with floating leaves. B, branch of xerophilous plant inhabiting littoral dunes. [Massart, J. (1910).] FIG. 100. Polygonum amphibium, L. A, upper epidermis, and B, lower epidermis of floating leaf, cf. Fig. 99^. C, upper epidermis, and D, lower epidermis of xero- philous leaf, cf. Fig. 99 B. The elements marked with a cross are reservoir cells. [Massart, J. (1910).] xi] SEEDLINGS OF WATER PLANTAIN 153 the next leaves formed had a smaller blade, a longer stalk, and an upper epidermis with chlorophyll and but few stomates. FIG. 101. Alisma Plantago, L. Seedlings found growing under the shade of a large A, Plantago plant in a dry ditch, May 31, 1911. (Nat. size.) [A. A.] FIG. 102. Alisma Plantago, L. One of the seedlings such as those shown in Fig. 101 which had germinated on dry land, but was planted in a pot on May 31, 191 1, and submerged in shallow water until August 9, IQII (two months, nine days). In this time it developed into a typical water form with three floating leaves (a, b, c) and others showing transitions from the submerged type. (Reduced.) [A. A.] The succeeding leaves were long and simple. Fig. 103, p. 154, illustrates this experiment. 154 HETEROPHYLLY [CH. The Pontederiaceae1 and some of the Aponogetons2 also have band-shaped, submerged leaves in addition to those that are aerial. Scirpus lacustris (Cyperaceae), in which the aerial leaves are very poorly developed, may produce strap-like floating leaves. They were first re- corded by Scheuchzer3 early in the eighteenth century. Some of the Pontederiaceae, e.g. Eichhornia crassipes, present a curious typeofheterophylly — the petioles being swollen into bladder-like, floating organs, when the plant grows in its normal free-swimming manner, but becoming slender and elongated when it is thrown upon a muddy shore and takes root there4. Experimental work shows that not only a floating life, but full light and low temperature, en- courage the spherical form of petiole, while heat, and poor illumination, tend to reduce it to a more ordinary shape5. The bladder-like swellings of the leaves of Pistia also fail to develop when the plant is stranded on mud6. Examples of heterophylly in aquatics might be multiplied almost without limit, but it is important to remember that they are not unique, and that we often meet with the same phenomenon in terrestrial plants. As Nehemiah Grew7 1Goebd,K. (1891-1893). 2 Krause, K. and Engler, A. (1906). 3 Scheuchzerus, J. (1719). 4 Spruce, R. (1908). 5 Treviranus, L. C. (1848*) and Boresch, K. (1912). 6 Hansgirg, A. (1903). 7 Grew, N. (1682). FIG. 103. Potamogeton na- tans, L. Land plant which has been transferred to water and has produced narrow water leaves. (Re- duced.) [Goebel, K. (1891- 1893)-] xi] THE MEANING OF HETEROPHYLLY 155 wrote in the seventeenth century, "there are some, which have Leaves (besides the two first Dissimilar ones1) of Two Kinds or Two distinct Figures ; as the Bitter- Sweet, the common Little Bell, Valerian, Lady-Smocks, and others. For the Under Leaves of Bitter-sweet, are Entire; the Upper, with two Lobes ; the Under Leaves of the Little Bell, like those of Fancy; the Upper, like those of Carnation, or of Sweet-William'' We find parallels to the heterophylly of hydrophytes not only among terrestrial Flowering Plants, but also in the case of the distinct * youth forms* of Conifers, and even — more remotely — in the Chantransia stage of such Algae as Batrachospermum. The conclusion to be drawn from our very brief survey, which only touches the fringe of the subject, is that heterophylly is so widespread that no interpretation can be valid unless the con- dition be treated broadly as a very general attribute of plant life, rather than as a rare and exceptional phenomenon, for which special and individual explanations will suffice. (3) THE INTERPRETATION OF HETEROPHYLLY To the earlier writers, such as Lamarck2, the problem of heterophylly presented no difficulties. They regarded the sub- merged or aerial type of leaf as representing a direct response, on the part of the plant, to the medium. The work of the last thirty years, has, however, rendered this simple conception untenable; the theory that now holds the field accords a much less prominent place to adaptation. The first observation that cast doubt upon the idea that leaf form necessarily depended directly on the milieu, was that of Costantin 3, who showed that, in the case of Sagittaria, the aquatic and aerial leaves were already distinguishable from one another in the submerged bud; he noticed auricles on a leaf which was only 2 to 3 mm. long. In Ranunculus aquatilis, also, the leaves destined to be aerial are differentiated in the bud. 1 I.e. cotyledons. 2 Lamarck, J. B. P. A. (1809). 3 Costantin, J. (18852) and (1886). 156 HETEROPHYLLY [CH. A large amount of experimental work has been published by various authors on the effect of conditions upon the leaf forms of heterophyllous plants, and, although some of the results are confused and conflicting, a study of the literature seems to justify one general conclusion — namely, that, in many cases, the submerged type of leaf is, in reality, the juvenile form, but can be produced later in the life-history in consequence of poor conditions of nutrition ; the air leaf, on the other hand, is the product of the plant in full vigour and maturity. This conclusion, which is primarily due to Goebel1 and his pupils, is substantiated not only by experiments but by observations in the field. In many heterophyllous plants, the first leaves produced by a seedling, whether it develops on land or in water, conform, more or less, to the submerged type. This is the case for instance in the Alismaceae. In Alisma Plantago (Fig. 101 yfand #, p. 153) and Sagittaria sagittifolia (Fig. 90, p. 141), the first leaves produced by the seedling, or the germinating tuber, are ribbon- like, even when the young plant is terrestrial. The formation of this type of leaf can be induced again, even in maturity, by conditions which cause a general weakening of the plant. Costantin2, thirty- four years ago, recorded that, when the leaves of Alisma Plantago were cut off in the process of clearing out a water-course, or in a laboratory experiment, the next leaves produced were ribbon-like, thus representing a regression to the submerged form. More recently, another worker3 tried the experiment of cutting off the roots of healthy, terrestrial plants of Sagittaria natans which bore leaves with differentiated laminae; it was necessary to cut the roots away every week, as they grew again so rapidly. The result of this treatment was that the plants were found to revert to the juvenile stage, the new leaves being band-shaped. When the experimenter ceased to interfere with the roots, the plants again formed leaves with laminae. Other plants, with uninjured roots, grown as water cultures in distilled water, also produced the juvenile leaf form, 1 Goebel, K. (1896), etc. 2 Costantin, J. (1886) 3 Wachter, W. xi] WATER LEAVES AND POOR NUTRITION [57 while those grown in a complete culture solution developed their laminae normally. The same observer recorded a case in which a plant of Hydrocleis nymphoides, Buchenau (Butomaceae), which had been bearing the mature form of leaf, was observed to revert to the ribbon form. On examination it was found that most of the roots had died off. When a fresh crop of roots was pro- duced, the mature type of leaf occurred again. Another writer1 demonstrated by a series of experiments upon Limnobium Eoscii (Hydrocharitaceae) that, in this case also, the heterophylly is not a direct adaptation to land or water life, but that the floating leaves are " Hemmungsbildungen " due to poor nutrition. In Stratiotes aloides^ also, he showed that the stomateless leaves were primary, and that their production could be induced at later stages by unfavourable conditions2. An experiment tried by Goebel3 on Sagittaria sagitttfoHa indicated that absence of light in this case inhibits the formation of leaves of the aerial type. An observation of Gliick's on Alisma graminifolium, Ehrh.4, also points to the same conclusion. But it seems probable that the effect produced in these cases was not due directly to the darkness, but to the state of inadequate nutrition brought about by the lack of light for carbon assimi- lation. Among the Potamogetons 5, again, experimental work has shown that reversion to juvenile leaves can be obtained under conditions of poor nutrition. For example, when a land plant of P. fluitanS) which had been transferred to deep distilled water, had its adventitious roots repeatedly amputated, regression was obtained to the floating type of leaf and then the submerged type (Fig. 104, p. 158). A similar reversion to thin, narrow leaves was brought about, in the case of P. natans^ by growing the upper internodes of a shoot as a cutting (Fig. 105, p. 159). Waterlily leaves respond to experimental treatment in just 1 Montesantos, N. (1913). 2 See pp. 51-52. 3 Goebel, K. (1891-1893). 4 See p. 280. 5 Esenbeck, E. (1914). HETEROPHYLLY CH FIG. 104. Potamogetonfluitans, Roth. A land plant transferred for about a month to distilled water with no substratum and the adventitious roots repeatedly re- moved. The internodes marked I were formed during land life; fej, £>2 and 63 are the surviving land leaves; the internodes i^ and «2 show some elongation as a result of the changed conditions ; 64 and 65 are floating leaves ; bQ-bw are leaves of the submerged type. [Esenbeck, E. (1914).] xi] WATER LEAVES AND LOW VITALITY 159 the same way as the Monocotyledons already mentioned. In the case of two species of Castalia, it has been found possible to induce the mature plants to form submerged leaves, either by removing the floating leaves or by cutting off the roots1. This confirms an earlier suggestion, made by an Italian writer2, that the development of the submerged leaves of Nymphaea lutea FIG. 105. Potamogeton natans, L. The uppermost interned es of a normal plant grown as a cutting. One floating leaf (s) survives, while the axillary shoots have produced leaves with thin narrow blades, representing a transition between the floating and submerged types. [Esenbeck, E. (1914).] was due to "un indebolimento o diminuzione di energia vitale." This suggestion has received independent, experi- mental confirmation from another worker3, who estimated that a well-developed floating lezfofNympkaea lutea was about eleven times the dry weight of a submerged leaf of the same area. Another Dicotyledon, Proserpinaca palustris^ which was in- 1 Wachter, W. (18972). 2 Arcangeli, G. (1890). 3 Brand, F. (1894). 1 60 HETEROPH YLLY [CH . vestigated by Burns1, gave results pointing to the same general conclusion as those observations already quoted. The primitive type of leaf in this plant is always a " water" leaf, but this form of leaf was also produced in the autumn by all the plants, regardless of any external conditions which the experimenter could control. On the other hand, at the time of flowering and in the summer generally, almost every plant, whether growing in water or air, produced the "land" type of leaf — the transi- tion from the "water" to the "land" type taking place earlier on strongly growing than on weak stems. The author considers it evident that the aquatic environment is not the cause of the division of the leaf, nor does it depend on light, temperature, gaseous content of the water or contact stimulus. The only con- clusion, which he considers justified by his experiments, is that Proserpinaca -palustris has two forms — adult and juvenile; under good vegetative conditions, it tends to produce the adult form with the undivided leaf, the flower and the fruit, while, if the vegetative conditions are unfavourably influenced, a reversion can be induced to the primitive form with the submerged type of leaf. These results are consistent with those of McCallum2, who had dealt with the same species at an earlier date, but his interpretation is slightly different. He is inclined to regard the occurrence of the aquatic form as induced by the checking of transpiration, and by the increased amount of water which hence accumulates in the protoplasm. This explanation is not incon- sistent with the more general view that any condition tending to lower the vitality may be responsible for a reversion to the submerged type of leaf. In nature, the regression to the juvenile type of leaf some- times occurs, not only in the case of an entire plant subjected to adverse conditions, but also in the case of lateral shoots from an individual which is otherwise producing the mature form of leaf. Goebel3, for instance, examined an old example of Eichhor- nia azurea (Pontederiaceae) which had wintered as a terrestrial 1 Burns, G. P. (1904). 2 McCallum, W. B. (1902). 3 Goebel, K. (1891-1893). xi] REVERSION IN LATERAL SHOOTS 161 plant in a greenhouse ; the leaves were of the mature form — differentiated into sheathing base, petiole and lamina — except in the case of a lateral shoot, which bore the grass-like, simple, leaves which characterise the young plant. Goebel1 also de- scribes the occurrence of subdivided leaves of the water type on lateral shoots of normal land plants of Limnophila hetero- phylla. A corresponding reversion has been observed in the case of the side branches of plants of Proserpinaca palustris* developing in the air from a plant whose main stem was pro- ducing the mature type of leaf; by removing the growing apex of the stem in June, these side branches of the * water* type were induced to develop. The interest of these lateral shoots, which show a reversion to an ontogenetically earlier type of leaf, is enhanced by the fact that C. and F. Darwin3 have recorded a case of the occurrence, on lateral shoots, of leaves whose characters are probably phylogenetically earlier than those which the species normally exhibits. Their observations related to the sleep habits of the allied genera, Melilotus and Trifolium. They noticed, in Melilotus Taurica, that leaves arising from young shoots, produced on plants which had been cut down and kept in pots during the winter in a greenhouse, slept like those of Trifolium^ with the central leaflet simply bent upwards, while the leaves on the fully-grown branches of the same plant afterwards slept accor- ding to the normal Melilotus method, in which the terminal leaflet rotates at night so as to present one lateral edge to the zenith. They suggest that Melilotus may be descended from a form which slept like Trifolium. The idea that the * juvenile' leaves, produced on lateral shoots, may in some cases represent an ancestral type, is con- sistent with the facts in the case, for instance, of the Alismaceae, provided that the ' phyllode theory ' of the Monocotyledonous leaf be accepted in the sense advocated by Henslow and the present writer. According to this theory, which will be dealt 1 Goebel, K. (1908). 2 Burns, G. P. (1904). 3 Darwin, C. and F. (1880). A.W. P. II 1 62 HETEROPHYLLY [CH. xi with in some detail in Chapter xxvm, the ancestral leaf of this family was band-shaped, while the oval or sagittate blade, or * pseudo-lamina/ is a later development — a secondary ex- pansion of the distal region of the sheath or petiole. The submerged youth-leaves of this family would thus represent a reversion to phylogenetically older forms. If the interpretation of heterophylly indicated in the present chapter holds good at all widely, the teleological view of the submerged leaf must be considerably modified. The present writer would like to suggest that, for the old conception of heterophylly as induced by aquatic life, we should substitute the idea that such a difference between the juvenile and mature forms of leaf as would render the juvenile leaf well suited to life in water, has been in many cases one of the necessary pre- liminaries to the migration from land to water, and that the aquatic Angiosperms thus include, by a process of sifting1, those plants whose terrestrial ancestors were endowed with a strong tendency towards heterophylly2. 1 Guppy, H. B. (1906) first emphasized the fertile idea that the habitats of plants were determined by their peculiarities of structure, and not 'vice versa. In relation to the occurrence of plants with buoyant seeds and fruits in water-side stations, he writes, " there are gathered at the margins of rivers and ponds, as well as at the sea-border, most of the British plants that could be assisted in the distribution of their seeds by the agency of water. This great sifting experiment has been the work of the ages, and we here get a glimpse at Nature in the act of selecting a station." 2 In addition to the references mentioned in this chapter, MacDougal, D. T. (1914) and Shull, G. H. (1905) may also be consulted; the results recorded in these papers emphasize the difficulty and complexity of the problem. [ 163 ] / CHAPTER XII THE ANATOMY OF SUBMERGED LEAVES1 THE majority of submerged leaves have certain charac- ters in common, the most obvious of which is their delicacy of structure. On removal from the water they gene- rally collapse rapidly, and in some cases, e.g. Hippuris vulgaris, when they are plunged into alcohol the chlorophyll begins visibly to pass into solution almost from the first moment. The general tenderness of the leaves is due to the thinness of the mesophyll and the absence of differentiation between spongy and palisade parenchyma, and also to the relative lack of me- chanical elements and the slight development of the cuticle2. It is indeed the epidermal characters — such as the reduction of cuticle — which most markedly distinguish submerged from aerial leaves. It will be remembered that, in general, the epidermal cells of the leaves of Dicotyledons tend to be sinuous in outline, while those of Monocotyledons are more rectangular. But in the case of such a plant as Callitriche verna (Fig. 1 1 1, p. 1 70) which has both aerial and submerged leaves, it is found that, though the aerial leaves show the characteristic Dicotyledonous sinuosity in the form of their epidermal cells, the corresponding elements in the submerged leaves have straight walls, and hence approach the Monocotyledonous type. An interesting hypothesis on this subject was put forward long ago by Mer3. He drew attention to the fact that the epidermis was the tissue most directly affected by transpiration, and suggested that variations in that function might exercise an influence upon the form of the epidermal cells. According to his view, when transpiration is 1 For a comprehensive account of this subject see Schenck, H. (1886), which has been largely drawn upon in the present chapter. 2 A cuticle, though thin, seems to be invariably present. See Geneau de Lamarliere, L. (1906). 3 Mer, E. (iSSo1). 1 64 SUBMERGED LEAVES [CH. feeble, as in the case of submerged plants, the epidermal cells are kept in a constant state of turgescence, and hence their growth takes a uniform course resulting in regularity of form. But, on the other hand, when transpiration is active, as in land life, the current is subject to great variations which react upon the form of the epidermal cells and produce sinuosity. It is scarcely possible to submit such a theory to direct proof, but it seems to the present writer that it is at least consistent with the fact, established at a much later date than Mer's work, that Monocotyledons with their rectangular epidermal cells, are in general, though with many exceptions, c sugar-leaved * and weak transpirers, while Dicotyledons, with their epidermal cells often resembling a Chinese puzzle, are * starch-leaved ' and strong transpirers1. The epidermal cells of submerged leaves differ from those of air leaves not only in form but also in contents. Chlorophyll grains, which are generally described as absent from the epi- dermis of terrestrial plants, are often present in great abundance in this tissue in submerged leaves2. Treviranus3, nearly a cen- tury ago, alluded to the lack of distinctively epidermal charac- ters— or, to use his own expression, the "absence of an epider- mis " — in the case of the lower surface of the leaf of Potamogeton crispus, while Brongniart4, a few years later, observed the pre- sence of chlorophyll in the leaf epidermis of P. lucens. Subse- quently, epidermal chlorophyll has been observed widely among aquatic plants5, though there are certain exceptions, such as Callitricht*. In some cases, e.g. Zoster a^ Cymodocea, Posidonia1, the epidermis is actually the part of the leaf richest in green corpuscles. The presence of chloroplasts does not constitute, however, so absolute a difference from land plants as is some- times assumed, since it has been shown that chlorophyll grains can be found in the epidermis of the green organs of the 1 Stahl, E. (1900). 2 Schenck, H. (1886). 3 Treviranus, L. C. (1821). 4 Brongniart, A. (1834). 5 Chatin, A. (1855!), etc. 6 Schenck, H. (1886). 7 Sauvageau, C. xn] CHLOROPHYLL AND STOMATES 165 majority of terrestrial Dicotyledons, though they are generally absent in the case of terrestrial Monocotyledons1. They are usually to be observed only in the lower epidermis of the leaf, but it seems probable that this is due to the destructive action of sunlight upon the chlorophyll in the upper epidermis. In support of this view it may be mentioned that, in diffused light, chlorophyll occurs in the upper epidermis of the leaves of Eellis perennis, whereas under normal conditions there is chlorophyll only in the lower epidermis. The presence of green plastids in the epidermis of submerged plants may thus be regarded as representing merely the elaboration of a character already existing in terrestrial plants, which finds favourable oppor- tunities for development in the relatively dim illumination which submerged plants receive. The statement, frequently made, that stomates are absent from submerged leaves, and from the lower surface of floating FIG. 1 06. Elodea canadensis, Michx. T.S. leaf; », intercellular air channels [Schenck, H. (1886).] leaves, needs considerable qualification2. It is, indeed, broadly true that stomates are much less frequent in submerged than in terrestrial leaves, and, moreover, in certain water plants, such as Elodea (Fig. 106), Vallisneria^ Thalassia, and other Hydrocharitaceae which always live entirely submerged, sto- mates never occur3. Among the Cryptogams, Isoetes lacustris is entirely free from stomates, and Goebel4 even found that it failed to produce any when grown for two years as a land plant. Submerged leaves in general are not only poor in stomates but also in hairs ; it has been suggested by Mer5 that this — like the ^tohr, A. (1879). 2 Costantin, J. (I8851). See also Porsch, O. (1903) for citations of a large number of cases in which the occurrence of stomates on submerged organs is mentioned in the literature. 3 Solereder, H. (1913). Goebel, K. (1891-1893). 5 Mer5 £ (jgSo1) and (1882^ 4 1 66 SUBMERGED LEAVES [CH. form of the epidermal cells — may be correlated with the feeble- ness and uniformity of the transpiration stream. He supposed that the active and variable flow of sap in land plants might bring about the accumulation of nutriment at certain points of the epidermis, thus favouring localised cell-multiplication and the production of hairs and stomates. It seems possible to the present writer that this suggestion contains an element of truth. But on the other hand it must be remembered that stomates have been observed in a large number of submerged leaves, such as those of Lobelia Dortmanna1^ Villarsia ovata* and Pontederia cor data?) and on the lower surfaces of certain floating leaves, such as Limnocharis Humboldtii* and Hydrocharis Mor- sus-ranae^. Porsch5, who has considered the subject compre- hensively, concludes that the stomatal apparatus must have been gradually evolved over a long period of time, so that its charac- ters have become fixed with great tenacity; for, in cases where its existence must be not only superfluous, but attended by a certain danger to the plant, instead of being discarded, it is often modified secondarily in such a way as to render it func- tionless. He shows that, in the case of submerged plants which retain their stomates, four different modifications are found, each of which must have the result of preventing water entering the tissues through the aperture between the guard cells : (1) The guard cells may close on submergence, even in full illumination, e.g. Callttriche verna and Hippuris vulgaris. (2) The aperture may be permanently closed, as in the case of Potamogeton natans (Fig. 1 07 B\ in which the whole stomatal apparatus remains roofed in with cuticle. (3) The development of each stomate may actually cease at an early stage. This is rare, but such abortive stomates are found in the submerged parts of a species of Oenanthe^. 1 Armand, L. (1912). 2 Costantin, J. (I8851). 3 Duchartre in discussion following Chatin, A. (1855*). 4 Goebel, K. (1891-1893). 5 Porsch, O. (1903) and (1905). 6 Porsch uses the specific name "Oenanthe aquatilis, L."; he is pro- bably referring to Oe. Phellandrium^ Lamk. var. fluviatilisy Colem. xn] WATER PORES AND AIR PASSAGES 167 (4) The stomates may develop normally, but the guard cells remain pressed together with their cuticular ridges interlocked, e.g. Calla pa- lustris (Fig. 107 A). In addition to ordinary stomates, which, in sub- '. '. FIG. 107. A. Calla palustns, L. T.S. stomate merged life, are incapable in submerged leaf stalk; the thickening bands of pvprrkino- their normal fit closely together. B. Potamogeton natans , L. T.S. submerged stomate from leaf stalk function, Submerged leaves of floating leaf. This stomate is entirely ! i i roofed in with cuticle. [Porsch, O. (1905).] also very commonly bear water stomates, which are probably of importance in keeping up the 'transpiration' stream by exudation1. A longitudinal section passing through the water pores of Pistia Stratiotes is shown in Fig. 53, p. 82, while the apical opening of Potamogeton densus — in which the tracheids communicate directly with the exterior without the intervention of water stomates — is represented in Fig. 108. The aerating system of submerged leaves is a very conspicuous feature. The mesophyll of such subcylindrical radical leaves as those of Littorella and Lobelia Dortmanna is traversed from end to end by air passages, interrupted only ^ iog Pota**geton by porous diaphragms, and the same densus, L. L.s. apex of feature is markedly developed in the medi^ner^e InT showing elongated petioles of such leaves as Sagit- the apical opening. (Upper . ,T^. * N <-ni j- i surface of leaf to right tana (Fig. 8, p. 19) Inese diaphragms hand.) fx 220<) [Sauva- form points d'appui for the secondary 8eau> c- N*9**M nerves connecting the longitudinal bundles2. The mesophyll of submerged leaves shows, as has been already indicated, little sign of differentiation into palisade and 1 This subject is considered more fully in Chapter xxi. 2 Duval-Jouve, J. (1872). i68 SUBMERGED LEAVES [CH. spongy parenchyma. In many cases the assimilatory activity seems, in great measure, confined to the epidermis, the meso- phyll serving rather for storage purposes. Myriophyllum^ shows this distinction clearly; the epidermis is rich in chloro- phyll, while the mesophyll contains large starch grains (Fig. 109). This leaf is a good example of the subdivided, submerged type, each limb of which exhibits a tendency to a radial arrangement of the T . 77 7-7- . 7 FIG. IOQ. Mvriophyllum spicatum. L. tissues. Ltttorella, UtriCUlana T.S> through 'a segment of the leaf of minor (Fig. 74, p. IO8) and the water form. The epidermis contains ^ \ „ , chloroplasts and the mesophyll is laden Ler atop hy Hum all Show the Same with large starch grains, only indicated approximately radial type of in a few cells- ^w) [Schenck, H leaf anatomy. The effect of environment upon this kind of leaf, is illustrated by a com- parison between the land and water forms of Myriophyllum. In the case of M. alter nifolium^ the land form, when growing in sunny situations, has shorter and thicker leaf segments than the water form; they are also dorsiventral and elliptical, instead of radial and cylindrical, while the xylem is more highly developed than in the water form. The epidermis contains only a few small chlorophyll grains, and stomates occur. The epidermal cells also have the sinuous outline which is lacking in the water form. The absence of marked dorsi- ventrality in the leaves of many submerged plants, such as Myriophyllum, may in part be attributed to the fact that they are perpetually being moved about by water currents, and thus they do not retain any constant position in relation to the incident light. The very young submerged leaves of Myriophyllum vertidl- latum and M. spicatum show a peculiarity which has^repeatedly iSchenck, H. (1886). xn] NON-RADIAL ANATOMY 169 attracted the attention of botanists1 — the occurrence, namely, of little colourless cellular plates, arising generally at the apex and base of each lobe, but sometimes elsewhere (Fig. 1 10 y/and By p. 170). The cells at their base (c in Fig. no B) become corky at an early stage, and the plates drop off. They are prob- ably best interpreted as caducous trichomes; their function, if they have one, is quite unknown. As examples of the flat, non-radial type of submerged leaf, Callitricfa Elodea and Alisma may be mentioned. In Fig. in, p. 170, the contrast between the aquatic and aerial leaf of Cal- litriche verna is indicated. The water leaf is thin, but still retains some mesophyll ; the outlines of the epidermal cells in the two forms show the distinguishing characters to which reference has already been made. Callitriche autumnalis2, which lives and flowers completely submerged, has a thinner leaf. The leaf of Hottonia resembles that of Callitriche. The ribbon-leaf of Alisma Plantago shows a slightly different type of structure. The chlorophyll-containing epidermis forms the essential part of the leaf, and the large air passages are bounded by it. There is one main bundle, accompanied by two tiny laterals placed close to the margins. In Elodea canadensis (Fig. 106, p. 165) we reach almost the ultimate phase in reduction of the meso- phyll, for here the entire assimilating tissue is reduced to the two epidermal layers. The extremely delicate leaf is strength- ened by some fibrous cells. Supporting sclerenchyma is cha- racteristic of a certain number of submerged leaves such as those of the Potamogetons (e.g. Fig. 38, p. 61). There is a strong tendency, in submerged leaves, to the reduction of the tracheal system. Among the Hydrocharitaceae, for instance, though typical spiral tracheids occur in the sub- merged leaves of Stratiotes^ the leaves of a number of other genera show either no tracheids at all, or else more or less ephemeral elements with annular thickenings, e.g. Elodea^ Halophila, Vallisneria and Thalassia*. 1 Irmisch, T. (1859!), Borodin, J. (1870), Magnus, P. (1871), and Perrot, E. (1900). 2 Hegelmaier, F. (i 864). 3 Solereder, H. (i 9 1 3). i yo SUBMERGED LEAVES [CH. FIG. no. Trichomes of Myriopkyllum verticillatum, L. A, diagram of a young leaf showing the arrangement of the trichomes. B, a single multicellular caducous trichome at leaf margin with corky cells, c. at its base. [Perrot, £. (1900)-] -I FIG. in. Callitriche verna, L. A, T.S. submerged leaf, x 80; B, T.S. leaf of land form, x 147; C, upper epidermis of submerged leaf, x 92; D, upper epi- dermis of land leaf, x 88; E, lower epidermis of submerged leaf, x 92; F, lower epidermis of land leaf, x 88. [Schenck, H. (1886).] xn] 'ADAPTATION' TO WATER LIFE 171 A consideration of the structure of submerged leaves opens up a series of perplexing theoretical problems. The idea that the submerged type of leaf arises as an adaptive response to the milieu^ proves on examination altogether inadequate. The general form of these leaves seems attributable to poor nutri- tion, while certain characters — thinness, lack of differentiation of spongy and palisade parenchyma, and presence of chlorophyll in the epidermis — are also common, in some degree, to terres- trial plants growing in the shade, and seem intimately con- nected with lack of sunlight1. We may perhaps suppose that the dimness of the light which reaches a plant living below the surface of the water may be directly responsible for these characters; the green pigment, for instance, may be present in the epidermis simply because the leaf is not exposed to direct sunlight, which in the case of terrestrial plants destroys the chlorophyll in the epidermis as fast as it is formed2. Now there is little doubt that a thin leaf with an epidermis rich in chlorophyll is particularly well adapted for the assimilation of dissolved carbon dioxide; how then are we to account for the singular coincidence that characters arising in this fortuitous and mechanical fashion prove definitely advantageous to the plant? It is perhaps conceivable that it is the very fact that terrestrial plants under conditions of poor illumination tend to develop this type of leaf, which has rendered possible the as- sumption of the submerged habit, and that it is those plants whose leaves happened under such conditions to develop on the lines particularly suited to water life, which have accomplished the transformation into thorough-paced aquatics. 1 Schenck, H. (1885). 2 Stohr, A. (1879). CHAPTER XIII THE MORPHOLOGY AND VASCULAR ANATOMY OF AQUATIC STEMS1 THE stems of plants that pass the greater part of their vegetative life entirely submerged, fall in general into two categories. The less common type is the abbreviated axis bearing a tuft of long narrow leaves (e.g. Stratiotes, Fig. 31, p. 49 and Fig. 32, p. 53) while, on the other hand, the majority of submerged plants are characterised by thin, elon- gated, branched stems rising wholly or partially into the water, clothed with leaves and often capable of rooting at the nodes (e.g. Potamogeton, Fig. 37, p. 60 and Myriophyllum, Fig. 144, p. 221). Owing to the high specific gravity of the water, and the lightness of the stems, due to the air in the intercellular spaces, each axis is to a large extent relieved of the task of supporting the weight of its branches. In consequence there seems to be no impulse to the relatively strong development of a single main axis, and, in conformity with this, the general system is often sympodial (e.g. Hippuris, Fig. 112). The plant frequently grows actively in front while it dies away behind, and may thus be regarded, to use Schenck's expression, as being in a state of perpetual youth. The older regions tend to become infested with a flora of epiphytic Algae and Fungi, among which a microscopic fauna makes its appearance. This is an obvious disadvantage, since no leaf thus laden can perform its functions successfully. Possibly the rapid growth of fresh leafy shoots at the apex serves as a compensation for a loss of activity in the older regions, traceable to this cause. The vascular system of submerged stems shows certain modifications upon the terrestrial type, the most striking differ- 1 For a detailed treatment of this subject see Schenck, H. (1886), which has been largely drawn upon in the present chapter. CH. xm] MEANING OF LIGNIFICATION 173 ence being that the xylem tends to be reduced in amount, while the lignification is often very poor. Spiral or annular vessels, when present in the neighbourhood of the growing apex, may, in some instances, be completely destroyed by the elongation of theinternodes, and may survive only at the nodes, e.g. Potamogeton lucens^^ Zannichellia patustris1, Althenia filiformis2, etc., while in the case viEhdea canadensis^ the tracheal thickenings do not even persist in the nodal tissues. Ceratophyllum (Fig. 56, p. 87) is an example of a further degree of reduction, since here ligni- fication is entirely lacking, even in the apical region. This loss of lignification has been sometimes regarded as a corroboration of the widely-held view that the transpiration stream has no existence in submerged plants. But, as we shall show in Chapter xxi, the idea that such a current is absent in these plants, seems often to have been accepted on totally inadequate grounds. In FIG. 112. Hippwis vulgaris, L. Diagram of the horizontal rhizome as seen from above to show sympodial growth; a-A ; b-B; c-C\ d-D, E, represent successive axes. [Irmisch, T. (1854).] this connexion it appears to the present writer that, when xylem and the part which it plays in water-conduction is being con- sidered, too much stress is often laid — almost unconsciously perhaps — on the question of lignification. It seems sometimes to be assumed that the functional importance of the xylem is proportional to its degree of lignification ; an idea which may perhaps be interpreted partly as a hypnotic impression con- veyed to the botanist's mind by the vividness of the xylem in 1 Caspary, R. (18582). 2 Prillieux, E. (1864). 174 AQUATIC STEMS [CH. stained sections, and partly as a survival from the old days of the 'imbibition theory/ when the ascent of water was sup- posed to be due to some mysterious property peculiar to the Signified membrane. But it is now universally recognised that water travels in the cavities of the vessels and tracheids rather than in the walls. What part then does lignification play in the ascent of water? It must be remembered that the water-con- ducting elements are dead and empty, and that in terrestrial plants they often contain air, which is more or less rarefied, and is thus at low pressure. These dead elements are generally in contact with turgid living cells, which exert a strong pressure against their walls. From the point of view of the ascent of water, the only function of the lignified walls of vessels and tracheids appears to be to prevent their being crushed by the neighbouring living elements. The way in which tyloses force themselves into vessels through the defenceless, thin places in their walls, gives some idea of the pressure which living cells are prepared to exert. In hydrophytes, however, the circum- stances are very different. The vessels, instead of frequently containing rarefied air, as in the case of land plants, are pre- sumably more continuously full of liquid, and are therefore less liable to be crushed and obliterated by the surrounding living elements. The conduction of water is not, in their case, conditioned by the possession of armoured walls. There is every reason to suppose that the non-lignified conducting elements of a submerged plant may be as effective in raising water as the woody vessels of a terrestrial tree ; that water does, as a matter of fact, travel freely in the non-lignified xylem spaces of the submerged Potamogetons has been shown by experiment1. Elongated, submerged stems, unless they grow in perfectly still water, must be subjected to some amount of tension from currents. It is probably more than a mere coincidence that the vascular system of aquatics is so often condensed into a central strand, recalling the central cylinder of roots and of climbing stems, both of which are organs subjected to pulling forces. 1 Hochreutiner, G. (1896); see pp. 261-263, Chapter xxi. xm] CONDENSED VASCULAR CYLINDER 175 The central strand, even when extremely simple as in the case of Cattitriche*) the Hydrilleae, Aldrovandia^ Naias, Hippuris*, etc., is not a single bundle, but represents an entire vascular system, in which the strands are not differentiated as indi- viduals. That the xylem reduction, to which we have already referred, is not itself the cause of the union of the single bundles into an axial strand, may be deduced from a comparison with the stems of colourless saprophytes or parasites. In such plants there is little transpiration and no assimilation and the xylem is proportionately reduced. But the simplified bundles retain their ancestral position and do not fuse into an axial strand3. Among the Dicotyledons there are certain hydrophytes, e.g. the Water Buttercups (Fig. 113, p. 176), in which the bundles remain perfectly separate, but in the majority some degree of condensation may be observed. The Potamogetons (Fig. 39, p. 62 and Fig. 40, p. 64) provide an exceptionally interesting series illustrating, within a single Monocotyledonous genus, stages in the concentration of the vascular cylinder. It must suffice here to draw attention to a few other typical examples, showing various grades in the reduction of the vascular system. In Pep Us For tula there is a well-marked axial strand, in which individual bundles can no longer be distinguished. In transverse section, an external ring of disconnected phloem groups is seen to enclose a ring of xylem, consisting of short radial rows of vessels separated by rows of parenchyma. The internal phloem characteristic of the Lythraceae is developed within the xylem, and a pith is formed. A cambial layer occurs, but does little work. The next stage of reduction may be illustrated by the stem of Callitriche (Fig. 1 14 y^and #, p. 176) which shows in trans- verse section a small ring of xylem surrounded by phloem; there is no cambium. In the water forms (Fig. 1 14 5) the pith is resorbed at an early stage and is represented by a space. Hippuris has travelled still further upon the road of speciali- 1 Hegelmaier, F. (1864). 2 Sanio, C. (1865). 3Schenck, H. (1886). 1 76 AQUATIC STEMS [CH. sation. The vascular tissue is concentrated into a definite cylin- der, with external phloem and internal xylem, enclosing what seems at first sight to be a pith. But Sanio1, who described the .-v.b. FIG. 113. Ranunculus trichophyllus, Chaix. T.S. young stem to show the numerous air spaces, s, in the ground tissue, v.b. = vascular bundle; h = hair, (x 47.) [A. A.] B FIG. 114. Callitriche stagnalis, Scop. Central cylinder of stem. A, land form. (X475-) B, water form, (x 290.) [Schenck, H. (1886).] anatomy of the stem, demonstrated that the central region, which, if the mature structure alone were examined, would 1 Sanio, C. (1865). xm] THE 'PITH' OF HIPPURIS 177 certainly be regarded as pith, is in reality to be interpreted as xylem parenchyma. He described the occurrence of a number of cauline tracheal elements in the 'pith' region of the embry- onic vascular cylinder near the growing point. These cauline elements were found by Sanio to be ephemeral and impersis- tent; he observed their first appearance at levels above the entry of the first lignified leaf traces. This account appeared to the present writer so singular, that she repeated Sanio's observa- tions in order to see whether the application of microtome methods, by which the history of the tissue in question could be traced element by element, would confirm or refute his conclusions. The result was in all essentials to confirm Sanio's description; the accuracy of his work is indeed remarkable, when it is considered that he was obliged to rely entirely on hand sections for the interpretation of this delicate piece of apical structure. In one stem-apex examined by the present writer, the first cauline xylem element appeared when the stele was only 0-08 mm. in diameter (Fig. 115 A^ p. 178). This harmonises with Sanio's statement that in one preparation he observed the first cauline element when the cylinder was about o-i mm. across. The cauline elements gradually increased (Fig. 1 1 5 B) and persisted for a distance of a few millimetres from the apex, becoming gradually less lignified and thinner-walled until they finally disappeared. At the level at which the first lignified leaf trace began to pass in towards the stele (Fig. 115 B\ there were twenty-one cauline tracheal elements. At a slightly lower level, at which the tracheids belonging to eight leaf traces (L) had entered and taken up a position at the periphery of the stele, twenty-one cauline elements could still be identified (Fig. 1 1 5 C). In this particular case, they were found to be just finally vanish- ing at the level at which the seventh set of lignified leaf traces (counting from the apex) entered the stele ; at this level the stele was only 0-2 mm. in diameter. However, a few of the outermost cauline elements were more persistent than the rest, and either themselves became part of the xylem ring, or fused with the leaf traces as they entered. That the lignified elements in the A. W. P. I2 1 78 AQUATIC STEMS [CH. 'pith* are actually xyl em, and not merely altered pith cells, is indicated by their possession of typical tracheal thickenings, and also by their occasionally identifying themselves, as just mentioned, with the xylem ring. U L \ / I _A, . N' / L I— FIG. 115. Hippuris vulgaris, L. Series of transverse sections of stele of a stem near apex to show relation of eauline and leaf trace xylem ; the dotted line in each case represents the periphery of the stele, (x 280 circa.) A, appearance of first eauline element when stele is 0-08 mm. in diameter. B, level at which first lignified leaf trace begins to pass in towards the stele, which contains 21 eauline xylem elements, but no leaf traces. C, the level at which eight lignified leaf traces (L) have taken up a position at the periphery of the stele, in which 21 eauline elements can still be counted. D, a lower level at which traces (I/) from a second node have entered. Fusion of traces from the two nodes or of eauline elements with either is indicated by (L + L'), (C + L), etc. [A. A.] Myriophyllum (Figs. 1 1 6 and 117) closely resembles Hippuris in vascular anatomy and has the same eauline tracheal elements in the pith, but the xylem is more reduced1. 1 Vochting, H. (1872). XI 1 1] MILFOIL AND HORNWORT 179 Ceratophyllum (Fig. 56, p. 87), as we have already shown, may be regarded as representing the extremest stage in the simplification characteristic of the stem-anatomy of Dicotyle- donous water plants. There is a central duct, surrounded by FIG. 1 1 6. Myriophyllum spicatum, L. T. S. moderately old axis, (x 30.) [Vochting, H. (1872).] FIG. 117. Myriophyllum spicatum, L. T.S. stele of young axis showing the scat- tered internal vessels and eight phloem groups near the periphery of the stele. (x2i5.) [Vochting, H. (1872).] elements whose walls are somewhat thickened, but consist of cellulose only1. These thick-walled cells are again surrounded by a broad zone of phloem2. In connexion with the strong tendency shown by aquatic 1 Sanio, C. (1865). 2 Schenck, H. (1886). 180 AQUATIC STEMS [CH. plants towards the condensation of the vascular system to a single strand, devoid of secondary thickening, and in which individual bundles cannot be distinguished, an interesting suggestion, put forward some years ago by Scott1, may be con- sidered. Expressed very briefly, this suggestion was that the cases of polystely2 occurring among the Angiosperms may be due to descent from aquatic ancestors, from which a reduced type of vascular system without cambium has been derived. If plants with this heritage at any stage of their phyletic history returned to terrestrial life, they probably experienced the need for an increase of vascular tissue; but the production of normal secondary thickening possibly presented difficulties, owing to the condensed nature of the vascular system and the loss of the cambial apparatus, and this may have led to the alternative expedient of multiplying the existing steles. Scott refers to two genera of flowering plants containing polystelic species — Auricula (Primulaceae) and Gunner a* (Haloragaceae). Both these genera include polystelic and monostelic species. The single steles of the monostelic species are exactly like the indi- vidual steles of the polystelic species; they have the vascular bundles crowded together and are almost devoid of pith and 1 Scott, D. H. (1891). 2 The word 'polystely' is used in this connexion in a descriptive sense, as a matter of convenience, irrespective of the possible validity of the objections to its use as a morphological term raised by Jeffrey, E. C. (1899), 3 For the case of Gunnera a somewhat similar interpretation had been proposed in 1 875 by Russow, who however did not perceive that a return from water to land life might be the factor initiating the polystelic con- dition. He suggested that the Gunneras were descended from ancestors whose vascular system had been condensed into a single central strand, and that in the course of generations this form of stele might have become so far stereotyped that it could no longer separate into its original con- stituents (collateral vascular bundles) when a more elaborate conducting system was required j it thus adopted the alternative of branching, and reproducing its structural peculiarities in each branch. (Russow, E. ('875)0 xm] POLYSTELY 181 secondary thickening. Both Auricula and Gunnera have near relatives which are aquatic in habit. The reduced aquatic stele of the submerged stem of Hottonia has much in common with an individual stele of Auricula. This comparison between Hottonia and Auricula has had its force greatly increased by Prankerd's1 subsequent discovery of a transient polystelic phase in Hottonia palustris in the base of the inflorescence axis — that is to say, in the region of transition from an aquatic to an aerial type of stem. It was observed by Scott that the stele of Myriophyllum or Hippuris agrees closely in structure with that of the monostelic Gunneras, or with a single stele from one of the polystelic species. The comparison of the stele of Myriophyllum with that of the Gunneras has been fully confirmed by more recent work2. In the case of Gunnera — assuming a descent from an aquatic ancestor — it is easy to realise how acute the need for increased vascular tissue in the rhizome must have become when the present type of habit was acquired, since the leaves grow in some cases to an enormous size. Darwin3, in the Voyage of the Beagle, describing the occurrence of Gunnera scabra on the Island of Tanqui, off Chili, remarks — " I measured one [leaf] which was nearly eight feet in diameter, and therefore no less than twenty-four in circumference! The stalk is rather more than a yard high, and each plant sends out four or five of these enormous leaves, presenting together a very noble appear- ance." It should be noted that Scott had overlooked one previous record of polystely due to Dangeard and Barbe4 — that of the occurrence of four or five steles in the axis of Pinguicula vul- garis. But this case introduces no difficulty so far as Scott's hypothesis is concerned, for Pinguicula is related to Utricularia 1 Prankerd, T. L. (1911). 2 Schindler, A. K. (1904). This author takes the view that Hippuris does not belong to the same cycle of affinity as Gunnera and Myrio- phyllum. 3 Darwin, C. (1890). 4 Dangeard, P. A. and Barbe, C. (1887). 1 82 AQUATIC STEMS [CH. xm with its numerous aquatic species. Further instances of poly- stely have been subsequently discovered among the Nymphaea- ceae1. Though the anomalous structures met with in this family cannot perhaps be explained on quite the same lines as those of Auricula and Gunnera, their existence does not invalidate Scott's view; they are of interest as furnishing another example of the tendency towards the development of distinct steles or vascular zones in aquatic plants in which secondary increase in thickness is lacking. The present writer would like to suggest that there is possibly some significance in the fact that nearly all the known cases of polystely in Angiosperms occur in plants whose main vegetative axis takes the form of a rhizome. This organ, not being sub- jected to the same mechanical strains as an erect stem which has to support leaves and branches, is not so irrevocably committed to the * continuous cylinder* type of vascular system, which is the best form of structure for withstanding bending forces. That the polystelic type of anatomy does not make for strength, is indicated by the recent observation, concerning a gigantic Hawaian species of Gunner 'a, that "the rhizome is very soft, and can be severed by a single machete stroke2." One special point of interest connected with the hypothesis of the origin of polystely through an aquatic ancestry, lies in the fact that, if it be accepted, it forms a particularly salient in- stance of the working of a certain principle of evolution which the present writer proposes to call "the Law of Loss3'*; this law will be discussed in Chapter xxvm. 1 Gwynne-Vaughan, D. T. (1897); see a^so Chapter in, p. 37. 2 MacCaughey, V. (1917). 3 Arber, A. CHAPTER XIV THE AERATING SYSTEM IN THE TISSUES OF HYDROPHYTES THE existence of a highly-developed system of inter- cellular spaces, is one of the most marked anatomical characters of water plants. It is generally assumed that this lacunar system serves for the storage of the oxygen evolved in assimilation, and its conveyance to the parts of the body that stand in especial need of it, more particularly the roots and rhizomes buried in the asphyxiating mud. The mesophyll of the lamina, the ground tissue of the petiole, and the cortex of the stem and root, are the regions in which the air spaces reach their greatest development. In the stem, the cortex, which is generally broad in propor- tion to the stele, is penetrated by lacunae, which may be so numerous as to render the whole organ extremely fragile in texture. Two features in the arrangement of the cortical cells, however, seem in some degree to obviate the dangers of this fragility. The air spaces are, in the main, confined to the middle cortex, while the outer cortex in many cases consists of elements which are more closely placed and thus form a firmer peripheral shell1; the septa, again, are radially arranged and thus are able to withstand pressures acting at right angles to the axis, which would otherwise be liable to crush the stem2. Support is also obtained by diaphragms3, occurring chiefly at the nodes, which divide the air spaces into sections; these diaphragms are not air-tight, but are more or less water-tight, so that they form a safeguard against the flooding of the entire aerating system in the case of accidental injury. Fig. 1 1 8, p. 1 84, represents part of a transverse section of a stem of Potamogeton natansy in which 1 Haberlandt, G. (1914). 2 Schenck, H. (1886). 3 Duval-Jouve, J. (1872), Blanc, M. le (19 12) and Snow, L. M. (1914). i84 AERATING SYSTEM [CH. the cortical lamellae are connected by a diaphragm (Z>) with small intercellular spaces (m) at the angles of the cells. Fig. 119, FIG. 118. Potamogeton natans, L. Part of T.S. of stem with diaphragm (D) penetrated by intercellular spaces (m). f.l.b. = vascular bundle. [Blanc M. le (1912).] FIG. 119. Hippuris vulgaris, L. Three stages in the development of the nodal diaphragms of the stem, seen in T.S. (all x 318). A, young stem, intercellular spaces small and walls scarcely thickened. C, old stem, 7 mm. in diameter; inter- cellular spaces so much enlarged that the cells are stellate, walls much thickened. B, same stem as C, but from a region 3-5 mm. across, which shows intermediate characters. [A. A.] Ay 5, C, shows the development of the nodal diaphragm-tissue in the case of Hippuris. The air spaces may be either formed by the separation of cells (schizogenous) or by their destruction (lysigenous). When xiv] LACUNAE IN PITH AND CORTEX 185 the air spaces are schizogenous, they may be arranged in the form of a single ring (e.g. Myriophyllum, Fig. 1 16, p. 179), or a number of rings may occur, giving a lace-like appearance to the stem, when seen in transverse section (e.g. Hippuris). The development of the air spaces in the cortex of Hippuris vu/garis1 is illustrated by Fig. 120 A and B. The Water Crowfoot forms a transition to those plants in which the air spaces are lysigenous, for, in the young stem, irregularly placed schizo- genous air spaces occur, especially in the pith (Fig. 1 1 3, p. 1 76), FIG. 1 20. Hippuris vulgaris, L, Parts of transverse sections through a younger stem (A) and an older stem (B) showing the origin of the cortical lacunae. [Barratt, K. (1916).] while, in the older stem, the whole of the central parenchyma becomes torn and destroyed, leaving a large axial lacuna. Pep Us Portu/a2 is an example of a plant whose air spaces are mainly lysigenous. In transverse sections of the internodes, four such spaces are visible, each containing the torn remains of cells. The aerating system of the roots of aquatics is to be found in the cortex. In some cases, e.g. Vallisneria^ the intercellular spaces may be small, but more frequently they are of con- spicuous size, and arranged with a regularity that gives a notable 1 Barratt, K. (1916). 2 Schenck, H. (1886). 1 86 AERATING SYSTEM [CH. symmetry of pattern to the transverse section. The process of development of the intercellular spaces has been followed by the present writer in the case of Stratiotes aloides'1 (Fig. 121). The whole inner region of the cortex in the root of this plant must be visualised as consisting of radially arranged plates, one cell wide, which in the early stages are so placed as to leave no spaces between. The cells composing the plates divide very rapidly, and a number of new cell-walls are formed, almost all in planes ..Lac. FIG. 121. Stratiotes aloides, L. Tangential section through middle cortex of a young root to show the origin of the lacunae (lac.), (x 318.) [Arber, A. (1914).] at right angles to the long axis of the root. The result is that the plates elongate in the direction of growth of the root, but, owing to the rapidity of their cell-divisions, the plates grow in length faster than the rest of the root, and are thus forced into un- dulations, since they become too long to retain their normal vertical position. The possibility of their taking up this sinuous form is due to the fact that the root enlarges in diameter and thus allows room for the separation of the plates. It will readily 1 Arber, A. (1914). xi v] SECONDARY AERENCHYMA 187 be seen that a series of plates, side by side, elongating indepen- dently, and at the same time prevented from stretching to their full length, will naturally become detached from one another at certain points, leaving spaces between. The result of these processes is that the middle cortex, as seen in transverse section, consists of radial plates of cells, like the spokes of a wheel, in contact or separated by lacunae, whereas in tangential section the plates are found to meet their neighbours at intervals, so as to form a network. In some plants, e.g. Myriophyllum and Callitriche ver#a\ the air spaces in the root cortex may be increased by the replace- ment of small schizogenous air spaces by large cavities of a partially lysigenous nature, due to the disruption of the septa. Remarkable as is the aerating system developed in the pri- mary tissues, that formed in the course of secondary growth is often even more conspicuous. This secondary aerating system, or aerenchyma, arises in some cases from a phellogen, in others from a typical cambium. We will first consider that which is produced by a phellogen, and may be regarded as a special modification of an ordinary periderm. It is well known that in land plants the impervious corky mantle, which so often covers the older parts, is interrupted at intervals by lenticels, or patches of powdery cork, in which the cells are slow in becoming suberised, and are separated by intercellular spaces, instead of being closely applied to one another as in normal periderm. These lenticels form a channel by which gaseous exchange takes place between the atmosphere and the interior of the plant. We have thus, in the lenticel tissue, an example of an aeren- chyma formed on a small scale by ordinary terrestrial plants, and, moreover, this aerenchyma has a tendency to become hypertrophied when the plant is submerged. The case has been described, for instance, of a Poplar branch which had been a long time under water, and in which masses of whitish tissue pro- truded from the surface in many places. On examination these protrusions proved to be due to a great development of the iSchenck, H. (1886). 1 88 AERATING SYSTEM [CH. aerenchyma of the lenticels1. Salix viminalis and Eupatorium cannabinum, again, have been shown to develop spongy tissue beneath the lenticels when grown in water or on marshy soil2. In the course of evolution, this tendency to hypertrophy of the lenticel tissue under the influence of water, may have formed the starting point for the development of the special air-con- taining phelloderm which is so marked a feature of a number of plants to which we must now refer. It was recorded more than forty years ago, by a Russian observer, that the stems and roots of Epilobium hirsutum^ Lycopus europaeus, and two species of Lythrum produced aerenchyma, when grown in water3. In Lythrum Salicaria the aerenchyma, which appears on the submerged parts when grown in shallow water, enlarges the stem to as much as four times its normal thickness2. It can be induced to occur in this and other cases (e.g. Lycopus europaeus) by merely keeping the cut branches in water for a few weeks1. The list of our native waterside plants, in which aerenchyma occurs under suitable conditions, includes Lysimachia, Lofus, Oenanthe, and Scutellaria, in addition to the genera already named4. Schenck2, to whom our knowledge of aerenchyma is largely due, showed that this tissue was particu- larly characteristic of Onagraceae, where it occurred in twelve species belonging to three genera; Leguminosae, where it was found in six species representing five genera; and Lythraceae, where it appeared in six species belonging to three genera. It was Schenck who proposed the useful term 'aerenchyma' for this non-suberised ventilating tissue produced by a phellogen. The cells are not dead and empty, as in normal cork, but are lined with a delicate protoplasmic pellicle and generally contain clear cell-sap; they are separated by extensive lacunae. That they are homologous with cork-cells is indicated by the fact that, in the roots of Jussiaea, the cork, formed when the plant grows on land, is replaced by aerenchyma when it grows in 1 Goebel, K. (1891-1893). 2 Schenck, H. (1889). 3 Lewakoffski, N. (I8731); on Epilobium see also Batten, L. (1918). 4Gluck, H. (1911). xiv] JUSSIAEA AND NEPTUNIA 189 water1. It has been suggested1 that the stimulus that causes the phellogen to develop aerenchyma in lieu of cork, is the lack of oxygen in the inner tissues. The present writer would prefer, however, to express the same idea somewhat differently, and to say that the presence of some minimum of oxygen is possibly a necessary condition for the process of suberisation, which is inhibited when the oxygen-content of the cell-sap falls below a certain point. Some remarkable cases of aerenchyma development are found in the tropical Onagraceous genus Jussiaea^\ in J. peruviana (Fig. 1 22, p. 1 90), the submerged parts of the shoots are clothed with this tissue, which is also developed on the normal roots which enter the mud (m.r.'), and in certain erect roots which seem to serve entirely for aeration (a.r.*). Fig. 1 22 B exhibits the origin of the stem aerenchyma (a) from a phellogen (pg). Special breathing roots also occur in the case of Jussiaea repens. They show, in transverse section, a tiny stele, surrounded by a volu- minous aerenchyma. That the modification of these roots is directly related to the aquatic environment, is indicated by the fact that Jussiaea grandiflora^ when cultivated for some years in the botanical garden at Marburg as a land plant, produced only normal adventitious roots, but when it was transferred to water it developed roots with aerenchyma3. The aerenchyma of certain members of the Leguminosae has been recognised for many years. Humboldt and Bonpland 4, for instance, more than a hundred years ago, recorded that in "Mi- mosa lacustris" (Neptunia oleracea^ Lour.), the Floating Sensitive Plant (Fig. 1 23, p. 19 1), the stems and branches were covered by "une substance spongieuse, blanchatre." They made the mis- take, however, of supposing that this tissue was a foreign body, and not an integral part of the plant. More recent observations5 1 Schenck,H.(i889). 2 Martins, C.( 1866). 3Goebel,K. (1891-1893). 4 Humboldt, A. de, and Bonpland, A. (1808). 5 Rosanoff, S. (i 87 1 ). This author uses the name " Desmanthus natans " for the plant now called Neptunia oleracea. 190 AERATING SYSTEM [CH. have made it clear that the spongy mass (f in Fig. 123) is an aerenchyma developed from a phellogen. That it also acts B FIG. 122. Jussiaea peruviana, L. A, habit drawing. The shoots are clothed with aerenchyma up to the water level (s.w.). m.r., mud roots; a.r., air roots. Aerenchyma occurs in both types of root. (Reduced.) B, Transverse section of submerged part of a stem to show aerenchyma (a) developed from phellogen (pg). The phloem (ph), normal cambium (c) and" xylem (xy] are also shown. [Adapted from Schenck, H. (1889).] as a float is indicated by Spruce's1 account of the plant as he saw it growing in South America. He describes the buoyant 1 Spruce, R. (1908). xiv] SESBANIA AND AESCHYNOMENE 191 "cottony felt" as serving to hold the delicate bipinnate leaves and the heads of pale yellow flowers above the surface of the water. In Sesbania^^ again, another Leguminous genus not at all closely related to Neptunia, a similar air-tissue occurs, arising from a cork-cambium in the inner cortex, just outside the endodermis. It is a curious fact that among the Leguminosae we not only meet with the case just described, in which an aerenchyma arises externally from a phellogen, but we also find instances in which a tissue of somewhat similar nature is produced internally from FIG. 123. Ncptuniaoleracea^our. Floating shoot. The two oldest internodes have lost their floating tissue, /, while the three youngest have not yet developed it. (Reduced.) [Adapted from Rosanoff, S. (1871).] a normal cambium, and is thus of the nature of secondary wood. In these cases, the air is contained within the xylem elements. Aeschynomene aspera^ Willd.2 is a Leguminous shrub, frequent in India on the margins of fresh waters, in which a pith-like tissue, white and homogeneous, occupies the greater part of the stem. This substance is, in fact, secondary xylem. It is so extremely light in weight that it is collected to make toys, floats for fishermen's nets, and 'pith' helmets. Another member of the same genus which grows in Venezuela, Ae. his- pidula, H. B. K.3, has remarkable swellings on the submerged 1 Scott, D. H. and Wager, H. (1888). * Moeller, J. (1879). 3 Ernst, A. (i8;22). 192 AERATING SYSTEM [CH. parts of its stem, said to be due to aerenchyma. A third Legu- minous plant, which has been described under the name of Herminiera elaphroxylon, G. and P.1 but which is perhaps better regarded as another member of the genus Aeschynomene^ also has aerenchyma2. The floating wood of this plant, which is known as the "Ambatsch," is employed on the Blue Nile to make rafts. The pieces used are as thick as a man's arm, and show under the bark a shining white woody mass, penetrated by numerous rays. The wood is exceedingly light; a segment of stem 2 1 feet long and about 4 inches in diameter, is described as weighing less than \\ ounces. It has been shown that the pits of the xylem are real perforations with no pit-closing mem- branes, so that there is free passage for gases3. The chief function served by the lacunar system of sub- merged stems seems to be aeration4, but there are also instances in which it plays a very important part in adding to the buoy- ancy of the plant. In Trapa natans^ for instance, the aquatic stem is formed exclusively of soft tissue, and would be unable, if it depended on its own stiffness, to rear itself to the surface of the water. It is entirely due to the increase of lacunae in the upper part of the stem, and the swelling of the petioles of the upper leaves, that the axis is enabled to raise the flowers into the air. In the deeper regions, the pith is a compact tissue, and there are only two circles of lacunae in the cortex, but in the upper part of the stem the pith is lacunate and the number of circles of air spaces increases to four or five5. The secondary lacunar tissues were always assumed by the earlier writers to serve for flotation alone ; in certain cases (e.g. some of the Leguminosae already mentioned) it is quite pos- sible that they were correct. Martins6, who long ago described and figured the air roots of Jussiaea, regarded them merely 1 Also called Aedemone mtrabilis^ Kotschy. 2Kotschy, T. (1858), Hallier, E. (1859), Jaensch, T. (1884!) and (i8842), Klebahn, H. (1891). See also Hope, C. W. (1902). 3 Goebel, K. (1891-1893). 4 Schenck, H. (1889). 5 Costantin, J. (1884). 6 Martins, C. (1866). xi v] NESAEA 193 as floating organs. For this particular case, this view can scarcely be maintained, since Goebel1 has shown that Jussiaea repens floats quite well, even if the roots be all removed. A good case has been made out, however, for regarding the aerenchyma of Nesaea verticillata 2, one of the Lythraceae, as a true floating tissue. Many of the wand-like stems of the plant, growing on the borders of ponds in America, are described as reaching a length of six to eight feet. In July and August they bend with their own weight until the stem apex touches the water, when it curves upwards again. In the region of contact between the stem and the water a swelling occurs, and roots also arise from b.w. FIG. 124. Nesaea verticillata, H. B. and K. Plant at beginning of August; s.w., surface of water; b.w., bottom of water;/./., floating tissue. [Adapted from Schrenk, J. (1889).] this region, anchoring the floating part of the stem to the ground (Fig. 124). The epidermis of the swollen region be- comes fissured, disclosing a snowy white, soft, elastic, spongy tissue, which arises from a pericyclic phellogen. Contraction of the roots draws the swollen part down into the water, and the spongy layer gradually extends over the submerged regions. In the autumn the long slender stems die, except those portions that have produced floating tissue around themselves, and have rooted in the mud. A new root-stock is thus developed, some- times at a considerable distance from the mother-plant. As 1 Goebel, K. (1891-1893). 2 Schrenk, J. (1889). A. W. P. 13 194 AERATING SYSTEM [CH. xiv evidence for the view that the aerenchyma in this plant is not respiratory in function, Schrenk, who described it, points out that in old stems the surface of this tissue is covered by a layer which is air-tight and suberised, and that a similar layer is also sometimes found separating it from the interior of the stem. He accounts for its occurrence in regions where it cannot serve for flotation, by supposing that the meristem spreads there automatically from the floating parts. To the present writer, however, the question whether the secondary air-containing tissues of water plants serve mainly for aeration or for flotation, seems to be a matter of minor import- ance. It appears to her that the evidence as a whole points rather to a fundamentally different interpretation — namely, that the formation of the secondary air-tissues is directly induced by environmental conditions, and that their serving any purpose is to be regarded as quite fortuitous. In the case of the -primary lacunar system, the position is somewhat different, and it seems difficult to escape the conclusion that we have here an example of the inheritance of acquired characters. There is some experimental evidence tending to show that this system was initiated as a direct response to the aquatic milieu; its elaboration may either be attributed to natural selection or to the inherited effects of use. There is no doubt that the habit of developing an elaborate aerating system has now become in many cases an inherited character, for though it can be modified and reduced by terrestrial conditions, it cannot be altogether eliminated. [ 195 J CHAPTER XV LAND FORMS OF WATER PLANTS, AND THE EFFECT OF WATER UPON LAND PLANTS THE majority of water plants, with the exception of those most highly specialised for aquatic life, are capable of giving rise to land forms. Those plants which, when mature, produce floating or air leaves, can obviously develop a land form with less change in their structure and mode of life than those which normally live entirely submerged. Limnanthemum nym- phoides^) for instance, has been found growing on damp ground with abbreviated internodes and petioles, and with reduced laminae. Land forms ofHydrocharis*, and many Nymphaeaceae3 and Alismaceae4 are known, either in nature or in cultivation. Successful terrestrial forms can also be produced by those Potamogetons which possess coriaceous, floating leaves, or have the power to develop such leaves on occasion. The land form of Potamogeton natans is shown in Fig. 125, p. 196. P. varians, a form allied to P. heterophyllus, Schreb., can exist for season after season without being under water at all, tiding over the winter by means of its bead-like tubers5. Even P. perfoliatus has also been recently stated to produce a land form6, though it is generally regarded as a typically submerged type, which is incapable of terrestrial life7. Myriophyllum, Callitriche* and the Batrachian Ranunculi (Fig. 126, p. 196) agree in producing land forms which are close-growing and tufted. When Myriophyllum spicatum1, for 1 Schenck, H. (1885). 2 Merj & (J8821). 3 Bachmann, H. (i 896), and Mer, £. (i 8821). See also p. 32, Ch. m. 4 See Chapter n and Gliick, H. (1905). 5 Fryer, A. (1887). « Uspenskij, E. E. (1913). 7 Fryer, A., Bennett, A., and Evans, A. H. (1898-1915). 8Lebel, E. (1863). 13—2 196 LAND FORMS OF WATER PLANTS [CH, FIG. 125. Potamogelon natans, L. or possibly P. polygonifolius, Pourr. Land form from a dried-up swamp, New Forest, September 2, 1911, after a very dry summer. Only the blades of the leaves, and sometimes not even the whole of these, were visible above ground. (Reduced.) [A. A.] 2.B. FIG. 126. Ranunculus aquatilis, L. lA, seedling which germinated in water, and which is shown in i B at a somewhat older stage. zA , seedling which germinated on land, and which is shown in 2.B at a somewhat older stage. (Nat. size.) [Aske- nasy, E. (1870).] xv] THE WATER VIOLET 197 instance, is left stranded, the water leaves are apt to dry up, but the ends of the shoots grow into a land form entirely different in habit from the water form. It develops as a minute turf, an inch high; the stems are frequently branched, the internodes are short instead of being elongated as in the water form, and many adventitious roots are produced from the nodes. The leaves are smaller than in the submerged form, and the segments are fewer, broader and thicker. A close connexion between submerged and aerial * forms * has in recent years been demonstrated in the case of Hottonia, the Water Violet. In this plant, which previous observers had Inflortftceue •-•'• FIG. 127. Hottonia palustris, L. Diagrammatic sketch of typical land and water forms. [Prankerd, T. L. (1911).] erroneously described as free-floating, it is now known1 that the oldest part of the rhizome is generally embedded in mud, and that from it arise vertical aerial branches, which may be- come detached by the dying off of the older part of the stem, thus giving rise to so-called 'land forms/ which are similar in anatomical structure to the submerged parts of the aquatic plant, rather than to the aerial inflorescence region (Fig. 127). The differences between the land and water leaves of Poly- gonum amphibium^ have already been mentioned, and are illus- trated in Figs. 99 and 100, p. 1 52. It is notable that in this case iPrankerdjT. L. (1911). 198 LAND AND WATER FORMS [CH. the plant reaches its optimum development as an aquatic, and flowers freely in water. As a land plant it rarely blossoms and, indeed, under xerophilous conditions, flowering seems to be entirely inhibited1. In the case of amphibious plants, which can produce land or water forms according to circumstances, the difference in external appearance is often very marked. Limosella aquatica, for instance, produces a land form with leaf-stalks half-an-inch to one inch long, while the water form may have petioles six inches long, terminating in tender trans- lucent blades2. Littorella lacustris is another striking example. The shallow water form, deep water form, and land form are shown in Fig. 128 A, B and C. Various land plants can grow and flower freely with their roots and the lower parts of their stems actually under water; Solanum Dulcamara (Bittersweet) is a species to which these condi- tions seem especially favourable. Such plants form a transition to those which frequent the margins of fresh waters, and are capable of responding to changes in the water level by producing, at need, actual aquatic forms. Gliick3, who has given great attention to this subject, has shown that, in nature, submerged forms, often with reduced vegetative organs, are produced not only by plants which normally inhabit damp or marshy situations, such as Ranunculus Flammula (Figs. 1 34 and 1 35> P'2O3)> Ca/thapa/ustris(Fig. 129), Cnicuspratensis* (Fig. 1 30 FIG. 128. Littorella lacustris, L. (=L. juncea, Berg.). A and B, water forms; C, land form. A is from water 30 to 40 cms. deep ; B is from water 100 cms. deep; C shows three male flowers one of which has lost its stamens. (Reduced.) [After Gliick, H. (1911), Wasser- und Sumpfgewachse, Bd. in, Fig. 34, p. 346.] 1 Massart, J. (1910). » Clack, H. (191 1). 2Schenck, H. (1885). 4 Gliick uses the name Cirsium anglicum> D.C. xv] WATER FORMS OF LAND PLANTS 199 A and 5) and Menyanthes trifoliata^ but also by typically terres- trial plants such as Achillea ptarmica, Trifolium resupinatum (Fig. 131 5) and Cuscuta alba (Fig. 131 A), Gluck1 has also produced experimentally a submerged form of Iris Pseudacorus. Seeds of terrestrial plants may sometimes germinate and reach a considerable development while entirely submerged. The FIG. 129. FIG. 130. FIG. 131, FIG. 129. Caltha palustris, L. The two leaves with long petioles belong to the sub- merged form : the middle leaf is a corresponding air leaf of the land plant. (Re- duced.) [After Gluck, H. (1911), Wasser- und Sumpfgewachse, Bd. in, Fig. 3, p. 65.] FIG. 130. Cirsium anglicum, D.C. (=Cnicus pratensis, Willd.). A, land form, B, water form. [After Gluck, H. (1911), Wasser- und Sumpfgewachse, Bd. in, Figs, i a and 6, p. 16.] FIG. 131. Cuscuta alba, J. and C. Presl, forma submersa. A, parasitic on water form of Echinodorus ranunculoides, (L.) Engelm. B, parasitic on the form of Tri- folium resupinatum, L. with floating leaves. (Reduced.) [After Gluck, H. (1911), Wasserund Sumpfgewachse, Bd. in, p. 114, Figs. 7 A and B.~\ present writer has noticed Horse Chestnuts sprouting freely in the mud at the bottom of a stream : one which was measured had a plumular axis more than i inch in length, and a primary root of 3! inches. In connexion with Gliick's record of a submerged form of 1 Gluck, H. (1911). 200 LAND PLANTS IN WATER [CH. Cnicus pratensis, it is interesting to note that a somewhat diffe- rent water form has been described in the case of C. arvensis1. The plant in question had suffered nine months' inundation in a fenland flood; when observed in November, at first sight " the leaf-rosette appeared normal ; . . . but on lifting it, it was found to be attached to the ground by about 2 or 3 feet of slender leafless stem of very soft and flexible consistency — exactly re- sembling the woodless stem of a true aquatic. During the flood Cnicus aruensis had evidently floated at the end of this aquatic stem, much in the manner of, say, a Potamogeton or Callitriche" The present writer has noticed Ranunculus repent* growing by the water-side and putting out long runners into the water; these runners bore leaves that were either submerged or rose approximately to the level of the surface. Hydrocotyle vu/garis2 is also not infrequently seen either more or less submerged or with a number of floating leaves (Fig. 132). A considerable amount of work has been done on the anatomical changes induced by growing terrestrial plants or amphibious plants in water instead of air. Among terrestrial plants, Vicia sativa, when grown in water, does not develop aquatic characters in its epidermis, but the xylem suffers marked diminution. This enfeeblement of the xylem is characteristic of various other land plants when grown in water, and, in the case of Ricinus and Lupinus, there is a similar reduction in the thickening of the bast fibres 3. Rubus fruticosuS) when grown in water, showed no change in the micro- scopic structure of its sub-aquatic leaves and stem, except that, in both organs, the chlorophyll was developed nearer the sur- face than in the normal condition in air, while the hairs on the stem tended to be unicellular instead of multicellular4; in the shoots of Salix, also, little anatomical change was induced by submergence5. 1 Compton, R. H. (1916). 2 The existence of these forms was noted by Gliick, H. (1911). On Hydrocotyle see West, G. (1910). 3 Costantin, J. (1884). 4 Lewakoffski, N. (18732). * Lewakoffski, N. (1877). xv] AMPHIBIOUS PLANTS IN WATER 201 In the case of amphibious plants, the comparison of air and water shoots gives results of greater interest. Costantin1 de- scribed the anatomy of a plant of Mentha aquatica growing on dry land, which happened to have the apex of one of its shoots plunged into water. The young part of the stem, which had thus grown in an aquatic milieu, when compared with the older part growing in air, was found to be glabrous and to have a greater diameter and larger air spaces. The same increase in the air FIG. 132. Hydrocotyle vulgaris, L. A branch sent out into water from a plant growing on the bank ; w, water level. The under surfaces of the five expanded leaves were examined for stomates, which were present on all. The petiole of the air leaf was more hairy than that of the succeeding leaves. July 14, 1910. (£ nat. size.) [A. A.] spaces and of the diameter of the stem, was observed in sub- merged shoots of Veronica Anagallis and Nasturtium amphibium. Costantin notes that, in general, when submerged plants are grown in deep water, the fibrous and tracheal elements diminish markedly. Cardamine fratensis is an example of an amphibious plant which seems to pass with remarkable ease from the water to the air condition. The present writer has found, on more than one 1 Costantin,J. (1884). 202 LAND AND WATER FORMS [CH. occasion, that an entirely submerged plant, when placed in soil under ordinary aerial conditions, rapidly developed into a typical land plant. Schenck1 has described the comparative anatomy of submerged and aerial plants of this species. The anatomy of the submerged stem showed several points of interest. The intercellular spaces and the diameter of the cortex were increased ; the vascular cylinder had approached nearer the centre of the stem; all mechanical elements were absent, and the xylem was reduced (Fig. 133, cf. A and 5). In the case of FIG. 133. Cardamine pratensis, L. A, T.S. stem of land form. B, T.S. submerged stem; rp = cortex, m = pith, mr = mechanical ring. C, T.S. leaf of land form. D, T.S submerged leaf. [Schenck, H. (1884).] the leaves, those that were submerged had developed no palisade tissue (Fig. 133, cf. Cand Z>). Such anatomical work as that briefly outlined above, leads to the general conclusion that when amphibious plants are grown in water they readily acquire the characters which we regard as typical of aquatic plants, but that, when terrestrial plants are grown under similar conditions, the changes which occur, though trending in the same direction, are very much less marked. There seem to be two possible, alternative explanations of this difference of behaviour. On the one hand it may be that 1 Schenck, H. (1884). xv] ORIGIN OF AQUATIC HABIT 203 amphibious plants were not originally gifted with any special aptitude for aquatic life, but that they have gradually acquired, and passed on to their descendants, the capacity for reacting in an advantageous way to the stimuli of an aquatic environment, and that we are thus dealing with a case of the inheritance of acquired characteristics. But the second alternative, which appears to the present writer to have most in its favour, is that, in general, those species which are capable of a suitable response to aquatic conditions have already been sifted out by nature, and now inhabit situations where such conditions, at least occasionally, arise; or, in other words, that the various species of flowering plants were all endowed, from the first moment of their appearance, with different constitutions which gave them varying degrees of capacity for the adoption of water life; and that their habitats have been determined by this capacity and not vice versa1. FIG. 134. Ranunculus Flammula, L. A , form with floating leaves. B, land form. (Reduced.) [After Gliick, H. (1911), Wasser- und Sumpfgewachse, Bd. in, Figs. 84 and 85, p. 494.] FIG. 1 35 . Ranunculus Flammula, L. Submerged form. The short up- right stem replaces the inflor- escence. (Reduced.) [ After Gliick, H. (1911), Wasser- und Sumpfge- wachse, Bd. in, Fig. 86, p. 496.] 1 See Footnote I, p. 162. CHAPTER XVI THE ROOTS OF WATER PLANTS THE roots of certain of the more specialised water plants, are extremely reduced or even in some cases entirely absent, e.g. Ceratophyllum, Aldrovandia and Utricularia. In other instances, such as Nymphaea, although the primary root is very short-lived, a considerable system of adventitious roots may be developed. As we shall show in Chapter xxi, among aquatics, absorption by the roots is by no means of such negli- gible importance as some writers have suggested; but at the same time, when plants rooted at the bottom of water are compared with those terrestrial herbaceous plants which they most closely resemble in size and habit, it becomes clear that, in the roots of the water plants, the function of anchorage has assumed a greater importance, while the function of absorption is less pre-eminent. A firm hold in the mud, and erectness of the flowering stem, are often a sine qua non for aquatics, and their roots help in various ways to bring this about. Some- times we merely get a richly ramifying root system, e.g. Ranun- culus aquatilis^. In other cases the type of arrangement of the adventitious roots is such as to hold the stem in position. This point is well illustrated in a description written more than seventy years ago2, of a certain amphibious plant, Oenanthe Phellandrium. "The flowering stem is remarkably fistulose, furnished under water with frequent joints, which become more distant upwards: it attains its greatest thickness two or three internodes from the base, where it is often an inch or more in diameter. From the joints proceed numerous whorled pecti- nated fibres [adventitious roots], of which the lower ones are as stout as the original fusiform root: these, descending in a conical manner to the bottom of the water, form a beautiful 1 Hochreutiner, G. (1896). 2 Coleman, W. H. (1844). CH. xvi] TENDRIL ROOTS 205 system of shrouds and stays to support the stem like a mast in an erect position, while the pressure on the soft mud is lessened by the buoyancy of the hollow internodes." There are other cases, again, in which anchorage depends on some modification of the adventitious roots. Brasenia Schreberi (peltatay, for instance, is fixed by its well-developed root-caps, which are of the nature of anchors, and prevent dislodgment of the buoyant plant, when it is swayed about by the agitation of the water surface. A still more remarkable method is the production of spirally twisted roots, which in some cases fully deserve the name of tendrils. Most of the known examples occur in the Potamogetonaceae, but they have also been recorded in the Hydrocharitaceae (Hydrilla)2, Fig. 136, and FIG. 136. Hydrilla verticillata, Presl. Tendril roots. [Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.).] Gentianaceae (Menyanthes)^, while the present writer has noticed them in Myriophyllum verticillatum (Haloragaceae). The first case among the Potamogetonaceae in which spirally twisted roots were observed, seems to have been Cymodocea antarctica^. At a later date the corkscrew roots of Zannichellia palustris were fully discussed by Hochreutiner5 (Fig. 137 A—F^ p. 206). He describes these roots as long, unbranched, and twining about other objects like tendrils — to use his own ex- iSchrenk,;. (1888). 2 Graebner, P., in Kirchner, O. von, Loew, E., and Schroter, C. (1908, etc.). 3 Irmisch, T. (1861). 4 Tepper, J. G. O. (1882). 5 Hochreutiner, G. (1896). 206 ROOTS OF WATER PLANTS [CH. pression, " elles grimpent en bas." He adds that Potamogeton densus (Fig. 137 G and //) shows the same peculiarity. A more recent writer1 has recorded that, when the turions of Pota- mogeton obtusifolius germinate, they produce spirally coiled roots, which apparently serve to anchor the plantlets in the mud. Twining roots are not confined to water plants; a case is recorded by Darwin2, on the authority of Fritz Miiller, in FIG. 137. Twining roots of Zannichellia palustris, L. (A—F) and of Potamogeton densus, L. (G, H). [Hochreutiner, G (1896).] which the aerial roots of an epiphytic Philodendron in the forests of S. Brazil, twined spirally downwards round the trunks of gigantic trees. That root tendrils merely represent a further development of the general tendency to nutation common to stems and roots, is indicated by C. and F. Darwin's3 record 1 Graebner, P., in Kirchner, O. von, Loew, E., and Schroter, C. (1908, etc.). 2 Darwin, C. (1891). 3 Darwin, C. and F. (1880). xvi] EQUILIBRIUM AND ASSIMILATION 207 of a slight and tentative circumnutation in the seedling roots of several ordinary terrestrial plants. When the radicles of Phaseolus, Vicia and Quercus "were compelled to grow and slide down highly inclined surfaces of smoked glass, they left distinctly serpentine tracks." Hildebrand 1 has described a differentiation between absorb- ing and anchoring roots in the case of Heter anther a zosteraefolia. He states that from each leaf-base two roots arise, one of which remains short and branches freely, while the other grows rapidly in length and serves for anchorage. Plants cultivated in England do not, however, so far as the present writer has been able to observe, show this distinction ; it would be interesting to know whether other botanists, who have seen this species growing in Brazil, can confirm Hildebrand's description. In the case of Phragmites communis 2, there is a similar differentiation be- tween long, thick, unbranched mud-roots, and thin water-roots, branched to the third degree. The roots of free-floating plants obviously do not serve for anchorage, but they seem sometimes to perform a corre- sponding role in preserving equilibrium; this is particularly obvious in the cases of Lemna and Stratiotes. Aquatic roots often exercise another function, which is more remote from those generally assumed in the case of terrestrial plants — namely, that of assimilation ; their colour is sometimes quite conspicuously green. In the Water Chestnut, Trapa natans*, the later roots, developed adventitiously below the leaf-bases, are free-floating and branched. These feathery structures have been supposed by some authors to be of foliar nature ; this is erroneous, although physiologically they correspond to the divided leaves of Myriophyllum 4. It is an indication of the extra- ordinarily acute mind of Theophrastus, the Father of Botany (born B.C. 370), that he avoided the morphological pitfall which has been fatal to so many subsequent writers, for in describing Trapa he says, "quite peculiar to this plant is the hair-like 1 Hildebrand, F. (1885). 2 Pallis, M. (1916). 3 Barneoud, F. M. (1848). 4 Goebel, K. (1891-1893). 2o8 ROOTS OF WATER PLANTS [CH. character of the growths which spring from the stalk ; for these are neither leaves nor stalk1." We have already alluded to the thal- loid roots of the Podostemaceae, which also serve for assimilation . Like the stems of aquatics, the roots show certain anatomical divergences from those of land plants2. Root hairs are occasion- ally absent, e.g. Lemna trisulca. The roots of Ekdea bear no absorbent hairs so long as they are immersed in water, but they develop them freely on entering the soil3. In other hydrophytes, e.g. Hydrocharis, the root hairs are unusually long. It is rather curious that in the roots of water plants the piliferous layer, and the layer immediately below it, are often cuticularised. The aerating system, which occurs in the primary cortex, or as a secondary formation, has been dealt with in Chapter xiv. As in the case of submerged stems, the vascular system of the roots tends to be very much reduced. The simplest root among Dicotyledonous water plants is that of Callitriche stagnalis (Fig. 1 3 8), which has two protoxylems — each consisting of a single tracheid — separated by a single median metaxylem element. This simple xylem group is flanked on either side by a single sieve-tube with companion-cells. In certain Monocotyledons, a still more extreme degree of simplification is reached. Vallis- neria spiralis (Fig. 139), for instance, has merely a central channel, corresponding to the central vessel of other forms, surrounded by a ring of cells, three of which are apparently sieve-tubes, each accompanied by a companion-cell. Naias, again, has a root of a very simple type, in which the phloem is more conspicuously developed than the xylem4 (Fig. 140). The reduction series in the roots of the Potamogetons is illustrated in Fig. 41, p. 65. Plasticity is certainly a marked feature of the roots of water plants, for though they have to some extent given up the work of absorption, they have assumed and developed various other functions to which their terrestrial ancestors must have been comparative strangers. 1 Theophrastus (Hort) (1916). 2 Schenck, H. (1886). 3 Snell, K. (1908). 4 Sauvageau, C. XVI ] ROOT ANATOMY 209 eni FIG. 138. Callitnche stagnatis, Scop. T.S. central cylinder of ad- ventitious root of water form ( x 470) ; s, sieve tube. ' [Schenck, H. (1886).] FIG. 139. Vallisneria spiralis, L. A, T.S. adventitious root (x 240). B, T.S. central cylinder (x 470); end, endodermis, 5, sieve tubes of which three are present. The central vessel is unthickened. [Schenck, H. (1886).] FIG. 140. T.S. central region of roots of Naias, sieve tubes shaded. A, Naias major, All., two central vessels. B and C, N. minor, All., one central vessel in B and two in C. [Sauvagean, C. (iSSg1).] A. w. P. CHAPTER XVII THE VEGETATIVE REPRODUCTION AND WINTERING OF WATER PLANTS THE conditions under which hydrophytes live — unlimited water supply, abundant carbon-dioxide and protection from sudden temperature changes — are favourable to acti- vity of growth1, and the luxuriance which this vegetation often attains is a matter of common note; indeed it sometimes becomes such a hindrance to navigation as to compel the atten- tion, not only of botanists, but also of those who normally take no interest in plants. Even in the rivers of countries with a temperate climate, such as our own, aquatics are liable to multiply at a rate which renders them a great embarrassment in boating. A letter, for instance, which appeared in the Morning Post of July 1 6, 1914, refers to a locality in the Thames above Whitchurch Weir, where the weeds were "20 ft to 30 ft long and close under the surface of the stream from one bank to another." The sluices of mills are liable to be choked, too, in the autumn, by the countless detached fragments of Pofamo- geton. But the classic example in England of the extremely rapid growth and multiplication of a water plant, is the behaviour of Elodea canadensis*, the American Waterweed, in the first decade or so after it made its debut in this country. Exactly when and how it was introduced from America remains a mystery. Its first appearance in Great Britain is said to have been in Ireland in 1836, while its first recorded occurrence in England was in Berwickshire in i8423. It travelled south, and by 1851 was so . (1885). 2 Marshall, W. (1852) and (1857), Caspary, R. (18582), and Siddall, J. D. (1885). For the continental history of the plant see Bolle, C. (1865) and (1867). 3 Johnston, G. (1853). CH.XVII] ELODEA IN BRITAIN 211 luxuriant at Burton-on-Trent — where it had been recorded in I8491 — tnat it bid fair to block up one of the two streams into which the Trent there divides. Unfortunately the Curator of the Cambridge Botanic Garden, who had received the plant from Professor Babington in 1 847, introduced it into a tribu- tary of the Cam in 1 848. By 1 852 it had spread into the river, and so completely choked it as to raise the water level several inches, and to prevent fishing, swimming and rowing, and greatly to hinder the towing of barges. At this date it first invaded the fen district, and in a few years so choked the dykes as seriously to impede drainage. The difficulties caused by the presence of excessive quantities of the plant were so acute that an adviser was sent down by the Government to consider the best method of dealing with the pest. No successful plan for coping with it was discovered, but in a few years the luxuriance of the Elodea diminished without any apparent cause. Siddall2, to whom we owe the most exhaustive treatment of the subject, concludes that, "The experience of those who have had most to do with it seems to indicate that if left alone, its habit is, upon first introduction into a new locality, to spread with alarm- ing rapidity; so much so as literally to choke other water plants out of existence. But this active phase reaches a maximum in from five to seven years, and then gradually declines, until at last the Anacharis [Elodea] ceases to be a pest, and becomes an ordinary denizen of the pond, river, or canal, as the case maybe." As has been already stated in Chapter iv, Elodea canadensis never reproduces itself sexually in this country, and the history of the plant suggests that possibly the whole Elodea population of England may be regarded, in one sense, as a single individual, with an enormous vegetative output, mechanically sub-divided into vast numbers of apparently distinct plants; in other words, it is not improbable that it may represent the soma developed from a single fertilised ovum. It would thus be a " major plant unit," whose soma consists of a vast number of minor indivi- duals. Pallis3, in a most suggestive study of the problem of 1 Caspary, R. (18582). 2 Siddall, J. D. (1885). 3 Pallis, M. (1916). 14—2 212 VEGETATIVE REPRODUCTION [CH. individuality in the case of Phragmites communis, has brought forward evidence which strongly suggests that the " major plant unit/* i.e. the total vegetative output which one fertilised egg is capable of initiating, is to be regarded as a constant for each species, its mass being the measure of specific vital energy. She has shown that, in the case of the reed swamps of the Danube, there are distinct indications of a definite life-cycle of vegetative growth, terminating in senescence and death, whose arrival is not fortuitous or due to external conditions, but is a necessity inherent in the very nature of the species itself. Such a thesis is obviously very difficult to substantiate, but the history of Elodea, outlined above, certainly seems to the present writer to lend itself more readily to some such interpretation, than to the accepted explanation, which sees in the aggressive phase of this introduced plant, merely the direct stimulating effect of change of environment. Elodea has passed through a period of great luxuriance, followed by a gradual diminution in vigour, occur- ring more or less contemporaneously in all the localities which have been colonised by its rapid vegetative multiplication. By 1883 its period of maximum abundance was apparently over. In 1 909 an enquiry1 was set on foot to determine the condition of the species at that date, i.e. sixty-seven years from its first recorded appearance in England. This enquiry resulted in reports from many localities indicating that Elodea had sunk every- where into the condition of a mere denizen, displaying no greater luxuriance than the other water plants with which it was associa- ted. Siddall, in this year, wrote that he had some difficulty in finding a specimen of Elodea in a locality where in 1873 all other vegetation was choked with it. He also made the extremely interesting statement that the circulation of the protoplasm was very feeble in 1909 as compared with its condition in 1873 — a statement which the present writer feels must be accepted with some reserve, for it is a point on which a really critical com- parison would be attended with obvious difficulties. The general history of Elodea seems at least to point towards 1 Walker, A. O. (1912). xvn] WATER HYACINTH & RIVER LETTUCE 213 the conclusion that the " individual," which was introduced into this country, has run its course, through an opulent maturity, to a point approaching senility, which may ultimately lead to complete extinction. Water plants certainly appear to offer a favourable field for the study of the "major individual," since, in this biological group, reproduction by sexual means is often deferred for long periods. In warmer climates the rapidity of growth of water plants is even more remarkable than in temperate regions. The way in which Eichhornia speciosa, Kunth1, the Water Hyacinth, may sometimes choke a wide river, forms a really startling example of excessive quickness of growth and multiplication. About the year 1890, this plant was accidentally introduced into the St John's River in Florida, which, being a sluggish stream, was particularly well-suited to serve as its home. After seven years, two hundred miles of the river bank had become fringed with a zone of Eichhornia from twenty-five to two hundred feet in width. In the summer of 1896, a strong north wind drove the plants up stream from Lake George, forming a solid mass entirely covering the river for nearly twenty-five miles. The growth was so dense that small boats with screw propellers could not get through the mass. Formerly, when the stream was clear, logs used to be rafted down the river, and it is esti- mated that, at the time when the Water Hyacinth was at its maximum, the lumber industry of the region suffered an approximate annual loss of $55,000 from the difficulty of rafting. In Africa, the River Lettuce, Pistia Stratiotes^ plays a similar part to the Water Hyacinth of America in hindering naviga- tion. Miss Mary Kingsley2 gives a characteristically racy description of its behaviour on the Ogowe and the neighbouring rivers in the French Congo. " It is," she writes, " very like a nicely grown cabbage lettuce, and it is very charming when you look down a creek full of it, for the beautiful tender green makes a perfect picture against the dark forest that rises from 1 Webber, H. J. (1897). » Kingsley, M. H. (1897). 2i4 VEGETATIVE REPRODUCTION [CH. the banks of the creek. If you are in a canoe, it gives you little apprehension to know you have got to go through it, but if you are in a small steam launch, every atom of pleasure in its beauty goes, the moment you lay eye on the thing. You dash into it as fast as you can go, with a sort of geyser of lettuces flying up from the screw; but not for long, for this interesting vegetable grows after the manner of couch-grass. I used to watch its method of getting on in life. Take a typical instance : a bed of river-lettuces growing in a creek become bold, and grow out into the current, which tears the outside pioneer lettuce off from the mat. Down river that young thing goes, looking as innocent as a turtle-dove. If you pick it up as it comes by your canoe and look underneath, you see it has just got a stump. Roots? Oh dear no! What does a sweet green rose like that want roots for? It only wants to float about on the river and be happy; so you put the precious humbug back, and it drifts away with a smile and gets up some suitable quiet inlet and then sends out roots1 galore longitudinally, and at every joint on them buds up another lettuce; and if you go up its creek eighteen months or so after, with a little launch, it goes and winds those roots round your propeller2." The luxuriance of hydrophytes as compared with other herba- ceous plants can be demonstrated not only by examples of their multiplication on a large scale, but also when the dimensions of individuals are considered. A striking instance is afforded by Caspary's3 measurements of the leaves of a plant of Victoria regia cultivated in a hot-house; the maximum growth of the lamina recorded in 24 hours was as much as 30-8 cms. in length and 36*7 cms. in breadth. Even in our climate the growth of aquatics must be rapid, to produce the length of stem some- times observed; in the case of Ranunculus fluitam^ shoots twenty or more feet in length have been recorded4, while floating 1 Botanically these " roots " are of course lateral stems. 2 For other cases of plant accumulations which are on a sufficient scale to form serious obstructions, see Hope, C. W. (1902). 3 Caspary, R. (18562). 4 Schenck, H. (1885). xvii] LUXURIANCE AND PERENNIATION 215 branches of Utricularia vu/garis may be six feet long1. The shoot system, as a whole, sometimes attains a remarkable development. The present writer examined, for instance, a plant of Polygonum amphibium growing at Roslyn Pits, Ely, on June 30, 1913, which showed at the surface of the water only one flowering branch with seven foliage leaves. The plant was pulled up with a boat-hook and inevitably somewhat mutilated in the process, but, notwithstanding the breakages, the various axes forming the shoot system were found to measure altogether approximately forty-two feet. Besides the two visible leafy shoots, eight of the branches terminated in leaf buds, which looked as though they would probably have reached the surface in the course of that season. The longest internode in the horizontal part of the stem measured as much as sixteen inches. The great development often reached by individual water plants is no doubt an expression of the same tendency as that which leads them so generally to perenniation. Annuals are quite rare among hydrophytes ; only a few examples are known, such as Naias minor, Naias flexilis and certain species of Elatine*. There is of course no dry season to be spanned, and many aquatics can continue their vegetation all the year round, in some cases paying little regard to the passage from summer to winter. Zannichellia palustris, for instance, may be found in flower in November, while Aponogeton distachyus^ cultivated out- of-doors in England, flowers sometimes in December and Janu- ary. The strength of the tendency to perenniation may be illus- trated by the fact that the following plants have at different times passed successfully through one or more winters in so unsympathetic a location as a rain-water tub in the present writer's garden — Hydrocharis Morsus-ranaey Stratiotes aloides^ Spirodela polyrrhiza, Lemna trisulca, Myriophyllum sp., Qenan- the Phellandrium v&r.fluviatiliS) CeratophyllumyHippuris,3.nd two species of Potamogeton. That the perennial habit is directly related to the environment, seems to be indicated by the fact that, in the case of Callitriche2'^ the land forms are annual while 1 Burrell, W. H. and Clarke, W. G. (1911). 2 Schenck, H. (1885). 216 VEGETATIVE REPRODUCTION [CH. the water forms are perennial. In the aquatic Callitriches, rooted internodes bearing lateral buds may remain in the mud and tide over the winter1. Montia fontana^ also, is biennial in places where the water is liable to dry up, but, in springs and permanent streams, it grows strongly and becomes perennial2. Those water plants which have not adopted special methods of perenniation, generally retain their leaves through the winter, e.g. Peplis Portula^ Ceratophyllum, Hottonia^ and the submerged species of Callitriche. In the case of such plants, any detached shoot will generally grow into a new individual with extreme readiness. In Hottonia the branches forming a whorl below the inflorescence become separated from the axis and give rise to new plants in the spring3. The present writer has noticed that, in the case of Peplis Portula and Ceratophyllum, the submerged stems are very brittle, and, in the early autumn, quantities of detached floating shoots may be observed. The behaviour of Callitriche* is particularly striking, for in this case new plants can be formed from a node with only a very small piece of inter- node attached. Lawia xeylanica^ Tul.5, one of the Podostema- ceae of Ceylon, can recommence its growth from any portion of the thallus, however small, if it be submerged under favour- able conditions, and other members of the family have a similar power. A very notable capacity for vegetative multiplication is exhibited by some Cruciferae. In the case of the North American Nasturtium lacustre^^ the pinnately dissected, sub- merged leaves become detached about the middle of August and float at the surface of the water ; an adventitious bud arises at the base of each leaf and develops into a new plant. The same production of buds from foliar tissue has long been known in Cardamine pratensis^ the Lady's Smock, where it can easily be observed at various times of year (Fig. 141). On May 21, 1919, the present writer saw countless plantlets growing from detached leaflets in a dyke in the fens near Lakenheath Lode. 1 VaucherJ.P.(i84i)andLebel,E.(i863). 2 Royer,C. (1881-1883). 3 Prankerd, T. L. (1911). 4 Hegelmaier, F. (1864). 5 Willis, J. C. (1902). 6 Foerste, A. F. (1889). xvn] AQUATIC GEOPHYTES 217 The caddice worms, which also abounded in this dyke, seemed to have a great fancy for using the leaflets in constructing their cases, and, in consequence, their armour was often elegantly crested with tiny adventitious plants of Lady's Smock. In addition to those aquatics which retain their leaves through the winter, there are others which perenniate in or upon the substratum by means of rhizomes or tubers. Plants which adopt this habit, may be described as aquatic geophytes. Limnanthemum (Figs. 22 and 23, p. 41), Castalia (Fig. 1 1, p. 26) and Nymphaea (Figs. 10, p. 25 and 1 2, p. 27) are rhizomatous. In some cases — e.g. Sagittaria^ certain Potamogetons and Nymphaeaceae — special tubers are formed which outlast the winter These afford a means of vegetative multipli- cation, since an individual plant may submerged type growing among Utricularia in shallow in some cases give rise to numerous pooi, Commissioners' Pits, tubers; a single plant of Sarittaria Upware, June 27, 1914- in . .* ., c . each case the terminal leaflet Saglttljoha^ for instance, may produce bears an adventitious plant- as many as ten tuber-bearing stolons. let tVhe bfe: C',s!ngle» ' . much-decayed pinnule bear- Another method of vegetative reproduc- ing a well-developed piantiet; tion is illustrated by Littorella lacustris\ ^ ^af si^eTTl AJ which puts out runners in the spring, bearing at their apices young plants not easily distinguishable from seedlings; these plantlets become independent by the late summer or autumn. A plant of this species with a runner is shown in Fig. 142, p. 218. The most distinctive mode of wintering and of vegetative reproduction found among hydrophytes, is, however, by means of winter-buds or turions ; these specialised shoots, which are 1 Buchenau, F. (1859). 218 VEGETATIVE REPRODUCTION [CH, FIG. 142. Littorella lacustris, L. Plant drawn in February 1912. The collapsed region at base of stem probably represents the part formed in 1910; r = runner arising in a leaf axil. (Reduced.) [A. A.] xvn] TURIONS 219 stored with food material, and protected externally in some way, become detached from the parent and pass the winter either floating, or resting at the bottom of the water. In the spring they expand, produce adventitious roots, and rapidly develop into full-fledged individuals. Certain plants, also, which do not actually produce independent turions of a specialised type, show transitions towards such a development. If shoots of the Greater Spearwort, Ranunculus Lingua^ •, are left in water over the winter, they rise to the surface in the spring in a partly decomposed state, but bearing healthy buds in the axils of their leaves ; these become detached to give rise to new plants. Elodea canademis (Fig. 34, p. 55) and Stratiotes aloides (Fig. 32, p. 53), again, produce primitive reproductive buds, which do not imme- diately become free, but germinate while attached to the parent plant2. The apices of the shoots of Ceratophyllum are clothed in autumn with leaves which are more crowded and of a deeper green than those of the rest of the shoot, but, as we have already pointed out3, they can scarcely be said to form definite winter- buds. Certain turions showing a high degree of specialisation have already been mentioned, e.g. those of Hydrocharis (pp. 47-49), Potamogeton (pp. 66-69), tne Lemnaceae (pp. 75-77), Aldro- vandia (p. 1 10), and Utricularia (pp. 101-103). The difference between the normal foliage leaf and the protective outer leaf of the turion, in the case of U. intermedia^ is shown in Fig. 143, p. 220. Among the British plants to whose wintering habits we have not yet referred,Myriopky//um verticillatum^ affords a striking example of turion formation. In August the plant may be found simul- taneously producing flowers and winter-buds (Fig. 144, p. 22 1). Early in October the ragged shoots may be seen floating, with here and there a compact turion (T), distinguished against the faded brownness of the parent plant by its vivid, dark-green hue. These winter-buds become detached during the cold season, and 1 Belhomme, (1862). 2 Gluck, H. (1906). 3 See p. 87. 4 The winter-buds of Myriophyllum were noted by Vaucher, J. P. 220 WINTERING HABITS [CH. in the spring they expand into graceful shoots (Fig. 145, p. 222). The germination normally occurs in March or April, but it can be induced at any time if the temperature is favourable ; if brought indoors and kept warm, the turions will develop into new plants in October, November, December or January1. Cold is ini- mical to the winter-buds, and, if frozen for a few days, many of them are killed. The turions of different aquatics vary very widely in their capacity to withstand freezing1. Those of Utricu- laria vulgaris are uninjured by inclusion in ice for as long as twelve days, while Hydrocharis Morsus-ranae, according to Gltick's experiments, is still more sensitive than Myriophyllum, FIG. 143. Utricularia intermedia, Hayne. A, winter-bud leaf (enlarged). B, summer leaf (less enlarged). [Goebel, K. (1891-1893).] for, after three to ten days in ice, nearly all the turions were killed. However, according to Guppy2, they are able to with- stand inclusion in ice for a period of some weeks; the discre- pancy between these results requires some explanation, which may perhaps lie in the particular conditions of the experiments. The turions of many hydrophytes are saved from the risk of becoming frozen by their habit of wintering at the bottom of fairly deep water. For many years botanists were inclined to interpret the development of * winter-buds ' on the simplest teleological Glttck, H. (1906). 2 Guppy, H. B. (i 893). XVII WATER MILFOIL 221 FIG. 144. Myriophyllum verticillatum, L. August 15, 1911. A, the inflorescence shows in succession female, hermaphrodite and female flowers. Three turions, T, occur on the lower part of the axis. (Reduced.) For the further development of one of these turions see Fig. 145, p. 222. B shows an hermaphrodite flower and its subtending leaf. (Enlarged.) [A. A.] 222 WINTERING HABITS [CH. lines. These turions were regarded as a definite adaptation devised by the plant to tide over the cold season, and to ensure vegetative propagation. But this position has been undermined by experimental work originating with Goebel's1 discovery that turion formation in Myriophyllum verticillatum is definitely the result of unfavourable conditions. This observer, for ex- ample, placed some of the buds in a glass vessel with water but without earth, where they grew into richly rooted plants, more than 30 cms. long. By April i, these plants had all formed new turions terminating the main and lateral shoots, while in the locality from which the original winter-buds had been collected, their contemporaries remained still ungerminated ! FIG. 145. Myriophyllum verticillatum, L. One of the turions shown in Fig. 144, p. 221, which had begun to germinate after the winter's rest and was found at the bottom of the water in this condition on March 16, 1912 ; b, base. (Nat. size.) [A. A.] Gliick2, who has carried Goebel's work on Myriophyllum fur- ther, has shown that if the plant is grown in a vessel of water, over-crowded with other aquatics so that there is much com- petition for food, 'winter' bud formation may occur even in the spring. He also planted turions of M. verticillatum in soil, and cultivated them for an entire summer as land plants. Numerous green shoots were formed, but, by the beginning of August, each individual plant had also produced four to ten pale green turions (Fig. 146 X), most of which were under the soil. This early development of turions is attributed by Gliick to the lack of water from which the plants suffered. On the other 1 Goebel, K. (1891-1893). 2 Gluck, H. (1906). XVI l] TURIONS 223 hand, luxuriant specimens growing in water in warm situations may vegetate throughout the winter without forming turions. It is most likely that, in normal life, it is the lowering of the temperature in the autumn which induces the formation of winter-buds. That it is unfavourable conditions which bring about the development of turions, seems to be true not only of the Water Milfoil but of aquatics in general. Some remarkable experiments on the effect of starvation upon Utricularia have been quoted on pp. 102—103. Similar results have been obtained in the case of Sagittaria sagittifolia^ in which, however, the vege- tative multiplication is effected by tubers and not by turions. Tuber formation in the Arrowhead normally occurs when the plant has exhausted itself by the forma- tion of inflorescences, and when cooler weather sets in. The land form, like that of Myriophyllum, produces tubers several weeks earlier than the form growing under the optimum aquatic conditions. Gliick1, one autumn, planted a tuber of the Arrow- head in a pot of earth and left it there, almost without water, until towards the end of the following July. The plant, which had failed to appear above the soil, was H. (1906), Wasser- und then examined, and it was found that the tuber had put out a few wretched-looking little ribbon-leaves, which had not possessed strength to pene- trate the earth. It had also formed four tiny stolons, 1-5 to 2 cms. long, each terminating in a small tuber, 8 to 10 mm. in length. This tuber formation had apparently occurred as a 1 Gliick, H. (1905). form with five subter- ranean turions, two of which are marked K. still attached to the base ^ timber *5 adventitious roots. The two lowest turions have grown out of the axis Sump& 226 VEGETATIVE REPRODUCTION [CH. xvn There is good reason to suppose that, as Schenck1 long ago suggested, the vegetative reproduction of water plants merely illustrates the general rule that vegetation and fructification stand in inverse ratio to one another. Orchards bear better when the trees are pruned, while in wet years when leafage is over-luxuriant, fruit formation diminishes. And thus the excessive vegetative activity of water plants acts, in all probabi- lity, as a deterrent to sexual reproduction. 1 Schenck, H. (1885). CHAPTER XVIII THE FLOWERS OF WATER PLANTS AND THEIR RELATION TO THE ENVIRONMENT THE most notable characteristic of the flowers of the majority of aquatic Angiosperms is that they make sin- gularly little concession to the aquatic medium, but display the utmost conservatism in form and structure. The plants which have, in the course of evolution, adopted water life, have, as we have already shown, profoundly modified their vegetative organs in connexion with their new environment, but their methods of sexual reproduction in general depart little from those which had already become stereotyped in their terrestrial ancestors. This sharp distinction, between the degree of modi- fication of the vegetative and reproductive parts, is particularly well shown in the case of so highly specialised a water plant as Utricularia vulgaris. Here the vegetative body is entirely submerged, but the aerial inflorescence axis and the flowers, which are adapted to entomophilous pollination, in no way differ from those of a terrestrial plant. The extreme divergence in mode of life, and even in internal structure, between the aerial reproductive region and the submerged vegetative region in this species, led an anatomist to speak of the plant as con- sisting of "an aquatic being, vegetating horizontally without roots," and "a vertical aerial being, producing flowers at its apex, and implanted in the first, which serves it as soil, or rather as roots1." Those hydrophytes which still retain a type of flower adapted for aerial life, are under the absolute necessity of raising their inflorescence axis well above the water level, if cross-pollination is to be secured. This is sometimes very incompletely achieved, iTieghem, P. van (1868). 15—2 228 FLOWERS OF AQUATICS [CH. and even within the same genus we find differing degrees of success in the avoidance of submergence of the flower. Ranun- culus fluitanS) for instance, which does not hold its peduncles well erect and grows in rapidly flowing water, very often suffers from the inundation of its flowers, and, in consequence, fails to set seed1. Sometimes the attempt to rise above the water surface seems to have been entirely given up. Ranunculus trichophyllus is described as growing in the River Inn in enor- mous masses, and frequently blooming under water, opening its flowers at a depth of i to i J feet, but whether it can set seed under these conditions does not seem to have been ob- served2. Those Batrachian Ranunculi which flower successfully in rapidly flowing water, prove to be species such as R. carinatus^ Schur. (R. confusus. Gen. et Godr.) which produce long flowering stalks rising erect above the water, and not readily submerged by slight changes in level1. In the case of the heterophyllous Water Buttercups, the leaves associated with the flower are often floating and relatively undivided; this must be an assistance in maintaining the equilibrium of the pedicel3. In Heter anther a zosteraefolia, also, the leaf next the inflorescence is described as always being of the floating type4. The association of floating leaf and flowers in Limnanthemum nymphoides, which is so close that the inflorescence appears at first sight to spring from the petiole, must also play a part in holding the flowers above water. If any locality in which Limnanthemum grows freely be visited in August, the way in which the fringed, yellow flowers are held clear above the water will be found to be one of their most striking characters. The early development and whorled arrangement of the branches springing from the base of the inflorescence axis in Hottonia palustris*, the Water Violet, serve to support it on all sides, and to keep it vertical, while the numerous adventitious roots arising from the base of the erect shoot probably have a 1 Freyn, J. (1890). 2 Overton, E. (1899). 3 Askenasy, E. (1870). 4 Hildebrand, F. (1885). 5 Schenck, rf. (1885) and Prankerd, T. L. (1911). xvm] INFLORESCENCE-FLOATS 229 similar effect (Fig. 127, p. 197). The part played by the roots in holding the stem of Oenanthe Phellandrium in an upright position has already been mentioned1, as well as the specialised branches which in some Bladderworts keep the inflorescence erect2. Fig. 150 shows the whorl of six branches surrounding FIG. 150. Utricularia inflata, Walt. Part of swimming water shoot, with an inflorescence axis bearing six floating organs. [Goebel, K. (1891-1893).] the flowering axis in Utricularia inflata. Spruce3, in his account of his travels in the Amazon region, mentions, as a general obser- vation, that those hydrophytes which rear themselves erect and thus raise the flowering part of their stem well out of the water, prove on examination to have the sub-aquatic leaves grouped 1 See p. 204. 2 See p. 99. 3 Spruce, R. (1908). 230 FLOWERS OF AQUATICS [CH. in whorls, even when their terrestrial relatives have a different arrangement. He states that Jussiaea amaxonica has the narrow submersed leaves so closely whorled as to resemble the Mare's- tail of our ponds, while the emersed leaves are solitary. Those water plants whose inflorescences rise into the air, depend for cross-pollination upon insects or the wind. Those which are entomophilous differ little from land plants in their methods of attraction, except that, speaking very generally, a blue colour perhaps occurs more rarely than in terrestrial plants, while white or yellow are common1. The frequency of white flowers among aquatics was noted long ago by Nehemiah Grew, who, in his little book, An Idea of a Phytological History Pro- pounded , published in 1673, wrote, "to Water-plants more usually a White Flower/* The rarity of blue flowers among hydrophytes may be accidental, but those who take a teleologi- cal view of these matters prefer to attribute it to the fact that blue does not contrast vividly with the colour of a water surface with its sky reflections. It is possible that some water plants, such as Lemna*, are pollinated by crawling insects, although they possess no special means of attraction. A certain number of aquatics appear to have given up insect pollination and taken to anemophily, often with concomitant simplification of the flower, e.g. Hippuris (Fig. 151) and Myrio- -phyllum (Fig. 144, p. 221). This change of habit may be associated with the fact that the number of insects flying over a water surface is probably less, on an average, than the number over a corresponding land surface. Peplis Portula (Fig. 152, p. 232) seems to be actually in a state of transition from entomophily to anemophily. There are six fugacious little white petals, and a small amount of honey is secreted3. But the flowers are very inconspicuous, and no insect visitors appear to be attracted. The stigma becomes ripe a little sooner than the stamens, but they bend inwards over it and pollinate it4. Myriophyllum is an example of a wind-pollinated genus, in 1 Schenck, H. (1885). * See p. 80. 3 MacLeod, J. (1894). 4 Willis, J. C. and Burkill, I. H. (1895). XVI 1 1] MARE'S-TAIL 231 - a.L FIG. 151. Hippuris vulgaris, L, A, shoot showing air^ leaves (a.?.), water leaves (w.l.} and roots (r.}. Whorls of flowers at the upper nodes; nlt node with flowers whose anthers have dehisced; n2, node with flowers whose anthers are still closed. B, whorl of flowers enlarged, leaves (I) cut away. C, a single flower seen from adaxial side; st = feathery style; an = anther; o = ovary. (Reduced.) [A. A.] 232 FLOWERS OF AQUATICS [CH. which the long anthers swing on flexible filaments (B in Fig. 144, p. 221). In M. spicatum1 the upper flowers of the spikes are generally staminate, and the lower pistillate, while perfect flowers often occur in the intermediate region. Littorella lacustris^ which is anemophilous, sets a full com- plement of seeds by this means; it does not, like Myriophyllum and Hippuris, raise its flowers out of the water, but is sterile except when it grows as a land plant (Fig. 1 2 8 C, p. 1 98). When submerged it develops no flowers, but reproduces itself by FIG. 152. Peplis Portula, L. Land form, Forest of Wyre, September 13, 1911. A , part of branch. (Nat. size.) B, flower and leaves. (Enlarged.) C, fruit with seeds showing through transparent fruit coat. (Enlarged.) [A. A.] runners (Fig. 1 28 y^and B, p. 198). Littorella has been described as flowering so luxuriantly, in the height of summer in a dried-up swamp, that the shaking of the white stamens in the wind gave the whole area a silken sheen2, while another record relates to a case of this plant flowering in a dry year, when it had only attained ta such minute dimensions that the length of the filaments actually exceeded that of the rest of the plant3! In this genus we are probably not dealing with a case of loss of 1 Knupp, N. D. (191 1). 2 Buchenau, F. (1859). 3 Preston, T. A. (1895). xvm] CLEISTOGAMY 233 entomophily associated with the water habit, since the immedi- ate ancestors of Littorella were most likely closely related to the typically wind-pollinated Plantagos. The difficulty of keeping entomophilous or anemophilous flowers above water seems to have led, in the case of certain aquatics, to the formation of cleistogamic flowers which can set seed even when submerged. But Prankerd's1 work has suggested that records of cases of cleistogamy among water plants ought to be received with some caution, unless they are based on evidence of a highly critical nature. Concerning the Water Violet, this author writes, "Cleistogamy has been attri- buted to Hottonia, but I have found no trace of it during three summers' field work. The idea is probably due to some small, closed flowers, which occur sometimes among those fully developed, but serial sections have shown that these are merely abortive." It is possible that similar detailed investigations of other water plants, which have the reputation of bearing cleisto- gamic flowers, might considerably reduce the list ; Subularia for instance, which has been called cleistogamic, seems to open its flowers even if submerged2. There are however a certain number of cases in which the existence of cleistogamy is adequately established. Hooker3, for example, described the phenomenon in detail in Limosella aquatica^ L. This plant in Kerguelen's Land was, he writes, "found in the muddy bottom of a lake, and probably flowers all the year round. I gathered it in the month of July (mid- winter), beneath two feet of water, covered with two inches of ice; even then it had fully-formed flowers, whose closely imbricating petals retained a bubble of air, the anthers were full of pollen and the ovules apparently impreg- nated. The climate of Kerguelen's Land being such, that this lake is perhaps never dried, it follows that the plant has here the power of impregnation when cut off from a free communication with the atmosphere, and supplied with a very small portion of atmospheric air, generated by itself." Ranunculus fluitans^ Lmk., 1 Prankerd, T. L. (1911). 2 Hiltner, L. (1886). 3 Hooker,}. D. (1847). 232 FLOWERS OF AQUATICS [CH. which the long anthers swing on flexible filaments (B in Fig. 144, p. 221). In M. spicatum1 the upper flowers of the spikes are generally staminate, and the lower pistillate, while perfect flowers often occur in the intermediate region. Littorella lacustris^ which is anemophilous, sets a full com- plement of seeds by this means; it does not, like Myriophyllum and Hippuris, raise its flowers out of the water, but is sterile except when it grows as a land plant (Fig. 1 2 8 C, p. 1 98). When submerged it develops no flowers, but reproduces itself by FIG. 152. Peplis Portula, L. Land form, Forest of Wyre, September 13, 1911. A , part of branch. (Nat. size.) B, flower and leaves. (Enlarged.) C, fruit with seeds showing through transparent fruit coat. (Enlarged.) [A. A.] runners (Fig. 1 28 y^and B, p. 198). Littorella has been described as flowering so luxuriantly, in the height of summer in a dried-up swamp, that the shaking of the white stamens in the wind gave the whole area a silken sheen2, while another record relates to a case of this plant flowering in a dry year, when it had only attained to such minute dimensions that the length of the filaments actually exceeded that of the rest of the plant3 ! In this genus we are probably not dealing with a case of loss of 1 Knupp, N. D. (191 1). 2 Buchenau, F. (1859). 3 Preston, T. A. (1895). xvm] CLEISTOGAMY 233 entomophily associated with the water habit, since the immedi- ate ancestors of Littorella were most likely closely related to the typically wind-pollinated Plantagos. The difficulty of keeping entomophilous or anemophilous flowers above water seems to have led, in the case of certain aquatics, to the formation of cleistogamic flowers which can set seed even when submerged. But Prankerd's1 work has suggested that records of cases of cleistogamy among water plants ought to be received with some caution, unless they are based on evidence of a highly critical nature. Concerning the Water Violet, this author writes, "Cleistogamy has been attri- buted to Hottonia^ but I have found no trace of it during three summers' field work. The idea is probably due to some small, closed flowers, which occur sometimes among those fully developed, but serial sections have shown that these are merely abortive." It is possible that similar detailed investigations of other water plants, which have the reputation of bearing cleisto- gamic flowers, might considerably reduce the list ; Subularia for instance, which has been called cleistogamic, seems to open its flowers even if submerged2. There are however a certain number of cases in which the existence of cleistogamy is adequately established. Hooker3, for example, described the phenomenon in detail in Limosella aquatica, L. This plant in Kerguelen's Land was, he writes, "found in the muddy bottom of a lake, and probably flowers all the year round. I gathered it in the month of July (mid-winter), beneath two feet of water, covered with two inches of ice; even then it had fully-formed flowers, whose closely imbricating petals retained a bubble of air, the anthers were full of pollen and the ovules apparently impreg- nated. The climate of Kerguelen's Land being such, that this lake is perhaps never dried, it follows that the plant has here the power of impregnation when cut off from a free communication with the atmosphere, and supplied with a very small portion of atmospheric air, generated by itself." Ranunculus fluitans^ Lmk., 1 Prankerd, T. L. (1911). 2 Hiltner, L. (1886). 3 Hooker,}. D. (1847). 234 FLOWERS OF AQUATICS R. aquatiliS) L. and R. divaricatus, Schr. are also said to flower under water, pollination occurring in a bubble of air formed within the perianth1. An Indian species of Podostemon, P. Barberi, Willis2, has cleistogamic flowers, with one stamen standing close up against the stigmas (Fig. 82, p. 121). Alisma natans* is described as being cleistogamic in deep water, while Echinodorus ranunculoides^ has an entirely submerged form which flowers under water at a depth of three feet. Other recorded cases of cleisto- gamy are Heteranthera dubia^ (Fig. 153) and Hydro thrix Gardner^ (Pon- tederiaceae), Euryale ferox1 (Nym- phaeaceae), Ilhcebrum verticillatunfi (Caryophyllaceae), Tillaea aquatica* (Crassulaceae), Trapella sinensis9 (Pe- daliaceae), and a number of species of Lythraceae with apetalous or sub- apetalous flowers, belonging to the genera Rotala^ Peplis and Nesaea10. The pollination of cleistogamic flowers, though it may occur beneath 1Royer, C. (1881-1883). 2 Willis, J. C. (1902). * 3Schenck, H. (1885). 4 West, G. (1910). 5Wylie, R. B. (^i;1). 6Goebel, K. (1913). 7Goebel, K. (1891-1893). 8Caspary, R. (1860). 9 Oliver, F.W. (1888). 10 Gin, A. (1909). FIG. 153. Heteranthera dubia, (Jacq.) MacM. A, L.S. through an immature flower cut slightly obliquely in the adaxial-abaxial plane. The tip of the stigma lies below the upper ends of the anthers, and the style at this stage is straight. B, upper por- tion of an older flower cut in the same general direction as A . The stigma has been shoved up into the upper end of the flower in contact with the tips of the anthers where the stigmatic hairs touch the pollen grains through the breaks in the sta- mens. Pollen tubes are passing from the anther into the stylar chambers. The style is beginning to fold on account of its exces- sive elongation. [Wylie, R. B. (I9I71)-] xvm] HYDROPHILOUS POLLINATION 235 the water surface, is no more truly aquatic than are the vital processes of a man in a diving bell, since, as Hooker points out in the case of Limosella, the transference of the pollen takes place within a bubble of gas. Certain plants, however, present transitional methods of pollination, which without being actually hydrophilous, show approaches to this state. The oft-quoted case of Vallisneria spiralis (Hydrocharitaceae) is perhaps the best instance of such a transitional method. The male and female plants are distinct. The female flowers are solitary within a spathe, and are carried up to the surface of the water by the elongation of the peduncle below the spathe. When mature they lie horizontally on the water surface1. The submerged male spathes contain over 2ooo2 small flowers each with two stamens; the perianths are hermetically sealed, each enclosing a bubble of air. These male flowers become detached and rise to the surface of the water, where they open. The float- ing male flowers were figured early in the eighteenth century by Micheli3, an Italian botanist. A later observer in India4 speaks of " seeing under a noonday sun the innumerable florets freed from their spathes and ascending like tiny air-globules till they reach the surface of the water, where the calyx quickly bursts — the two larger and opposite sepals, reflex, forming tiny rudders, with the third and smaller recurved as a miniature sail, conjointly facilitating in an admirable manner the florets' mis- sion to those of the emerging females. " The male flowers are thus conveyed over the water surface by air currents, and some of them get carried into the neighbourhood of the female flowers, where the sticky pollen of the dehiscing anthers is likely to be rubbed off against the exposed stigmas. Each female flower, owing to its weight, is surrounded by a minute depression in the surface film of the water; the male flowers easily slide down the slope thus produced, and so approach the female2. After pollination the spiral peduncle contracts, carrying the maturing fruit deep down into the water; it is said that the 1 Chatin, A. (18552). 2 W7lie> R- B- 3 Micheli, P. A. (1729). 4 Scott, J. (1869). 236 FLOWERS OF AQUATICS [CH. contraction does not actually bring it to the bottom of the water, but that the last stages in the descent are accomplished by its own weight, when it is ripe1. Other Hydrocharitaceae, e.g. the marine genus Enhalus^^ possess a pollination mechanism resembling that of Fallisneria. Others again, e.g. Elodea calli- trichoides*, have, by a further modification, arrived at a type of pollination which is strictly hydrophilous, for the pollen, instead of being rubbed off against the stigmas, is shed explo- sively and falls on to the surface film, reaching the stigmas by flotation. The ultimate stage in the series of the Hydrocharita- ceae is reached by the marine genus Halophila, in which neither male nor female flowers emerge from the water, and the process of pollination takes place in complete submergence4. The stigmas are thread-like and the pollen-grains, being united into strings, adhere readily to the stigmas, which present elongated receptive surfaces. The family Hydrocharitaceae is, indeed, of unique interest from the standpoint of the evolution of submerged pollination, since it includes within itself all stages in the transition from entomophily to hydrophily5. It contains insect-pollinated flowers, such as Hydrocharis Morsus-ranae and Elodea densa^ with attractive perianths, and, sometimes, nectaries ; flowers in which the unwetted pollen is conveyed over the water by the ' boat mechanism/ e.g. Vallisneria; flowers in which the pollen floats on the surface of the water, e.g. Elodea callitrichoides ; and, finally, flowers with entirely submerged pollination, such as Halophila. Callitriche^) among the Dicotyledons, provides another group of species in which the transition from aerial to aquatic pollina- tion can be followed. The genus is subdivided into two sections : Eu-callitriche^ to which the ordinary amphibious species of Water Starwort belong, and of which C. verna is the type, and 1 Royer, C. (1881-1883). 2 Delpino, F. and Ascherson, P. (1871). 3 Hauman-Merck, L. (i9i32). See p. 55. 4 See p. 130. 5 See pp. 55-57. 6 Hegelmaier, F. (1864), Jonsson, B. (1883-1884), and Schenck, H. (1885). 3) flower. In the case of the male flower both bracts can be seen. (Enlarged.) [A. A.] xvm] HYDROPHILOUS POLLINATION 237 Pseudo-callitriche, which consists of submerged plants grouped round the species C. autumnalis. C. autumnalis has no land form, but vegetates, flowers and fructifies below the level of the water surface. Throughout the genus the simple male and female flowers occur separ- ately (Fig. 1 54) ; the female flowers are commonly found lower down the inflorescence than the male, but, in C. autumnalis^ several male and female regions may alternate with one another. Insects, and possibly wind, carry the pollen of the Eu-callitriches, which is r . FIG. 154. Calhtnche verna, L. of the terrestrial type and is clothed July 19, 1910. Flowering shoot with an exine insoluble in sulphuric acid. That of the Pseudo-callitriches, on the other hand, is of the aquatic type; it has no differentiated exine and contains oil globules which render it lighter than water. It is carried to the stigmas by water currents. The aquatic pollination of Ceratofhyllum (Hornwort) has already been considered1, as well as that of three members of the Potamogetonaceae, Cymodocea^^ Zoster a* , and Zannichellia^. In connexion with the submerged pollination of Naias graminea — also belonging to the Pondweed family — a picturesque incident which has been placed on record by Bailey5, suggests that aquatic animals may occasionally play a part in the pollination of submerged plants. He writes, " While . . . examining portions of a living plant on which were ripe anthers, I noticed a colony of Vorticellidae attached to one of the fascicles of leaves; the grace and activity of its movements led me to watch it for a considerable time, and whilst so watching it I witnessed grains of pollen whirled in all directions, or drawn into the vortex of the animal by its marginal cilia. The alternate contraction and 1 See pp. 84-85. 2 See p. 126. 3 See pp. 127-129. 4 See pp. 70-71. 5 Bailey, C. (1884). 238 FLOWERS OF AQUATICS [CH. xvm elongation of the elastic and thread-like pedicles of the colony kept the pollen-grains in constant motion, which left me no doubt that at times the grains would be directly borne to the stigmatoid appendages of the pistilliferous flowers/' It seems to the present writer conceivable that, in future phases of evolution, if more Angiosperms reach the highly specialised stage of complete submergence, the water fauna may come to play an important part in their pollination. There may even arise a parallelism of development and an interdependence between aquatic animals and submerged plants comparable with that which has obtained in the case of aerial insects and the flowers which they pollinate! In general, the consideration of the flowers of hydrophytes seems to lead to the conclusion that submerged pollination is a relatively modern development. It is, from some points of view, merely a further advance on lines similar to those already marked out in the case of anemophily. The great majority of hydrophilous plants have near relatives — sometimes even mem- bers of the same genus — which retain anemophilous or entomo- philous habits ; this may be regarded as a proof that plants with submerged pollination have arisen in comparatively recent times from ancestors with the aerial type of flower. Ceratophyl- lum forms an exception, since it is entirely hydrophilous, and has no intimate affinities with any other genus. It is probable, from its extreme adaptation to aquatic conditions and its isolated position in the relatively primitive Ranalean plexus, that it is a genus whose ancestors took to aquatic life at a very early stage in the race history of the Angiosperms. [ 239 I CHAPTER XIX THE FRUITS, SEEDS AND SEEDLINGS OF WATER PLANTS1 AS we have shown in the preceding chapter, submerged pol- jt~\. lination represents an advanced stage in acclimatisation to water life, to which only a small proportion of hydrophytes have attained. But it is by no means so rare to find the events subse- quent to pollination taking place beneath the water surface. A great many aquatics — not only those which are hydrophilous, but also a number of those which raise their flowers into the air for pollination by wind and insects — after fertilisation draw down their gynaeceum into the water where the ripening pro- cesses take place. In fact, the water plants which retain an entirely aerial method of fruit-ripening are relatively few; examples of these exceptions are Utricularia, Hottonia and Lobelia^ all of which lift their many-seeded capsules on long infructescence axes above the water level. Numerous examples of those aquatics which are pollinated in air but ripen their fruit in water, might be quoted, but it will suffice to recall Aldro- the Aponogetonaceae3, Limnanthemum Humboldtia- Victoria regia5, the Batrachian Ranunculi (Fig. 93, p. 145), Pontederia rotundifolitfi (Fig. 155, p. 240) and other members of the Pontederiaceae4. Among the Hydrocharitaceae7, the ripening ovary is conveyed down into the water by several different methods; in Limnobium and Qttelia the flower-stalk bends down, in Vallisneria it contracts spirally, while in Stratiotes 1 For a good general account to that date, see Schenck, H. (1885). 2 Caspary, R. (1859 and 1862). 3 Krause, K. and Engler, A. (1906). * Muller, F. (1883). « See p. 34. ' 6 Hauman-Merck, L, (I9I31). 7 Montesantos, N. (1913). 24o FRUITS OF AQUATICS [CH. the fruit is carried down by the sinking of the entire plant. The lowering of the fruit must not, however, be regarded as a special innovation due to aquatic conditions, since countless examples FWT!f- FIG. 155. Pontederia rotundifolia, L. Branch bearing inflorescence (negatively geotropic) and infructescences (positively geotropic). (Reduced.) [Hauman- Merck, L. (191 3 1).] occur among terrestrial plants, e.g. the spiral contraction of the fruit stalk of Cyclamen and the downward curve of the peduncle of Linaria Cymbalaria. In those submerged fruits which are many-seeded, the method of dehiscence is necessarily different from that obtain- ing among terrestrial plants, since desiccation can play no part. The irregular opening of the fruit ofNymphaea lutea has already been described1. In the case of Limnanthemum nymphoides^ A 3 FIG. 156. Limnanthemum nymphoides, Hoffmgg. and Link. A, fruit from surface of water, October i, 1914 (nat. size). B, fruit kept in water in unheated greenhouse since October i, which had dehisced by November 23 (nat. size). C, seed, Novem- ber 24, 1914 (x 2). [A. A.] dehiscence takes place in a somewhat similar fashion. The present writer found a number of infructescences of this plant with green fruits (Fig. 156 A) floating on the surface of the 1 See p. 35. xix] INDEHISCENT FRUITS 241 water at Roslyn Pits, Ely, on October i, 1914. At this stage the seeds were unripe and white. The fruits were brought to the laboratory and kept in water. After a considerable time the pericarp split irregularly, after a fashion closely recalling Nymphaea\ by November 24, the fruits were in this bursting condition and the seeds, which had darkened in colour, had all the appearance of being ripe. The embryos are said to be pro- tected by the cuticularised epidermis of the testa1. The seeds are flat and ciliated at the edge (Fig. 156 C). That these hairs serve for flotation is indicated by the fact that if they are cut off the least touch makes the seeds sink1. It has also been ascer- tained that the seeds may become firmly attached to the downy plumage of a bird's breast, by means of this fringe of hairs2. The splitting of the ovary wall takes place mostly near the base — the lobes that are thus produced curving up until the outer epidermis of the pericarp, which was originally convex, becomes concave. This curvature is due to decay and loss of tissue on the inner surface of the fruit-wall, followed by swelling of the rest of the tissues, with the exception of the outer epidermis and adjacent layers (Fig. 157 A and J5, p. 242). The fruits of Stratiotes aloides and Hydrocharis Morsus-ranae are said to be burst open by the swelling of mucilage produced from the testa of the enclosed seeds. A remarkably large proportion of aquatics, on the other hand, have fruits which are either one-seeded and indehiscent, or else take the form of schizocarps or heads of achenes, separating into one-seeded segments. The seeds are thus protected both by pericarp and testa, which is possibly of value in enabling them to resist the rotting effect of prolonged submergence3. It is interesting in this connexion to compare, for instance, the fruits of Plantago major and of the closely related aquatic, Littorella /acustris1. The Plantain has a pyxidium capsule, with a thin elastic wall, opening by means of a lid and containing a number 1 Fauth, A. (1903). 2 Guppy, H. B. (1906). 3 The protection of the embryo in certain aquatics is considered by Marloth, R. (1883). A.W.P. 16 242 FRUITS OF AQUATICS [CH. of seeds. The fruit of Littorella on the other hand is reduced to a nut developed from the two-celled gynaeceum. Only one chamber is fertile and the embryo is protected by means of the sclerised fruit wall, with its aperture closed by a stopper formed from the funicular region of the seed. A protective endocarp, with an opening closed by a plug, is also found in the four one- seeded segments of the schizocarp of Myriophyllum spicatum^ B FIG. 157. Limnanthemum nymphoides, Hoffmgg. and Link. A, T.S. wall of fruit represented Fig. 156 A, p. 240, October i, 1914. B, T.S. wall of fruit represented Fig. 156-6, November 23, 1914. (Both x 78 circa.} ep. = epidermis. [A. A.] and in the drupe-like one-seeded nutlet of Hippuris vu/garis1. The seeds of the latter species winter in mud at the bottom of the water, protected by the stony endocarp. At germination the radicle emerges from the stone through a foramen which was previously filled by a cuticularised stopper, formed from part of the funicle and integument. In Alisma Plantago the embryo is protected by a chaffy carpel wall and a testa described by different authors as suberised1 or as composed of pectic substances2. In the case of the four nutlets into which the 1 Fauth, A. (1903). 2 Crocker, W. and Davis, W. E. (1914). xix] DELAYED GERMINATION 243 schizocarp of Callitriche divides, the same function is performed by the pericarp, which is thin, but tough and elastic. The well- protected seeds of hydrophytes can in many cases withstand inclusion for a considerable length of time in ice or frozen mud. The fruits of Sagittaria sagittifolia^ Alisma Plantago and Myrio- phyllum spicatum, and the seeds of Castalia alba and Nymphaea lutea can tolerate freezing for a week or two, or, in some cases, V \ much longer1. ^\ With the particularly effective protection of the embryo in ^hydrophytes, their characteristic habit of delayed germination is probably to be associated. The sprouting of the seed may in some cases be deferred until the third, fourth, or fifth year2, the embryo remaining uninjured by this prolonged period of dormancy. Several investigators have studied the subject of delayed germination, and the rather curious fact has emerged that this delay only occurs if the seeds are continuously immersed in water; if they are subjected to a period of drying, they germinate promptly. It has been noted, for example, that the seeds of MayacafluviatiliS) a Brazilian water plant, which were dried for six weeks after gathering, germinated at once, while seeds harvested at the same time, but put immediately into water, showed no sign of sprouting at the end of three months3. The seeds of some water plants can tolerate drying for a very long period, e.g. thirty months in the case of Limnanthemum nymph- oides*. The result of experimental work on the subject seems to be to show that the delayed germination of undried seeds is due to the mechanical pressure exerted by the seed coats5; if these are artificially ruptured, the development of the embryo presents no further difficulties. It has been found, for instance, that in 1 Guppy, H. B. (1893) and (1897). 2 Guppy, H. B. (1897) ; on delayed germination in Potamogetons see pp. 71, 72, and in Nymphaea, p. 36. 3 Ludwig, F. (1886). " 4 Guppy, H. B. (1897). 5 Sauvageau, C. (1894), Crocker, W. (1907), and Crocker, W. and Davis, W. E. (1914). For a somewhat different viewsee Fischer, A. (1907). 1 6— 2 244 SEEDLINGS OF AQUATICS [CH. comparative cultures of the achenes of Alisma Plantago, ex- amined at the end of ten days, those in which the protective coats were intact, had not germinated at all, while 98 per cent, of those whose walls had been ruptured, had begun to sprout. The reason why preliminary drying favours germination, may possibly be that it gives rise to some cracking of the seed coats; a speeding-up of germination also occurs, in some cases, if the seed passes through the alimentary canal of a bird1, a result which again may be due to some disintegrating chemical or mechanical action exerted on the wall. Freezing may also assist germination by means of its effect on the outer covering of the seed2. It should be noted, that delayed germination, though specially characteristic of water plants, is by no means peculiar to them. That the causes which bring it about are of a similar nature in aquatics and terrestrial plants, is indicated by the fact, well known to gardeners, that a large proportion of such seeds as those of Canna^ fail to germinate unless the shell is filed through. The phenomena of delayed germination suggest that Nature, in her solicitude for the protection of the embryo, is liable to defeat her own ends by enclosing it in a prison from which it can only escape with difficulty. The germination and development of the seedling in aquatics vary according to the natural affinities of the plants in question, and are characterised by few peculiarities related to the environ- ment, except a very frequent reduction of the primary root. In Utricularia (Fig. 67, p. 100), Stratiotes aloides*, Hydrocharis, Ruppia, Ceratophyllum (Fig. 55, p. 86), the Podostemaceae, Nymphaea lutea, Aldrovandia*, Hippuris, Naias, Trapa5, etc., the radicle is either quite undeveloped or very short-lived. In Aponogeton distachyus* the primary root does not attain to more than 0-5 cms. in length, and eventually it disarticulates by 1 Guppy, H. B. (1897). 2 Guppy, H. B. (1893). 3 Irmisch, T. (1865). 4 Korzchinsky, S. (1886). 5 Queva, C. (1910); see also Fig. 160, p. 247. 6 Sergueeff, M. (1907). xix] FIXATION OF SEEDLINGS 245 means of an absciss layer. There are exceptions, however, to the general rule that the radicle of water plants is poorly developed : in Lobelia Dortmanna, for example, it attains fair dimensions1. In the case of those water plants which grow rooted in the soil, the poor development of the radicle is often compensated, at an early seedling stage, by the production of a garland of very long root-hairs, which grow out from the 'collet,' or junction of hypocotyl and root, e.g. Htppuris*) Elatine hexandra* (Fig. 158) and many Helobieae3'4, such as Zannichetlia (Fig. 159 C, p. 246). This type of seedling is, however, by no means confined to hydrophytes, but is also found in a number of land plants. The weight of the large seed of Nelumbo 3, and of the achene wall in the case of the small seedling of Zanxichellia* (Fig. 1 59), are sufficient to keep the seedling steady at the bottom of the water until the epicotyl and first leaves are produced. Other FIG. 158. Elatine kexandra, D.C. Germination of seed; s, seed- s~*L^ ^f\ coat; h, wreath of hairs growing from collet and surrounding v^ C*S the primary root which forms a minute conical structure. [Klebs, G. (1884).] seedlings are anchored for some time by the fruit wall and associ- ated structures : the grappling apparatus of Cymodocea antarctica, for instance, has been already described6. In Trapa natans1 (Fig. 1 60, p. 247) the fixation of the seedling is accomplished in an unusual way, for here the heavy nut sinks to the bottom of the water, where it is held by hooks derived from the calyx. Two structures of very unequal size (Co1 and C02) are generally interpreted as the two cotyledons, though possibly this view is open to revision. The hypocotyl, including even its extreme apex, which presumably is of root nature, is negatively geo- tropic. The first lateral roots, borne by the hypotocyl, curve downwards and anchor the plant in the soil, while many of the 1 Buchenau, F. (1866). 2 Irmisch, T. (I8591). 3 Klebs, G. (i 884). * Warming, E. ,(i 883!). 5 Hochreutiner, G. (1896). 6 See p. 127. 7Goebel, K. (1891-1893). 246 SEEDLINGS OF AQUATICS [CH. later roots borne on the hypocotyl and plumule are negatively geotropic. An exceptional case is that of Littorella lacustris^^ in which the seeds remain in situ. The gynaecea are borne close to the axis, between the leaves, near the base of the little plant. On FIG. 159. Zannichelliapolycarpa, Nolte. A, L.S. fruit (x 15) ; a = stigma; 6 = coty- ledon; h = hypocotyl; A = vascular tissue; r = primary root; p = plumule. B, cotyledon emerging from fruit (x 6). C, seedling (x 4); rh = root hairs. [Raunkiaer, C. (1896).] the death of the parent, the fruits are left surrounded by the decaying remains; they germinate where they were produced, only being dislodged in rare instances. The somewhat similar behaviour of Cymodocea aequorea has been discussed on p. 127. 1 Fauth, A. (1903). xix] BULL NUT 247 FIG. 160. Trapa natans, L. i, L.S. through seed (Enlarged); Co^, the larger and Co2 the smaller cotyledon; St, stalk of larger cotyledon. 2 and 3, seedlings (Re- duced) ; A, shoot arising in axil of smaller cotyledon; W, roots arising in the region of the leaf insertions. [Goebel, K. (1891-1893).] 248 SEEDS OF AQUATICS [CH. In the case ofFarmeria metzgerioides, one of the Podostemaceae, germination of the two-seeded, indehiscent fruit also occurs in situ'1. The seedlings belonging to certain floating plants owe their station at the water surface to the early development of some type of buoyant organ : in the case of Lemna, for instance, the cotyledon itself acts as a float (Fig. 52, p. 81). The seedlings of certain plants which are rooted at maturity, are capable of developing to a considerable extent while still unattached. Some seeds of Limnanthemum nymphoides* were kept in water over a winter by the present writer, and on February 1 1 , one of them was observed to have germinated while floating. It is a somewhat remarkable fact that the large group of the Monocoty- ledons which are known collectively as the Helobieae or Fluviales — the Alis- maceae, Butomaceae, Hydrocharita- ceae, Juncaginaceae, Aponogetonaceae, Potamogetonaceae and Naiadaceae — are uniformly characterised by the absence of endosperm and by a *ma- cropodous ' embryo, in which the hypo- cotyl reaches excessive proportions (e.g. Zannichellia, Fig. 159, p. 246, Zostera, Fig. 1 6 1, Ruppia, Fig. 1 66, p. 3 1 9) ; in almost all other features the members of the group show great range and diversity. Except the Helobieae, the Monocotyledons may be said, in general, to be characterised by the possession of endo- sperm. The surmise suggests itself that possibly there may be some connexion between water life and an exalbuminous seed with an enlarged hypocotyl. The predominance among aquatics 1 Willis, J. C. (1902). 2 Fauth, A. (1903). B FIG. 161. Zostera marina, L. Fruit in longitudinal section. (XI5-) /£= fruit coat; fs= seed coat; r= primary root; rh= hypocotyl which at its base enwraps the cotyledon a. [Raunkiaer, C. (1896).] xix] MACROPODOUS EMBRYOS 249 of seeds with elaborate and impervious coats, seems to indicate that plants with imperfectly protected embryos have been unable to enter upon aquatic life. Possibly there is a danger of rotting if the contents of the seed are at all freely exposed to the water. If this is so, it may be that an embryo which keeps its reserves inside its own tissues is better adapted for water life than one whose storehouse is outside its own body, even if it is enclosed in a resistant coat; the food is probably more secure from the depredations of Bacteria and from other harmful external in- fluences, if it is incorporated within the cells of the embryo instead of being merely surrounded by the testa. In the opinion of the present writer, Monocotyledons have, in general, re- duced their seed-leaves to a single cylindrical or tubular struc- ture by means of the fusion of the petiolar or sheathing regions and the loss of the blades. They are thus not in a position to store food in the laminae of the cotyledons, as is done, for in- stance, in the case of such Dicotyledons as the Pea or the Bean. The radicles of aquatic seedlings are, as we have already shown, markedly reduced, so a second possible location for food storage is thus eliminated. In this connexion we may recall the fact that, whereas mature Dicotyledons often store food in their tap roots (e.g. Carrot, etc.) this method is unsuitable for Mono- cotyledons, owing to the ephemeral nature of the primary radicle, and they are hence almost wholly restricted to storage in leaf structures, stem structures, or adventitious roots. We are thus left with the fact that if a Monocotyledonous embryo is to store its food in its own body, the only region where this can be conveniently accomplished is the hypocotyl, since both cotyledon and primary root have suffered reduction. From these considerations we may perhaps conclude that the non- endospermic type of seed with a macropodous embryo, whose hypocotyl has become enlarged for food storage, represents a form of Monocotyledonous seed which is particularly well fitted for aquatic life. PART III THE PHYSIOLOGICAL CONDITIONS OF PLANT LIFE IN WATER " For the student of the conditions of aquatic life, the real inquiry has yet to be begun." H. B. Guppy, 1896. 253 CHAPTER XX GASEOUS EXCHANGE IN WATER PLANTS THE problems which a water plant has to solve, in connexion with its assimilation and respiration, differ widely from those which confront a terrestrial plant, since, instead of being surrounded by atmospheric air, it passes its life in water holding only a certain amount of air in solution. Owing to the varying solubility of the atmospheric gases, the dissolved air differs from free air in composition. At 15° C., the proportions in which the constituents should occur have been calculated to be as follows1 : FREE AIR DISSOLVED AIR Carbon dioxide 0-04% 2-19% Oxygen 2O-8o% 33-98% Nitrogen 79- 1 6% 63-82% In practice, however, the air dissolved in the surface layers of the water of lakes and streams, under natural conditions, yields varying figures when analysed, but all observers appear to agree that, as regards carbon dioxide it is supersaturated, sometimes highly so2. It seems clear that the excess cannot be obtained by diffusion from the air, for an American writer3, who has experimented with Elodea canadensis^ has shown that sufficient carbon dioxide to keep this plant growing, or even alive, does .not diffuse into water exposed to atmospheric air at Baltimore during the winter months. He demonstrated that all the carbon 1 Devaux, H. (1889). The proportion of nitrogen given in this table naturally includes the other inert gases which were not distinguished in Devaux's time; the amount would be more correctly stated as including approximately 78 per cent, of Nitrogen and I per cent, of Argon. 2 Forel, F. A. (1892-1904); Regnard, P. (1891). 3 Brown, W. H. (1913). 254 GASEOUS EXCHANGE [CH. dioxide which a 3 -litre jar of water would absorb from the air at ordinary temperatures, could be used up by ten shoots of Elodea in two minutes. His view is that the substratum serves as the chief source of carbon dioxide for submerged plants, the amount of this gas given off into the water from soil containing organic matter being greater than that obtained by diffusion from the air. Whether the excess of carbon dioxide is, in general, derived from the substratum, or whether it is due to the oxidation of carbonaceous substances in the water or to other causes, the fact remains that hydrophytes growing under natural conditions live in an environment particularly rich in carbon dioxide. This advantage tends to be neutralised, however, by the slow diffusion of gases in water. There is also the further drawback that the absorption capacity of water sinks as the temperature rises, so that, in warm weather, when the life processes of the plant are proceeding most vigorously, the supply of carbon dioxide is reduced1. Assimilation is nevertheless remarkably active among water plants, several features which they commonly show being well suited to the prevailing conditions ; one of these is the deve- lopment of chlorophyll in the epidermal cells, so that the epi- dermis forms part of the assimilating system, which is thus not shut off from the surrounding medium by a layer whose func- tion is purely protective, as in the case of terrestrial plants. Cuticle is relatively little developed, and the cell-walls seem to offer no more hindrance to the direct passage of dissolved gases than if they were merely thin plates of water2. That the waxy cuticle of such leaves as those of the submerged Potamogetons is no obstacle to the entry of liquids, has been proved by plas- molysis experiments in which the whole leaf was used3. Submerged plants show various characteristics which have the effect of increasing the surface relatively to the volume of the leaf, and thus bringing a large proportion of the assimilating cells into direct contact with the dissolved carbon dioxide. The 1 Goebel, K. (1891-1893). 2 Devaux, H. (1889). 3 Sauvageau, C. (iSgi1). xx] LACK OF OXYGEN 255 leaves may, for instance, be very thin, but extensive in area, a and a few obliquely placed (0J.), A small part of the transverse section is shown in greater detail in Fig. 1 70 C. In this drawing, the central and largest bundle is seen to be normally orientated, but the bundles on either side of it have the xylem below and phloem above. In the heart-shaped * lamina ' of Heter anther a reniformis, Ruiz, arid Pav., a very similar bundle arrangement is found (Fig. 170^). Here, only the midrib and main laterals are normally placed, the remaining bundles being inverted. The ' lamina ' of Eichhornia speciosa, Kunth (Fig. 170^) differs from that of the other members of the family here con- sidered, in its much greater thickness. Inverted bundles occur, ' not only in the thick basal region in which the transition from petiole to ' lamina ' takes place quite gradually, but also near the margin. Here, there is only a single series of vascular strands, among which inversely orientated bundles are very numerous. Some of the lateral veins in the ' lamina ' consist of a single, normally orientated bundle, while others include a pair of bundles, one normal and one inverted. Among the Pontederiaceae, we not only find leaves, such as those just described, in which there is a differentiation between petiole and * lamina/ but others, which are ribbon-like, with no distinction of blade and stalk. For comparison with the more highly differentiated leaves, sections were cut of the ribbon-leaf of Heter anther a zoster aefolia^ Mart. Here the midrib and main laterals proved to be normal, but the others — i.e. the majority of the laterals — were inverted. Fig. 1 70 F shows two adjacent bundles orientated in opposite ways. The structure of this ribbon-leaf is closely similar to that of the * lamina ' in H. reni- f or mis. 344 ' LAW OF LOSS ' [CH. It may be worth noting that a peculiar submerged member of this family, Hydrothrix Gardner^ Hook, f., described by Goebel1, has leaves with a sheathing base and hair-like upper region, whose external morphology distinctly suggests a phyllodic origin. In this case anatomical evidence cannot be sought, since the extremely slender leaves are said to be tra- versed by a single bundle only. The presence of inverted bundles in all species of Pontederia- ceae of which material has been available to the present writer, is a remarkable anomaly which calls for some explanation. It is difficult to see how such a structural peculiarity can be ex- plained as an adaptation, since it is common to leaves otherwise differing notably in type and mode of life. It is equally con- spicuous in the very delicate ribbon-leaf of Heteranthera zos- teraefolia and in the well-defined, thick * lamina * of Eichhornia speciosa; it occurs both in Heteranthera reniformis, in which palisade parenchyma is confined to the upper side and in Pontederia cordata, in which this tissue occurs towards both surfaces. In the present writer's opinion, this anatomical ano- maly is best interpreted on the view that the * laminae ' of the Pontederiaceae, instead of being homologous with the blades of Dicotyledons, are merely the expanded apices of pre- existing phyllodes : the inverted bundles are thus an indication of the petiolar nature of the organ, and are regarded as an ancestral feature rather than as an adaptation. The Pontederiaceae are not the only family in which we meet with phyllodic anatomy of the 'lamina/ The present writer has found, in the arrow-head blade of Sagittaria monte- vidensis, Cham, and Schlecht. (Fig. 171 B\ that, besides the normal main bundles (n.b^) and a series of smaller bundles running near the lower surface (n.b^ there is a third series of small inverted bundles near the upper surface (/.£.). In Sagittaria sagittifolia, L., inverted bundles are a less striking feature, but the lateral ribs, one of which is represented in Fig. 171 A) show both normal and inverted bundles. 1 Goebel, K. (1913). xxvm] SUBMERGED POLLINATION 345 If the view here advocated regarding the nature of the blades of Monocotyledonous leaves be accepted, it forms a particularly salient instance of the working of the * Law of Loss/ since we have here an instance of a discarded organ (the lamina) being replaced by a modification of another (the petiole) in lieu of being re-acquired. FIG. 171. A, Sagittaria sagittifolia, L., T.S. lateral vein of lamina, next but one to midrib. B, Sagittaria montevidensis, Cham, and Schlecht., small part of T.S. of leaf near margin. The lower of the two bundles belonging to the normal series (n.b2) is irregularly placed, (n.b^ = bundle of main normal series; i.b. = inverted bundle; xy = xylem; ph = phloem; a.t. = assimilating tissue; st = stomate; o.d. = oil duct.) [Arber, A. (1918).] The pollination methods of submerged Angiosperms may also possibly be regarded as illustrating the Law of Loss. The ciliation of the male gamete in the great group of the Pterido- phyta — from which it is supposed that Flowering Plants are ultimately derived — is associated essentially with aquatic fertili- sation ; with the adoption of terrestrial life this feature was lost, and is now unknown either in the higher Gymnosperms or the 346 'LAW OF LOSS ' [CH. Angiosperms. It might well have been expected that when certain Angiosperms adopted water-life so completely as even to revert to the remotely ancestral habit of submerged fertili- sation, they would also simultaneously revert to ciliated sperms, associated with a broad stylar canal and open micropyle. Such a trumpet-shaped stigma as that possessed by Zannichellta seems, indeed, exactly adapted for the entry of swimming sperms. But no such ciliated Angiospermic gametes have come into existence; those Flowering Plants which are pollinated beneath the water, go through all the processes of making pollen-grains as for aerial pollination, with such slight modifica- tions as will permit them to be carried passively to the stigma by gravity or water currents. It seems that cilia once lost cannot be recovered, even when the circumstances in which they were formerly of use again recur, and the plant has, as it were, to patch up some substitute. If the Law of Loss be accepted as of general application, it furnishes a clue to certain phylogenetic problems. We have already alluded to the light which it throws on the difficult question of the interpretation of the flower of Naias1. Again it is highly unlikely, on the Law of Loss, that a naked unisexual flower could evolve into a hermaphrodite flower with a peri- anth, and hence the law points to the primitiveness of such floral types as those found among the Ranales and Alismaceae. We have already2 considered Dr Scott's suggestion that the anatomical peculiarities of the polystelic genus Gunnera might lie in an ancestral history in which an original terrestrial period, followed by an aquatic phase, has been succeeded by a second terrestrial period. Expressing this example in terms of the Law of Loss, we may say that the cambial system, once dis- carded under the influence of water-life, could not be regained even when the plant reverted to terrestrial conditions; the expedient of adding to the number of the existing reduced steles represents a device for repairing this irrevocable loss of means by such substitutes as are to hand. 1 See p. 315. 2 See p. 180. xxvm] 'LAW OF IRREVERSIBILITY ' 347 Some time after the present writer had deduced the Law of Loss from a consideration of the structure of the water plants living to-day, she learned that zoologists had already arrived, on fossil evidence, at very similar conclusions regarding animals. The Law of Loss covers part of the same ground as Dollo's 'Law of Irreversibility.' That this law should have been arrived at independently for plants and for animals is perhaps an indica- tion of its probable validity. With current Mendelian conceptions, the ' Law of Loss ' harmonises without apparent difficulty. If evolution has pro- ceeded by variations due to successive losses of factors, we should certainly expect that the complete loss of an organ might be associated with inability to recall it, even when circum- stances seem to put a premium upon its reappearance. If we accept the views of Samuel Butler so far as to admit that there is at least an analogy of a highly intimate nature between heredity and unconscious memory, each example of the ' Law of Loss ' may perhaps be visualised as representing a lapse or failure of memory. If an organ be lost, the remembrance of it presumably in course of time becomes more and more remote, until finally, even if circumstances renew the need for it, the memory has so entirely faded that the plant cannot, as it were, recall how to reconstruct it. It is thrown, so to speak, on its own resources, and is thus compelled to discover for itself some method of responding upon new lines to the ancient need. [ 349 ] ALPHABETICAL LIST OF BOOKS AND MEMOIRS BEARING ON THE STUDY OF AQUATIC ANGIOSPERMS [This list is far from exhaustive, being merely intended to indicate the principal sources. Each title is followed by a brief note on the contents and scope of the memoir. In the case of works cited in the body of the text, or from which figures have been reproduced, references to the pages in question will be found beneath the authors' names.] Agardh, C. A. (1821) [P 123] Anon., (1828) [P- i?] Anon., (1895) [P- 17] Arber, A. (1914) [pp. 50, 1 86 and Figs. 31, p. 49 and 121, p. 186] Arber, A. (1918) [PP- 52, 336 and Figs. 169, p. 341, 1 70, p.342andi7i,p.345] Arber, A. (I9I91) [P- Arber, A. (I9I92) [pp. 182, 336] Arber, A. (I9I93) [P. X43] Arber, A. (i9i94) [pp. 74, 82, 316] Species Algarum, Vol. I. 1821, 531 pp. Gryphis- waldiae. (Amphibolis zoster aefolia [ — Cymodocea antarctica] included under the Algae.) Honzo Zuju (Phonzo Zoufou). Yedo, 1828. (A large series of volumes with fine illustrations of Japanese plants. Vols. 69-76 contain coloured figures of Nymphaeaceae, Trapa, Trapella and other water plants. There is a copy in the Library of the Kew Herbarium.) Useful Plants of Japan, described and illustrated. Agricultural Society of Japan, Tokyo, 1895. (Trapa, Nelumbo, Euryale, Sagittaria and Scirpus tuber osus are figured and their uses described.) On Root Development in Stratiotes aloides L. Proc. Camb. Phil. Soc. Vol. xvn. 1914, pp. 369-379, 2 pis. (The development of the adventitious roots is discussed in this paper, and attention is called to the frequently bi-lobed character of the nuclei in their stelar tissues.) The Phyllode Theory of the Monocotyledonous Leaf, with Special Reference to Anatomical Evidence. Ann. Bot. Vol. xxxn. 1918, pp. 465-501, 32 text-figs. (In this paper the nature of the leaves in the Pontederiaceae, Sagittaria and other aquatic Monocotyledons is discussed.) Aquatic Angiosperms and their Systematic Distri- bution. Journ. Bot. Vol. 57, 1919, pp. 83-86. (See Chapter 25 of the present book.) The 'Law of Loss' in Evolution. Proc. Linn. Soc. Session 131, 1918-1919, pp. 70-78. (See the last chapter of the present book.) Heterophylly in Water Plants. Amer. Nat. Vol. 53, 1919, pp. 272-278. (A general discussion of this question.) On the Vegetative Morphology of Pistia and the Lemnaceae. Proc. Roy. Soc. B, Vol. 91, 1919, pp. 96-103, 8 text-figs. (It is here shown that the leaf of Pistia is phyllodic in anatomy, and that its sheath forms a lateral pocket in which a bud is produced, in a position comparable with that of a young frond of Lemna.) 350 Arber,E. A.N.) and Parkin, J. j [PP- 308, 315] Arcangeli, G. (1890) [pp. 27, 159] Areschoug, F. W. C. (I8731) Areschoug, F. W. C. (i8732) [pp. 302, 303] Armand, L. (1912) [p. 166] Ascherson, P. (1867) [pp. 123, 124] Ascherson, P. (1870) [P- 135] Ascherson, P. (1873) [p. 146] Ascherson, P. (1874) [P- 3033 BIBLIOGRAPHY On the Origin of Angiosperms. Linn. Soc. Journ. Bot. (1907) Vol. 38, 1907, pp. 29-80, 4 text-figs. (This paper is partly devoted to a reconstruction of the primitive type of Angiospermic flower. Among aquatics, the Nym- phaeaceae, Alismaceaeand Butomaceae are regarded as showing certain primitive features of flower structure.) Sulle foglie delle piante acquatiche e specialmente sopra quelle della Nymphaea e del Nuphar. Nuovo Giornale Botanico Italiano, Vol. xxn. 1890, pp. 441- 446. (A study of heterophylly in these genera.) Om Trapa natans L. och dess i Skane annu lefvande form. Ofversigt af k. vet. akad. Forhandl. xxx. 1874 (for 1873), No. i, pp. 65-80, i pi. [An account of this Swedish paper was given in the same year in the Journ. of Bot. See Areschoug, F. W. C. (i8732).] On Trapa natans L., especially the form now living in the southernmost part of Sweden. Journ. Bot. Vol. xi. N.S. Vol. ii. 1873, pp. 239-246, i pi. [This paper is a translation, revisedby the author, of Areschoug, F.W.C.(i873').] Recherches morphologiques sur le Lobelia Dortmanna L. Revue gen. de Bot. T. xxiv. 1912, pp. 465-478, 1 8 text-figs. (A description of the anatomy of this species, and, for com- parison, of the terrestrial species, L. urens and L. erinus.} Vorarbeiten zu einer Uebersicht der phanerogamen Meergewachse. Linnaea, Bd. 35, N.F. Bd. i. 1867- 1868, pp. 152-208. (A systematic account of the marine Hydrocharitaceae and Potamogetonaceae, the synonymy and distribution being dealt with in detail.) t)ber die Phanerogamen des rothen Meeres, besonders Schizotheca Hemprichii Ehrb., Phucagrostis rotundata Ehrb. und Phucagrostis ciliata. Sitzungs-Berichte d. Gesellsch. naturforsch. Freunde zu Berlin, Dec. 20, 1870, pp. 83-85. [This brief descriptive account of the marine Phanerogams of the Red Sea should be read in conjunction with Magnus, P. (iSyo2).] Ueber Schwimmblatter bei Ranunculus sceleratus. Sitzungs-Ber. d. Gesellsch. naturforsch. Freunde zu Berlin, May 20, 1873, pp. 53-55. (The first record of the occurrence of floating leaves in this species.) Vorlaufiger Bericht iiber die botanischen Ergebnisse der Rohlfs'schen Expedition zur Erforschung der libyschenWiiste. (Schluss.) Bot. Zeit. Jahrg.32, 1874, pp. 641-647. (In this paper mention is made of the occurrence of Naias graminea, Del. in the rice fields both of Egypt and Upper Italy.) Ascherson, P. (1875) [PP- i35» 302] Ascherson, P. (1883) Ascherson, P.I and V (1907) Graebner, P. ) [pp. 133, 291, 315] Ascherson, P.j and V (1889) Gurke, M. ) Ascherson, P. Askenasy, E. (1870) [pp. 144, 228 and Fig. 126, p. 196] Aublet, F. (1775) [P- Auge de Lassu (1861) [p. 109] Bachmann, H. (1896) [pp. 32, 195] BIBLIOGRAPHY Die geographische Verbreitung der Seegraser, in Dr G. von Neumayer's Anleitung zu wissenschaft- lichen Beobachtungen auf Reisen, 1875, pp. 358-373 (also later editions). (A detailed and suggestive account of the distribution of the marine members of the Potamogetonaceae and Hydrochari- taceae.) Bemerkungen iiber das Vorkommen gefarbter Wurzeln bei den Pontederiaceen, Haemodoraceen und einigen Cyperaceen. Ber. d. deutsch. Bot. Gesellsch. Bd. i. 1883, pp. 498-502. (The author describes the blue or pale lilac colouring of the roots of several genera of Pontederiaceae.) Potamogetonaceae, in Das Pflanzenreich, iv. n (herausgegeben von A. Engler), 184 pp., 221 text-figs. Leipzig, 1907. (An authoritative account of all the species, Ascherson being responsible for the marine forms.) Hydrocharitaceae, in Die Natiirlichen Pflanzen- familien, n. i (Engler, A. and Prantl, K.). Leipzig, 1889, pp. 238-258, ii text-figs. (A systematic treatment of the family.) See Delpino, F. and Ascherson, P. (1871). Ueber den Einfluss des Wachsthumsmediums auf die Gestalt der Pflanzen. Bot. Zeit. Jahrg. 28, 1870, pp. 193-201, 209-219, 225-231, 2 pis. (An account of the structure and development of Ranunculus aquatilis, L. and R. divaricatus, Schr. The chief feature of the work is the experimental investigation into the effect of land or water conditions on these two species.) Histoire des plantes de la Guiane Fran£oise, T. i. London and Paris, 1775. (On pp. 582-584 there is the first account of the Podoste- maceous genus Mourera. The author notes that the plant grows on rocks in rapidly running water and is entirely sub- merged with the exception of the flowers.) Analyse du memoire de Gaetan Monti sur I'Aldro- vandia, suivie de quelques observations sur 1'irrita- bilite des follicules de cette plante. Bull, de la Soc. bot. de France, T. vm. 1861, pp. 519-523. (An analysis of Monti's original memoir on this plant, published between 1737 and 1747, followed by the first record of the closure of the leaves when irritated.) Submerse Blatter von Nymphaea alba. Landformen von Nymphaea alba. Ber. d. Schweiz. bot. Gesellsch. Heft vi. 1896 (Jahresber. d. ziircher. bot. Gesellsch.), pp. [n] and [12]. [The author describes certain cases of the occurrence of the submerged leaves of Castalia (Nymphaea) alba, and also of a land form which he found in three locali ties in the dry summer of 1895.] Bailey, C. (1884) [pp. 237, 275, 303] Bailey, C. (1887) [P- Baillon, H. (1858) [P- Balfour, I. B. (1879) [p. 129 and Fig. 87, p. 130] Barbe, C. (1887) BIBLIOGRAPHY Notes on the Structure, the Occurrence in Lancashire, and the Source of Origin, of Naias graminea Delile, var. Delilei Magnus. Journ. Bot. Vol. xxn. 1884, PP- 3°5~333> 47 text-figs., 4 pis. [This account of an Egyptian species, which has been introduced into Lancashire, in some points supplements Magnus, P. (I8701). Magnus, P. (1883), Ascherson, P. (1874) andWeiss, F. E. and Murray, H. (1909) deal with the same plant.] Forms and Allies of Ranunculus Flammula L. Journ. of Bot. xxv. 1887, pp. 135-138. (In this paper the existence of a form of Ranunculus Flammula with floating leaves is recorded.) Recherches sur 1'organogenie du Callitriche et sur ses rapports naturels. Bull, de la Soc. bot. de France, T. v. 1858, pp. 337-341. (A defence of the Euphorbiaceous affinity of Callitriche, based upon the structure and development of the gynaeceum.) On the Genus Halophila. Trans, and Proc. Bot. Soc. Edinburgh, Vol. xm. 1879, pp. 290-343, 5 pis. [A full account of two species of this genus collected by the author on the reefs surrounding the island of Rodriguez; Solereder, H. (1913), pp. 46, 47, discusses Balfour's material from the systematic standpoint.] See Dangeard, P. A. and Barbe, C. (1887). Barber, C. A. (1889) On a change of Flowers to Tubers in Nymphaea [pp. 36, 225 and Lotus, var. monstrosa. Ann. Bot. Vol. iv. 1889- Fig. 19, p. 37] l89i. PP- I05-H6, i pi. (An account of a case of the replacement — under cultivation — of flowers by tubers, which, when detached were capable of reproducing the plant.) Barneoud,F. M. (1848) Memoire sur Tanatomie et 1'organogenie du Trapa [p. 207] natans (Linn.). Ann. d. sci. nat. Ser. in. Bot. T. ix. 1848, pp. 222-244, 4 pis. (This early description of Trapa natans includes a study of the germination, anatomy and floral development.) Barratt, K. (1916) [p. 185 and Fig. 120, p. 185] in the Stem of 1916, pp. 91-99, The Origin of the Endodermis Hippuris. Ann. Bot. Vol. xxx. 6 text-figs. (The author's results regarding the apical anatomy of the stem of Hippuris are in general agreement with those of Schoute.) Barthelemy, A. (1883) Sur la respiration des plantes aquatiques ou des plantes aquatico-aeriennes submerges. Comptes rendus de 1'acad. des sciences, Paris, T. 96, 1883, pp. 388-390. (An account of experiments on the assimilation and respiration of aquatic plants, from which the author concludes that "la respiration speciale des organes verts ne peut a voirl' importance cosmique qu'on lui attribue. ") BIBLIOGRAPHY 353 Batten, L. (1918) [p. 188] Bauhin, G. (1596) [p- 9] Bauhin, G. (1620) [p. 9 and Fig. 3, p. u] Bauhin, G. (1623) [P- 27] Belhomme, (1862) [p. 219] Benjamin, L. (1848) [PP- 97, 99, 101] Bennett, A. (1896) Bennett, A. (1913) Bennett, A. (1914) [P- 551 Bennett, A. Observations on the Ecology of Epilobium hirsutum. Journ. Ecology, Vol. 6, 1918, pp. 161-177, 15 text- ligs. [A fully illustrated account of the "aerenchyma" of this species — a tissue whose existence had previously been recorded by Lewakoffski, N. (I8731) and Schenck, H. (1889).] Phytopinax seu Enumeratio Plantarum...Basileae per Sebastianum Henricpetri 1596. (Bauhin describes the germinating tuber of Sagittaria *as "Gramen bulbosum," p. 21.) Prodromes Theatri Botanici...Francofurti ad Moe- num, Typis Pauli Jacobi, impensis Joannis Treudelii, 1620. [Bauhin gives a figure (p. 4) of " Gramen bulbosum aquaticum" to which he has already referred in Bauhin, G. (1596).] Pinax Theatri Botanic! . . .Basileae Helvet. Sumptibus et typis Ludovici Regis, 1623. [The submerged leaves of Nymphaea (Castalia) alba are described on p. 193.] Note sur les bourgeons reproducteurs du Ranunculus Lingua. Bull, de la Soc. bot. de France, T. ix. 1862, p. 241. (A note on the wintering of this species.) Ueber den Bau und die Physiologic der Utricularien. Bot. Zeit. Jahrg. 6, 1848, pp. 1-5, 17-23, 45-50, 57-61, 81-86. (This paper, which contains some interesting observations, was written before the insectivorous nature of the bladders was recognised.) Fortschritte der schweizerischen Floristik. Potamo- geton. Ber. d. Schweiz. bot. Gesellsch. Heft vi. 1896, PP- 94-99- (A systematic enumeration of the results obtained by the author in the course of a revision of the principal Swiss herbaria.) Remarks on Some Aquatic Forms and Aquatic Species of the British Flora. Trans. Bot. Soc. Edinb. Vol. xxvi. 1917 (for 1911-1915), Part n. 1913, pp. 21-27. (Notes relating to the occurrence and nomenclature of some of the aquatic forms and species described by West, Gliick, etc.) Hydrilla verticillata Casp. in England. Journ. Bot. Vol. LII. 1914, pp. 257-258, i pi. (This plant, which is new to the British flora, has been found growing at Estwaite Water associated with Naias flexilis, etc.) See Fryer, A., Bennett, A. and Evans, A. H. (1898- A. w. P. 23 BIBLIOGRAPHY Geologic History indicated by the Fossiliferous Deposits of the Wilcox Group (Eocene) at Meridian, Mississippi. U.S. Geol. Survey. Professional Paper 1 08 E. Shorter contributions to general geology, 1917, Washington, pp. 61-72, 3 pis., i text-fig., i map. (This memoir contains an account with map of the past and present distribution of the genus Nelumbo.) The Prickle-pores of Victoria regia. Ann. Bot. Vol. i. 1887-1888, pp. 74-75. [The author criticises the account of these structures given by Trecul, A. (1854), and concludes that the function of the spines is probably merely protective.] Sur les diaphragmes des canaux aeriferes des plantes. Revue gen. de Bot. T. 24, 1912, pp. 233-243, i pi. (In this paper the diaphragms crossing the intercellular spaces of the stems aad leaves of certain aquatics are described, and they are figured in the cases of Sagittaria sagittifolia, Pontederia cor data and Potamogeton natans.) Ueber die durchsichtigen Punkte in den Blattern. Flora, N. R. Jahrg. XLII. (G. R. Jahrg. LXVII.) 1884, PP- 49-57, 97-H2, 136-144, 204-210, 223-225, 275- 283, 291-299, 339-349, 355-37°, 37I~386- (The transparent dots on the leaves of Nymphaeaceae are referred to on pages 100-102.) See Paillieux, A. and Bois, D. (1888). Weitere Mittheilung iiber die wasserleitenden Gewebe. Pringsheim's Jahrb. f. wissen. Bot. Bd. xxi. 1890, PP. 505-519. (An account of an experimental investigation of the transpira- tion stream in Myriophyllum proserpinacoides, when the plant is growing with its leafy shoots above water.) Bolle, C. (1861-1862) Notiz iiber die Alismaceenformen der Mark. Ver- handl. d. bot. Vereins Provinz Brandenburg, Heft. in. and iv. 1861-1862, pp. 159-167. [An account of certain forms of Sagittaria andAlisma found by the author. A more modern discussion of the subject will be found in Gliick, H. (1905).] Bolle, C. (1865) Eine Wasserpflanze mehr in der Mark. Verhandl. d. [p. 210] bot. Vereins Provinz Brandenburg, Jahrg. 7, 1865, pp. 1-15. [See note on Bolle, C. (1867).] Bolle, C. (1867) Weiteres iiber die fortschreitende Verbreitung der [p. 210] Elodea canadensis. Verhandl. d. bot. Vereins Provinz Brandenburg, Jahrg. 9, 1867, pp. 137-147. [This paper and Bolle, C. (1865) record the way in which Elodea, at that date a comparative rarity, was spreading over Germany.] Bonpland, A. See Humboldt, A. de, and Bonpland, A. (1808). 354 Berry, E. W. (1917) [p. 38 and Fig. 21, P- 39] Blake, J. H. (1887) Blanc, M. le (1912) [p. 183 and Figs. 8, p. 19, and 1 1 8, p. 184] Blenk, P. (1884) [P- 37] Bois, D. Bokorny, T. (1890) [p. 261] BIBLIOGRAPHY 355 Boresch, K. (1912) [P- 154] Bornet, E. (1864) [p. 125 and Fig. 83, p. 124] Borodin, J. (1870) [pp. 86, 169 and Fig. 163, p. 268] Bottomley, W. B. (1917) [p. 287] Boulger, G. S. (1900) [P- 321] Brand, F. (1894) [pp. 27, 159] Brongniart, A. (1827) [P- 309] Brongniart, A. (1833) [P- 76] Brongniart, A. (1834) [p. 164] Die Gestalt der Blattstiele der Eichhornia crassipes (Mart.) Solms in ihrer Abhangigkeit von verschie- denen Faktoren. Flora, N.R. Bd. 4 (Ganze Reihe, Bd. 104), 1912, pp. 296-308, i pi., 3 text-figs. (This paper describes a series of experiments which show that the inflated form of petiole in Eichhornia crassipes can be induced by full light, low temperature and a free-swimming life, whereas the converse conditions tend to be associated with the elongated form of petiole.) Recherches sur le Phucagrostis major Cavol. Ann. d. sci. nat. Ser. v. Bot. T. i. 1864, pp. 5-51, n pis. (This finely illustrated memoir gives a singularly complete account of the structure and life-history of the plant now called Cymodocea aequorea, Kon.) Ueber den Bau der Blattspitze einiger Wasser- pflanzen. Bot. Zeit. Jahrg. 28, 1870, pp. 841-851, i pi. [A description of the stomates which occur in small numbers near the apices of the submerged leaves of Callitriche and Hippuris. Mention is also made of the peculiar oil-containing processes at the tips of the leaves of Myriophyllum and Cerato- phyllum. For a criticism of this paper see Magnus, P. (1871).] Some Effects of Organic Growth-Promoting Sub- stances (Auximones) on the Growth of Lemna minor in Mineral Culture Solutions. Proc. Roy. Soc. B, Vol. 89, 1917, pp. 481-507, 2 pis. (By means of comparative cultures it is shown that Duckweed cannot be kept healthy in solutions with only mineral salts — soluble organic matter is essential.) Aquatic Plants. Journ. Roy. Hort. Soc. Vol. 25, 1900, pp. 64-77. (A suggestive general account of hydrophytes, with a systematic appendix showing the independent origin of the aquatic habit in a comparatively small number of Cohorts.) Ueber die drei Blattarten unserer Nymphaeaceen. Bot. Centralbl. Bd. LVII. 1894, pp. 168-171. (A brief account of the submerged, floating and air leaves of Nymphaea lutea and Castalia alba.) Memoire sur la Generation et le DeVeloppement de 1'Embryon dans les vegetaux phan6rogames. Ann. des sci. nat. Vol. 12, 1827, pp. 14-53, 145-172, 225-296, ii pis. (On p. 253 et seq. the author compares the embryo of Cerato- phyllum with that of Nelumbo.) Note sur la structure du fruit des Lemna. Archives de Botanique, T. n. 1833, pp. 97-104. (An account of the structure of the seed and fruit in Lemna minor and L. gibba.) Nouvelles recherches sur la structure de 1'^piderme des Ve"ge"taux. Ann. d. sci. nat. Ser. n. T. i. Bot. 1834, pp. 65-71, 2 pis. [On p. 68 the author records the discovery of chlorophyll in the epidermis of the leaves of Potamogeton lucens and the existence of "une pellicule tout-a-fait incolore" (=cuticle) on the surface of the epidermal layer. In PL III, Fig. 5, the characters of the epidermis are clearly demonstrated.] 23—2 356 Brown, C. Harrington (1876) Brown, R. (1814) [P- Brown, W. H. (1911) [p. 286] Brown, W. H. (1913) [pp. 253, 264, 265] Bruyant, C. (1914) [p. 291] Buchenau, F. (1857) Buchenau, F. (1859) [pp. 217, 232] Buchenau, F. (1865) Buchenau, F. (1866) [P- 245] Buchenau, F. (1882) [P- i?] BIBLIOGRAPHY Canoe and Camp Life in British Guiana, xi + 400 pp., 10 pis. and map. London, 1876. (On p. ii some Podostemaceae occurring in the Cuyuni River are described under the name of Lads spp.) General remarks on the Botany of Terra Australis. 89 pp. Reprinted in the Miscellaneous Botanical Works of Robert Brown, Vol. I. 1866. (The author includes CaUitriche in the Halorageae; see p. 22.) The Plant Life of Ellis, Great, Little, and Long Lakes in North Carolina. Contributions from the U.S. National Herbarium, Vol. 13, Part 10 (Misc. Papers), Washington, 1911, pp. 323-341, i text-fig. (An account from the ecological standpoint of the plant life of these lakes, special attention being paid to the relation of soils to aquatic vegetation.) The Relation of the Substratum to the Growth of Elodea. The Philippine Journal of Science, C, Botany, Vol. vin. 1913, pp. 1-20. (An important experimental study on the factors affecting the growth of Elodea, especially the CO8 supply.) Les Tourbieres du massif Mont-Dorien. Annales de Biologic Lacustre, T. vi. Fasc. 4, 1914, pp. 339-391, i map, 14 text-figs. (This memoir contains an ecological study of the peat bogs of this region.) Ueber die Bliithenentwickelung von Alisma und Butomus. Flora. N.R. Jahrg. xv. (G.R. Jahrg. XL.) 1857, pp. 241-254, i pi. (A description of the development of the parts of the flower in Alisma Plantago and Butomus umbellatus, with a briefer mention of Sagitlaria sagittifolia.) Zur Naturgeschichte der Littorella lacustris L. Flora, N.R. Jahrg. xvn. (G.R. Jahrg. XLII.) 1859, pp. 81-87, 464, 705-706, i pi. (A study of the external morphology of the flowering land form and the sterile water form of this species.) Morphologische Studien an deutschenLentibularieen. Bot. Zeit. Jahrg. 23, 1865, pp. 61-66, 69-71, 77-80, 85-91, 93-99, 2 pis. (In the 3rd and later parts of this memoir the branching and flower development of Utricularia are dealt with.) Morphologische Bemerkungen uber Lobelia Dort- manna L. Flora, N.R. Jahrg. 24 (G.R. Jahrg. 49), 1866, pp. 33-38, i pi. (An account of the germination and general morphology of this species.) Beitrage zur Kenntniss der Butomaceen, Alismaceen und Juncaginaceen. Bot. Jahrbucher (Engler's), Bd. n. 1882, pp. 465-510. [This paper is intended to supplement and correct Micheli's monograph of the same group; see Micheli, M. (1881).] BIBLIOGRAPHY 357 Buchenau, F. (I9031) Alismataceae, in Das Pflanzenreich, iv. 15 (heraus- [PP- 9. 3*4] gegeben von A. Engler), Leipzig, 1903, 66 pp., 19 text-figs. (The standard systematic account of this family.) Buchenau, F. (i9<>32) Butomaceae, in Das Pflanzenreich, iv. 16 (heraus- gegeben von A. Engler), 12 pp., 5 text-figs. 1903. (An authoritative account of the species of this family which includes water plants such as Hydrocleis nymphoides.) Burgerstein, A. (1904) Die Transpiration der Pflanzen. x + 283 pp., 24 [pp. 266, 267] text-figs. Jena, 1904. [This critical compilation contains a chapter (xxvi. " Guttation, Hydathoden") dealing with the elimination of liquid water from the leaves. The case of water plants is discussed on pp. 195-197.] Burkill, I. H. See Willis, J. C. and Burkill, I. H. (1895). Burns, G. P. (1904) Heterophylly in Proserpinaca palustris. L. Ann.Bot. [pp. 160, 161] Vol. xvm. 1904, pp. 579-587, i pi. [An account of experimental work on the conditions deter- mining the formation of leaves of the "land- type" and "water- type." This paper should bereadin conjunction withMcCallum, W. B. (1902), on which it is based.] Burrell, W. H.I Botanical Rambles in West Norfolk, with notes on and >• (1911) the genus Utricularia. Trans. Norfolk and Norwich Clarke, W. G.) Naturalists' Society, Vol. ix. 1914 (Pt n. 1911), [p. 215] pp. 263-268. (These notes contain a reference to remarkably luxuriant growth observed in Utricularia.) Biisgen, M. (1888) Ueber die Art und Bedeutung des Thierfangs bei [pp. 93, 94, 95] Utricularia vulgaris L. Ber. d. deutsch. bot. Gesellsch. Bd. vi. 1888, pp. Iv-lxiii. (The author discusses the function of the bladders in this species and shows experimentally that the carnivorous habit is an advantage.) Caldwell, O. W. (1899) On the Life-history of Lemna minor. Bot. Gaz. Vol. [p. 76] xxvn. 1899, pp. 37-66, 59 text-figs. (In this memoir special attention is paid to the gametophytes and fertilisation.) Cambessedes,J. (1829) Note sur les filatinees, nouvelle famille de plantes. [p. 311] Mem. du museum d'histoire nat. T. xvm. 1829, pp. 225-231. (The author proposes to remove Elatine, Bergia and Merimea from the Caryophyllaceae and to place them in a separate family. He remarks on certain resemblances which they show to the Hypericineae.) Campbell, D.H. (1897) A Morphological Study of Naias and Zannichellia. Proc. Cal. Acad. Sci. Ser. in. Botany, Vol. i. 1897— 1900, pp. 1-70, 5 pis. (In this memoir special attention is paid to the anatomy and the gametophytes.) 358 Candolle, Alphonse P. de (1855) [p. 296] Candolle, Auguste P. de (1827) [pp. 12, 337] Cario, R. (1881) Caspary, R. (1847) Caspary, R. Caspary, R. (1856 2) [p. 214] Caspary, R. (1857) Caspary, R. (I8581) Caspary, R. (i8s82) [PP-55,56, 173, 210, 211] Caspary, R. (i8s83) BIBLIOGRAPHY Geographic Botanique. Paris, T. n. 1855. (Pages 998-1006 deal with the distribution of aquatic species. After showing how widely these plants are distributed, the author concludes that the facts are scarcely explicable except on the ground that there have been multiple centres of creation.) Organographie vegetale. Paris, 1827. (Vol. i. Book 2, Chap. in. contains the first enunciation of the phyllode theory of the Monocotyledonous leaf.) Anatomische Untersuchung von Tristicha hypnoides Spreng. Bot. Zeit. Jahrg. 39, 1881, pp. 25-33, 41-48, 57-64> 73-82, i pi. [The author obtained material of this plant in Guatemala. The present paper forms an anatomical monograph of the species which was incompletely treated in Tulasne, L. R. (1852). The part of the plant which Cario describes as the "thallus" is now generally regarded as representing the root-system.] Ueber Elatine Alsinastvum und Trapa natans, Ver- handl. des naturhistorischen Vereines der preuss. Rheinlande, Jahrg. 4, 1847, pp. in, 112. (A brief note on a new locality for Elatine, and on the absence of Trapa in the neighbourhood of Bensberg.) Les Nympheacees fossiles. Ann. des sci. nat. Ser. iv. Bot. T. vi. 1856, pp. 199-222, 2 pis. (An account of the remains of this family found in Tertiary beds.) Ueber die tagliche Periode des Wachsthums des Blattes der Victoria regia Lindl. und des Pflanzen- wachsthums iiberhaupt. Flora, N.R. Jahrg. xiv. (G.R. Jahrg. xxxix.) 1856, pp. 113-126, 129-143, 145-160, 161-171. (A detailed study of the growth of the leaves of Victoria regia in a hot-house. The maximum growth in 24 hrs was 30-8 cms. in length, and 36-7 cms. in breadth.) Note sur la division de la famille des Hydrocharidees, proposee par M. Chatin. Bull, de la Soc. bot. de France, T. iv. 1857, pp. 98-101. [A criticism of views expressed in Chatin, A. (1856).] Eine systematische Ubersicht der Hydrilleen. Mon- atsber. d. Konig. Preuss. Akad. d. Wiss. Berlin, 1858 (for 1857), pp. 39-5 1- (A systematic account of the tribe of the Hydrocharitaceae which includes Elodea, etc.) Die Hydrilleen (Anacharideen Endl.). Pringsheim's Jahrb.'f. wiss. Bot. Bd. i. 1858, pp. 377~5I3, 5 Pls- (A very important monograph of that tribe of the Hydro- charitaceae which includes Hydrilla, Elodea and Lagarosiphon. The standpoint is systematic, but a good deal of anatomical work is included.) Die Bliithe von Elodea canadensis Rich. Bot. Zeit. Jahrg. 16, 1858, pp. 313-317, J P1- (A description of the female flower based on living material.) BIBLIOGRAPHY 359 Caspary, R. (1858*) Sur YAldrovanda vesiculosa. Bull, de la Soc. hot. de [p. in] France, T. v. 1858, pp. 716-726. [The observations in this paper are expanded and illustrated in Caspary, R. (1859 and 1862).] Caspary, R. (1859 Aldrovanda vesiculosa Monti. Bot. Zeit. Jahrg. 17, and 1862) 1859, pp. 117-123, 125-132, 133-139, 141-150, 2 pis. [pp. no, 239 and Aldrovandia vesiculosa. Bot. Zeit. Jahrg. 20, 1862, Fig. 75, p. in] pp. 185-188, 193-197, 201-206, i pi. [These papers form a monograph of this species. An abstract of part of Caspary's work on the subject is also to be found in Flora, N.R. Jahrg. xvn. (G.R. Jahrg. XLII.) 1859, pp. 140-143.] Caspary, R. (1860) Bulliarda aquatica D.C. Schriften d. konig. phys.-6k. [p. 234] Gesellsch. zu Konigsberg, Jahrg. i. 1861 (for 1860), pp. 66-91, 2 pis. (A monograph of this aquatic member of the Crassulaceae, now known as Tillaea aquatica L.) Caspary, R. (1861) Nuphar luteum L. var. rubropetalum. Schriften d. [p. 276] konig. phys.-6k. Gesellsch. zu Konigsberg, Jahrg. n. 1862 (for 1861), pp. 49-50, i pi. (A description, illustrated with a coloured plate, of a variety of Nymphaea lutea with red petals.) Caspary, R. (iSyo1) Neue und seltene Pflanzen Preussens. Schriften d. konig. phys.-6k. Gesellsch. zu Konigsberg, Jahrg. xi. 1871 (for 1870), pp. 61-64. (These field notes include an account of certain varieties of Castalia alba.) Caspary, R. (iSyo2) Welche Vogel verbreiten die Samen von Wasser- [p. 300] pflanzen? Schriften d. konig. phys.-6k. Gesellsch. zu Konigsberg, Jahrg. xi. 1871 (for 1870), Sitzungsber. p. 9. (This note emphasizes our ignorance of the part played by water birds in the distribution of water plants.) Caspary, R. (1875) Die geographische Verbreitung der Geschlechter von [p. 54] Stratiotes aloides L. Sitzungs-Ber. d. Gesellsch. Natur- forsch. Freunde zu Berlin, 1875, pp. 101-106. (An account of the distribution of this species, supplementing and criticising previous work, and showing that though in some regions female plants alone are present, no region is known in which male plants appear exclusively.) Cavolini, F. j, ^ Zosterae Oceanicae Linnei AN0H2IS. Contem- (Caulinus,P.)J ' ' platus est Philippus Caulinus Neapolitanus. Annis [p. 125] 1787 et 1791, 20 pp., i pi. Neapoli, 1792. [An account of the flowering and vegetative organs of Posidonia Caulini = " Zoster a oceanica." This paper and Cavolini, F. (1792 2) are analysed in Delpino, F. and Ascherson,P. (1871).] Cavolini, F. \ , 2> Phucagrostidum Theophrasti AN®H^I2. Contem- (Caulinus,P.)j ^'^ ' platus est Philippus Caulinus Neapolitanus. Anno [p. 125] 1792, 35 pp., 3 pis. Neapoli, 1792. (An account with good figures of the vegetative and flowering structure of Cymodocea aequorea = " Phucagrostis major" and Zostera nana = " Phucagrostis minor.'") 36° Chamberlain, C. J. Chatin, A. (1855*) [pp. 164, 1 66] BIBLIOGRAPHY See Coulter, J. M. and Chamberlain, C. J. (1904). Note sur la presence de matiere verte dans 1'epiderme des feuilles de I'Hippuris vulgaris, du Peplis portula, des Jussiaea longifolia et /. lutea, de I'lsnardia palustris et du Trapa natans. Bull, de la Soc. bot. de France, T. n. 1855, pp. 674-676. (The object of this note is to draw attention to the existence in many water plants of an epidermis supplied with stomates and also containing chlorophyll. The author points out that this type of epidermis is well adapted to amphibious life.) Memoire sur le Vallisneria spiralis, L. 31 pp., 5 pis. Paris, 1855. (The morphology, anatomy and floral structure are dealt with in detail, and there is a habit drawing showing male and female plants.) Anatomic comparee des vegetaux, Livraison i and 2, pp. 1-96, 20 pis. Paris, 1856. (The first part of this work deals with Monocotyledonous water plants. It is fully illustrated but singularly inaccurate.) Note sur le cresson de fontaine (Sisymbrium Nastur- tium. L., Nasturtium officinale R. Br.) et sur sa culture. Bull, de la Soc. bot. de France, T. v. 1858, pp. 158-166. (This economic paper deals with the cultivation of the Water- cress.) Faits d'anatomie et de physiologic pour servir a 1'histoire de I'Aldrovanda. Bull, de la Soc. bot. de France, T. v. 1858, pp. 580-590. (This paper is of less importance than those of Caspary dealing with the same subject. Chatin and Caspary obtained the main part of their material from the same source.) Chrysler, M. A. (1907) The Structure and Relationships of the Potamo- [pp. 63, 65, 135 and getonaceae and allied Families. Bot. Gaz. Vol. XLIV. Fig- 39, P- 62] 1907, pp. 161-188, 3 text-figs., 5 pis. (A discussion of the affinities of these families is based upon a study of the anatomy of Potamogeton, Ruppia, Zostera, Phyllospadix, Cymodocea and Zannichellia, etc.) Chatin, A. (i8552) [pp. 134, 235] Chatin, A. (1856) Chatin, A. Chatin, A. (i8s82) Clarke, W. G. Clavaud, A. (1876) [P- 78] Clavaud, A. (1878) [p. 127] See Burrell, W. H. and Clarke, W. G. (1911). Sur une particularite du Lemna tvisulca L. Actes de la Soc. Linn, de Bordeaux, T. xxxi. (Ser. iv. T. i.) 1876, pp. 300-311. (A note on the occurrence of raphides in this species and their possible biological significance.) Sur le v6ri table mode de fecondation du Zostera marina. Actes de la Soc. Linn, de Bordeaux, T. xxxn. (Ser. iv. T. n.) 1878, pp. 109-115. (An account of the pollination of Zostera growing in situ, from observations made from a boat.) BIBLIOGRAPHY 361 Cloez, S. (1863) Observations sur la nature des gaz produits par les [p. 256] plantes submergees sous I'influence de la lumiere. Comptes rendus de 1'acad. des sciences, Paris, T. LVII. 1863, pp. 354-357- (The author describes experiments showing that the gas given off by aquatic plants exposed to light is a mixture of oxygen and nitrogen: he holds that this nitrogen is produced by decomposition of the substance of the plant.) Cloez, S. and) , g , Recherches sur la vegetation. Comptes rendus de Gratiolet, P. j Vfr5»J r academic des sciences, Paris, T. xxxi. 1850, pp. [p. 256] 626-629. (An early account of the gaseous exchange in submerged plants.) Clos, D. (1856) Mode de propagation particulier au Potamogeton [p. 67] crispus L. Bull, de la Soc. bot. de France, T. in. 1856, pp. 350-352. (The first account of the peculiar turions of this plant. According to the author, they are unique among organs of vegetative reproduction in their horny consistency, and also in the fact that the detached shoot grows no further, but its whole vitality is concentrated in its axillary buds.) Cohn, F. (1850) UeberAldrovandavesiculosaMonti. Flora, N.R.Jahrg. [p. no] vin. (G. R. Jahrg. xxxm.) 1850, pp. 673-685, i pi. [A description of the anatomy and morphology of this species, less detailed than that of Caspary, R. (1859 and 1862). A brief account of early references to the plant is given in an appendix.] Cohn, F. (1875) Ueber die Function der Blasen von Aldrovanda und [PP- 93, 96, no, 270] Utricularia. Cohn's Beitrage zur Biologic der Pflanzen, Bd. i. Heft 3, 1875, pp. 71-92, i pi. (The earliest memoir in which the existence of the carnivorous habit in these two genera is fully established.) Coleman, W. H. (1844) Observations on a new species of (Enanthe. Annals [pp. 150, 204] and Mag. of Nat. Hist. Vol. xm. 1844, pp. 188-191, i pi. (The author makes out what appears to be a good case for regarding Oenanthe fluviatilis as a species distinct from Oe. Phellandrium, Lamk., instead of as a mere variety of it.) Compton, R. H. (1916) The Botanical Results of a Fenland Flood. Journ. of [pp. 200, 289] Ecology, Vol. iv. 1916, pp. 15-17, 2 pis. (This paper gives an account of the effect of a nine months' period of submergence upon the flora of an area of fenland in E. Anglia, 24 square miles in extent.) Cook, M. T. (1906) The Embryology of some Cuban Nymphaeaceae. [p. 309] Bot. Gaz. Vol. 42, pp. 376-392, 3 pis. (The author's study of several genera leads him to the con- clusion that the Nymphaeaceae are anomalous Monocotyledons.) Costantin, J. (1884) Recherches sur la structure de la tige des plantes [pp. 192, 200, 201, 259] aquatiques. Annales des sci. nat. vi. Ser. Bot. T. xix. 1884, pp. 287-331, 4 pis. (A comparison of the anatomy of stems of different individuals of the same species, or of different parts of the same stem, grown in water, in air, or embedded in soil beneath water A very important contribution to the experimental anatomy of water plants.) 362 Costantin, J. (1885 [pp. 165, 1 66] Costantin, J. (i88s2) Costantin, J. (i88s3) Costantin, J. (1886) [pp. 12, 28, 30, 51, 145, 151, 155, 156] Coster, B. F. (1875) [P- 67] Coulter, J. M.) and , Chamberlain, [(I9°4) C.J. J [pp. 322, 325] Coulter, J.M. ) and Land,W.J.G.) [P- 15] Crocker, W. (1907) [P- 243] BIBLIOGRAPHY Observations critiques sur I'dpidenne des feuilles des vegetaux aquatiques. Bull, de la Soc. hot. de France, T. xxxii. (Ser. n. T. vii.) 1885, pp. 83-88 (followed by an account of a discussion in which £. Mer and P. Duchartre took part, pp. 88-92). (The author attempts to show that the influence of the aquatic medium is one of the causes of the loss of stomates in sub- merged leaves. He also maintains that submerged plants possess a true epidermis, even if stomates are absent and chlorophyll present in this layer.) Recherches sur la Sagittaire. Bull, de la Soc. bot. de France, T. xxxii. (Ser. n. T. vn.) 1885, pp. 218-223. (Observations on the heterophylly of Sagittaria sagittifolia and a comparison of the anatomy of the submerged and aerial leaves.) Influence du milieu aquatique sur les stomates. Bull, de la Soc. bot. de France, T. xxxii. (Ser. n. T. vn.) l885> PP- 259-264. [This paper forms a continuation of Costantin, J. (I8851). The author criticises Mer's view that the presence or absence of stomates is partly an hereditary character and partly due to variations in illumination and nutrition, and brings forward further evidence to show that the milieu has a great influence on the distribution of stomates.] Etudes sur les feuilles des plantes aquatiques. Ann. d. sci. nat. Ser. vn. Bot. T. 3, 1886, pp. 94-162, 5 pis. (A memoir on the morphology and anatomy of the leaves of water plants, with special reference to the effect of the environ- ment upon their structure.) Om Potamogeton crispus L. och dess groddknoppar. Botaniska Notiser, Lund, 1875, pp. 97-102, i text-fig. [This paper, which deals with the winter buds of Potamogeton crispus, is reviewed in Bot. Jahresber. (Just) Jahrg. in. 1877 (for 1875), p. 425.] Morphology of Angiosperms. x + 348 pp., 113 text- figs. London and New York, 1904. (This general work contains a number of references to water plants.) The Origin of Monocotyledony. Bot. Gaz. Vol. 57, 1914, pp. 509-519, 2 pis., 2 text-figs. (In this paper the seedling of Sagittaria variabilis is described.) Germination of Seeds of Water Plants. Bot. Gaz. Vol. 44, 1907, pp. 375-380. [The author shows experimentally that the delay in "germination of the seeds of water plants, which have not been subjected to a period of desiccation, is due to the impossibility of absorbing sufficient water through the intact seed coats. Drying followed by a soaking seems to induce rupture of the coats, and thus to allow growth to begin. The paper contains a criticism of Fischer, A. (1907).] BIBLIOGRAPHY 363 Crocker, W. and Davis, W. E. [pp. 242, 243] Cunnington, H. M. (1912) [P- 135] iard, | and > >,C. ) Dangeard, P. A. Barbe, [P- 181] 1887) Delayed germination in seed of A lisma Plantago. Bot. (1914) Gaz. Vol. 58, 1914, pp. 285-321, 8 text-figs. (A detailed study of one case, A lisma Plantago, illustrating the delay in germination so common among water plants; the dormancy of the achenes is here due to the mechanical restraint exercised by the seed coats.) Crouan (Freres) (1858) Observations sur un mode particulier de propagation [P- 93] des Utricularia. Bull, de la Soc. bot. de France, T. v. 1858, pp. 27-29. (These notes on U. minor are written without knowledge of the previous literature.) Anatomy of Enhalus acoroides (Linn, f.), Zoll. Trans. Linn. Soc. Lond. Ser. n. Bot. Vol. vn. Pt 16, 1912 (1904-1913), pp. 355-371, i pi., 13 text-figs. (A detailed account of the anatomy of this marine Angio- sperm, in which special attention is paid to the development of the various tissues.) La Polystelie dans le genre Pinguicula. Bull, de la Soc. bot. de France, T. 34, ^1887, pp. 307-309. (The authors show that the old axes of Pinguicula vulgaris may contain four or five steles, each surrounded by a well- marked endodermis.) Darwin, C. (1859) On the Origin of Species, ix + 502 pp. London, [pp. 296, 298, 300, 324] 1859. (Chapter xn. contains a section dealing with the distribution of fresh-water animals and plants, pp. 383-388.) Insectivorous Plants, x + 462 pp. 30 text-figs. London, 1875. (Chapter xiv. deals with Aldrovandia and Chapters xvu. and xvin. with Utricularia.) Insectivorous Plants. Second Edition revised by Francis Darwin, xiv + 377 pp., 30 text-figs. London, 1888. [This edition contains a certain number of additional facts and references not found in Darwin, C. (1875).] Journal of Researches into the Natural History and Geology of the... voyageof...H. M.S. 'Beagle.' London, 1890. (See reference to Gunnera on p. 298.) The Movements and Habits of Climbing Plants. ix + 2o8 pp., 13 text-figs. London, 1891. (Darwin's references to climbing roots are of interest in con- nection with the tendril roots of certain water plants.) The Power of Movement in Plants, x + 592 pp., 196 text-figs. London, 1880. [On p. 21 1 the observations made by Rodier on the movements of Ceratophyllum are discussed. See Rodier, 6. (i877x) and (18772).] Darwin, C. (1875) [PP- 93. 95, in] Darwin, C. (1888) [P- 95] Darwin, C. (1890) [p. 181] Darwin, C. (1891) [p. 206] Darwin, C. and F. (1880) [pp. 90, 161, 206] 364 Davie, R. C. (1913) [pp. 50, 287] Davis, W. E. Delpino, F. (1870) [P- 135] Delpino, F. (1871) [p. no] Delpino, F. and) Ascherson, P. J(I87I) [pp. 84, 135, 236] Desmoulins, C. (1849) [P- 27] Devaux, H. (1889) [pp. 253, 254, 256] Dodoens, R. (1578) [p. 144] Dollo, L. (1912) [P- 39] BIBLIOGRAPHY Stratiotes Aloides, Linn., near Crieff . Trans, and Proc. Bot. Soc. Edinb. Vol. xxvi. 1913, pp. 180-183, x pi- (The author regards this plant as introduced in all Scottish localities. Water more or less richly charged with lime seems to suit it best.) See Crocker, W. and Davis, W. E. (1914). Ulteriori osservazioni et considerazioni sulla dico- gamia nel regno vegetale n. Atti della Soc. Ital. di Scienze Naturali, Vol. xin. 1870, pp. 167—205. [Pp. 168-187 deal with hydrophilous plants, giving a resume of the work on their pollination up to 1870. For a German version with some additions see Delpino, F. and Ascherson, P. (1871).] Sulle Piante a Bicchieri. Nuovo Giornale Botanico Italiano, Vol. in. 1871, pp. 174-176. (A footnote on p. 175 deals with the carnivorous habits of Aldrovandia.) Federico Delpino 's Eintheilung der Pflanzen nach dem Mechanismus der dichogamischen Befruchtung und Bemerkungen iiber die Befruchtungsvorgange bei Wasserpflanzen. Mitgetheilt und mit einigen Zusatzen versehen von P. Ascherson. Bot. Zeit. Jahrg. 29, 1871, pp. 443-445, 447-459, 463-467. (This paper is based on Delpino, F. (1870) with certain additions: it consists of a critical compilation from the literature dealing with the pollination of Posidonia, Cymodocea, Halodule, Zostera, Halophila, Ruppia, Vallisneria, CcratopKyllum and Enhalus.) Feuilles du Nymphaea et du Scirpus lacustris. Actes de la Soc. Linneenne de Bordeaux, T. xvi. (Ser. n. T. vi.) 1849, pp. 63-64. (A record of the fact that the submerged leaves of Castalia were known to Gaspard Bauhin, and that the floating leaves of Scirpus lacustris were described by Scheuchzer.) Du mecanisme des echanges gazeux chez les plantes aquatiques submergees. Ann. d. sci. nat. Ser. vii. T. 9, 1889, pp. 35-179, 8 text-figs. (This may be regarded as the classic memoir on the physics of the gaseous exchange in submerged plants. It includes a dis- cussion of earlier works on the subject.) A Nievve Herball, or Historic of Plantes:... no we first translated out of French into English, by Henry Lyte Esquyer. At London by me Gerard Dewes... 1578. (This herbal contains an account of the heterophylly of the Water Buttercup.) Les Cephalopodes adaptes a la Vie Nectique Secon- daire et a la Vie Benthique Tertiaire. Zool. Jahrb. Suppl. 15, Bd. i. 1912, pp. 105-140. [In this paper Dollo applies the Law of Irreversibility to certain aquatic plants; see also Arbor, A. (1919 2).] BIBLIOGRAPHY 365 Douglas, D. (1880) [P- 55] Duchartre, P. (1855) Duchartre, P. (1858) [p. 261] Duchartre, P. [P- [1872) Notes on the Water Thyme (Anacharis alsinastrum, Bab.). Science Gossip (Hardwicke's), Vol. xvi. 1880, pp, 227-229, 4 text-figs. (The male flowers of Elodea canadensis, hitherto unknown in Britain, are here recorded from Scotland and are described and figured.) Quelques mots sur la fecondation chez la Vallisnerie. Bull, de la Soc. bot. de France, T. n. 1855, PP- 289-293. (An historical account of the different views which have been held on the question whether the male flowers of Vallisneria do or do not become detached from their pedicels and float to the surface of the water.) Recherches experimentales sur la transpiration des plantes dans les milieux humides. Bull, de la Soc. bot. de France, T. v. 1858, pp. 105-111. (The author concludes from his experiments that the transpira- tion of a terrestrial plant can continue when it is grown in a saturated atmosphere or even when it is completely immersed in water.) Quelques observations sur les caracteres anatomiques des Zostera et Cymodocea, a propos d'une plante trouvee pres de Montpellier. Bull, de la Soc. bot. de France, T. xix. 1872, pp. 289-302. [The author shows that, in the absence of the organs of fructification, Zostera and Cymodocea can be distinguished by their anatomy. This analysis of the anatomical characters of marine Angiosperms was carried much further by another French observer about twenty years later; see Sauvageau, C. and following titles.] Dudley, W. R. (1894) Phyllospadix, its systematic characters and distribu- [p. 123] tion. Zoe, San Francisco, Vol. iv. 1894, No. 4, pp. 381-385- (A revised diagnosis of this genus, and of the two species, P. Scouleri, Hook, and P. Torreyi, Wats.) Sur la nature reelle de la " fronde " et du " cotyledon " des Lemna. Bull. mens. de la Soc. Linneenne de Paris, T. i. 1874-1889, No. 19, 1878, pp. 147-149. (This author regards the thallus of Lemna as "un sympode d'embryons disposes a la suite les uns des autres.") La fecondation chez les Ceratophyllum. Bull. mens. de la Soc. Linneenne de Paris, No. 132, 1892, p. 1056. (The author describes the rising to the surface of the detached anthers, and the descent of the pollen through the water.) Duval-Jouve, J, (1864) Lettre sur la de"couverte du Coleanthus subtilis en [pp. 299, 301] Bretagne. Bull, de la Soc. bot. de France, T. xi. 1864, pp. 265, 266. (Notes on the part played by birds in the dispersal of aquatic plants.) Dutailly, G. (1878) [P- 73] Dutailly, G. (1892) [P. 85] 366 BIBLIOGRAPHY Duval-Jouve, J. (1872) Diaphragmes vasculiferes des monocotyledones aqua- [pp. 167, 183] tiques. Academic des Sciences et Lettres de Mont- pellier. Mem. de la section des sciences, T. vm. 1872-1875, pp. 157-176, i pi. (The author of this paper shows that the occurrence of dia- phragms crossing the lacunae of the leaves of aquatic Angio- sperms is more general than has hitherto been supposed, and that transverse vascular connexions between the longitudinal veins are commonly associated with such diaphragms.) Ehrhart, F. (1787) Wiedergefundene Bliite der dicken Wasserlinse (Lemna gibba L.). Ehrhart's Beitrage zur Natur- kunde, Bd. I. 1787, pp. 43-51. [An account of the finding of the flowers of Lemna gibba which had not been seen since they were described in Micheli, P. A. (1729).] Engler, A. (1877) Vergleichende Untersuchungen iiber die morpho- [pp. 74, 82] logischen Verhaltnisse der Araceae. n. Theil. Ueber Blattstellung and Sprossverhaltnisse der Araceae. Nova Acta der Ksl. Leop. -Carol. Deutschen Akad. der Naturforscher, Bd. 39, No. 4, 1877, PP- x59-232» 6 pis. (The author explains the nature of the shoot of the Lemnaceae on the basis of a close comparison with Pistia, after an exhaustive discussion of the morphology of the Araceae in general.) Engler, A. (1879) Notiz iiber die Befruchtung von Zoster a marina und [pp. 135, 315] das Wachsthum derselben. Bot. Zeit. Jahrg. 37, 1879, pp. 654-655. [A criticism of Hofmeister, W. (1852), with remarks on the method of pollination, the branching of the sterile and fertile shoots, etc.] Engler, A. (1892) Die systematische Anordnung der monokotyledoneen [?• 3*4] Angiospermen. Abhandl. d. k. Akad. d. Wiss. Berlin, 1892, Abh. ii. 1892, 55 pp. (The systematic relationships of the Helobieae are dealt with on pp. 11—20.) Engler, A. See Krause, K. and Engler, A. (1906). Ernst, A. (I8721) Ueber Stufengang und Entwickelung der Blatter von Hydrocleis nymphoides Buchenau (Limnocharis Hum- boldtii C. L. Richard). Bot. Zeit. Jahrg. 30, 1872, pp. 518-520. (A brief account of heterophylly in this species.) Ernst, A. (1872*) Ueber die Anschwellung des unter Wasser befind- [p. 191] lichen Stammtheiles von Aeschynomene hispidula H. B. K. Bot. Zeit. Jahrg. 30, 1872, pp. 586-587. (A description of the aerenchyma found in this Leguminous shrub — a native of Venezuela.) BIBLIOGRAPHY 36? Esenbeck, E. (1914) [pp. 151, 157 and Figs. 104, p. 158, and 105, p. 159] Evans, A. H. Fauth, A. (1903) [pp. 15, 18, 241, 242, 246, 248, 271, 297] Fenner, C. A. (1904) [p. in] Ferrero, F. Fischer, A. (1907) [P- 243] Fischer, G. (1907) Beitrage zur Biologie der Gattungen Potamogeton und Scirpus. Flora, N.F. Bd. 7 (G.R. Bd. 107), 1914, pp. 151-212, 59 text-figs. (An account of experimental and anatomical work on the land forms of Potamogeton and on leaf development in Scirpus lacuster and other Cyperaceae which are normally leafless. The author follows Goebel in regarding the water leaves of all these plants as youth leaves, to which the plant reverts under con- ditions of poor nutrition, rather than as direct adaptations to the medium.) See Fryer, A., Bennett, A. and Evans, A. H. (1898- 1915). Beitrage zur Anatomic und Biologie der Friichte und Sarnen einiger einheimischer Wasser- und Sumpf- pflanzen. Beihefte zum Bot. Centralblatt, Bd. xiv. 1903, pp. 327-373> 3 p!s- (The fruit and seeds of Alisma, Elisma, Sagitlaria, Butomus, Callitriche, Hippuris, Myriophyllum, Limnanthemum, Meny- anthes and Littorella are dealt with, and certain land plants are included for comparison.) Beitrage zur Kenntnis der Anatomic, Entwicklungs- geschichte und Biologie der Laubblatter und Driisen einiger Insektivoren. Flora, Bd. 93, 1904, pp. 335- 434, 16 pis. (One section of this paper is devoted to Aldrovandia.} See Gibelli, G. and Ferrero, F. (1891). Wasserstoff- und Hydroxylionen als Keimungsreize. Ber. d. deutsch. Bot. Gesellsch. Bd. xxv. 1907, pp. 108-122. [A study of the delayed germination characteristic of many water plants, which the author attributes to the lack of certain chemical stimuli. For a criticism see Crocker, W. (1907).] Focke, W. 0. (1893!) [p- Die bayerischen Potamogetonen und Zannichellien. Ber. d. Bayer. Bot. Gesellschaft, Mimchen, Bd. xi. 1907, pp. 20-162. (A detailed systematic monograph of the Bavarian Potamo- getonaceae, without illustrations.) Eihe Fettpflanze des siissen Wassers. Abhandl. naturwiss. Vereine zu Bremen, Bd. xn. Heft in. 1893, p. 408. (This paper deals with Montia rivularis Gm. and its possibly xerophytic ancestry.) Focke, W. O. (i8932) Fehlen der Schlauche bei Utricularia. Abhandl. naturwiss. Vereine zu Bremen, Bd. xn. 1893, P- 5^3. (In this brief note the author reports the discovery of a form of Utricularia vulgaris without bladders. He considers that it cannot be a hybrid between U. vulgaris and U. intermedia because it resembles U. vulgaris in all points except the absence of bladders.) 368 Foerste, A. F. (1889) [p. 216] Forel, F. A. (1901) [p. 255] Forel, F. A. (1892- 1904) [pp. 253, 278] Frank, A. B. (1872) [pp. 281, 283] Freyn, J. (1890) [p. 228] Fries, E. (1858) [p. 276] Fryer, A. (1887) [PP- 195, 330] Fryer, A., Bennett, A. and Evans, A. H. (1898-1915) [pp. 58, 195, 303] BIBLIOGRAPHY Botanical Notes. Bull. Torr. Bot. Club, Vol. xvi. 1889, pp. 266-268, i pi. (On p. 266 there is a note on the adventitious buds which arise from the base of the submerged leaves in Nasturtium lacustre. In this species marked heterophylly occurs, the submerged leaves being pinnately dissected and the air leaves simple.) Handbuch der Seenkunde. Allgemeine Limnologie (Bibl. Geog. Handbiicher herausgegeben von F. Ratzel). Stuttgart, 1901. [This general treatise on Limnology contains a chapter (pp. 161-241) on the biology of lakes.] Le Leman. Monographic limnologique. 3 vols. Lausanne, 1904. (This elaborate monograph of the Lake of Geneva throws much light on the physics and chemistry of fresh waters. The Biology of the Lake is dealt with in Vol. in. pp. 1-408.) Ueber die Lage und die Richtung schwimmender und submerser Pflanzentheile. Cohn's Beitrage zur Biologie der Pflanzen, Bd. i. (1870-1875) Heft 2, 1872, pp. 31-86. [This memoir is the record of a series of experiments which the author undertook in order to examine the influences which regulate the position and direction of floating and submerged leaves. He chiefly employed Hydrocharis, Trapa and Callitnche. For criticisms of the work see Karsten, G. (1888) and Vries, H. de(i873).] Beitrage zur Kenntniss einiger Arten der Gattung Ranunculus. Bot. Centralbl. Bd. XLI. 1890, pp. 1-6. (On p. 5 the author gives some observations on the pollination of the aquatic species of Ranunculus.) Kiirzere briefliche Mittheilungen. Ueber A vena, Datura und Nymphaea. Bot. Zeit. Jahrg. 16, 1858, P- 73- [These notes contain the record of the occurrence in a lake in Sweden (Fagersjo in Nerike) of a (Nymphaea) Castalia with rose-purple flowers, which is regarded by the author as a variety of C. alba.] Notes on Pondweeds. 6. On Land-forms of Potamo- geton. Journ. of Bot. Vol. xxv. 1887, pp. 306-310. (This paper forms one of a series of contributions made by the author to the study of this group, the majority of which are not included in this bibliography, as their interest is almost exclusively systematic. In the present paper the land forms of Potamogeton natans, P. fluitans, P. plantagineus, P. hetero- phyllus and P. Zizii are described.) The Potamogetons (Pond Weeds) of the British Isles, x + 94 pages, 60 pis., 2 text-figs. London, 1898-1915. (A systematic monograph of the genus, as far as it is represented in Britain, with fine coloured plates by R. Morgan.) BIBLIOGRAPHY 369 Gardiner, W. (1883) On the Physiological Significance of Water Glands [pp. 267, 322] and Nectaries. Proc. Camb. Phil. Soc. Vol. v. 1886 (for 1883-1886). Paper read, Nov. 12, 1883, pp. 35- 50, i pi. [In the course of this paper the author suggests (p. 43) that Dicotyledons are typically land plants while Monocotyledons are of an essentially aquatic nature.] Travels in the Interior of Brazil, xvi + 562 pp., i map, i pi. London, 1846. (This volume of travels by the .Superintendent of the Royal Botanic Gardens of Ceylon contains an account on pp. 527, 528 of the curious Utricularia nelumbi folia.) Observations on the Structure and Affinities of the Plants belonging to the natural order Podostemaceae, together with a Monograph of the Indian species. Calcutta Journ. of Nat. Hist. Vol. vn. 1847, pp. 165- 189. (This paper is chiefly systematic, but points connected with the life-history are also touched upon. The author suggests that there is an affinity between the Podostemaceae and Nepenthes.) Gaudichaud, C. (1826) Voyage autour du monde, par Louis de Freycinet. [p. 130] Botanique. vii + 522 pp. [On p. 430 the filamentous pollen of Halophila ovata and Ruppia antarctica (=Cymodocea antarctica) is mentioned.] Gardner, G. (1846) [p. 108] Gardner, G. (1847) [pp. 112, 310] Geldart, A. M. (1906) [pp. 50, 54] Stratiotes A hides L. Trans. Norfolk and Norwich Naturalists' Society, Vol. vm. 1905, pp. 181-200, i pi. [This paper forms a useful account of the Water Soldier, partly drawn from Nolte, E. F. (1825) and other sources, but also con- taining original observations on the life-history of the plant.] Geneau de Lamarliere, Sur les membranes cutinisees des plantes aquatiques. Revue gen. de Bot. T, 18, 1906, pp. 289-295. (A micro-chemical study of the epidermis and of the cells in contact with the internal lacunae in the cases of Ranunculus fluitans, Caltha palustris, Castalia alba, Myriophyllum spicatum, Hottonia palustris, Elodea canadensis, Potamogtton densus Glyceria spectabilis and Equisetum limosum.) L. (1906) [pp. 163, 260] Gibelli, G. ) and j- (1891) Ferrero, F.j Gin, A. (1909) [pp. 234, 295, 303] A. W. P. Intorno allo sviluppo dell' ovolo e del seme della Trapa natans L. Ricerche di anatomia e di morfologia. Malpighia, v. 1891, pp. 156-218, n pis. (An elaborate and fully illustrated monograph dealing with the ovary, ovule and seed of Trapa natans. The vascular anatomy of the ovary is fully described, and the development of the embryo. The authors regard the embryo as a degraded structure which cannot be homologised with normal embryos.) Recherches sur les Lythracees. 166 pages, 13 pis., 28 text-figs. These Doct. Univ. Paris, 1909. (This memoir contains information about the structure, dis- tribution, etc. of the aquatic Lythraceae.) 24 Gliick, H. (1901) [P- 44] Gluck, H. (1902) Gluck, H. (1905) [pp. 9, 19, 195. 223, 280 and Figs. 147, p. 224, 148 and 149, P- 225] Gluck, H. (1906) [Passim and Figs. 44, P. 69, 57, P- 89, 58, P. 89, 59, p. 92, 63, p. 96, 64, p. 96, 66, P- 99, 69, p. 102, 146, p. 223] Gliick, H. (1911) [pp. 145, 188, 198, 199, 200, and Figs. 95, p. 147, 128, p. 198, 129, 130 and 131, p. 199, 134 and 135. P- 203] Gliick, H. (1913) BIBLIOGRAPHY Die Stipulargebilde der Monokotyledonen. Verhandl. d. Naturhist.-Med. Vereins zu Heidelberg, N.F. Bd. 7, Heft i, 1901, pp. 1-96, 5 pis., i text-fig. (In this work the morphology and biology of the stipular structures of many Monocotyledons are described, including those found in a number of aquatic forms such as Potamoge- tonaceae, Hydrocharitaceae, etc.) Ueber die systematische Stellung und geographische Verbreitung der Utricularia ochroleuca R. Hartman. Ber. d. deutsch. bot. Gesellsch. Bd. xx. 1902, pp. 141-156, i pi. (This paper contains a good deal of information about the submerged species of Utricularia in general.) Biologische und morphologische Untersuchungen iiber Wasser- und Sumpfgewach.se. I. Die Lebens- geschichte der europaischen Alismaceen. xxiv + 312 pp., 7 pis., 25 text-figs. Jena, 1905. [The species studied were Alisma Plantago, (I..) Michalet, A . graminifolium, Ehrh., Elisma natans, Buchenau, Echinodorus ranunculoides, (L.) Engelm., E. ranunculoides var. repens,(Lam.) , Caldesia parnassifolia, (Bassi) Par!., Damasonium stellatum, (Rich.) Pers., and Sagittaria sagittifolia, L. An elaborate series of culture experiments was carried out, to determine the effect of external conditions upon these plants.] Biologische und morphologische Untersuchungen uber Wasser- und Sumpfgewachse. II. Untersuchun- gen iiber die mitteleuropaischen Utricularia- Arten, iiber die Turionenbildung bei Wasserpflanzen, sowie iiber Ceratophyllum. xvii + 256 pp., 28 text-figs., 6 pis. Jena, 1906. (An admirable account of the genus Utricularia, of 'winter- bud' formation in general, and of the biology of the genus Ceratophyllum, with special reference to the formation of 'rhizoids.') Biologische und morphologische Untersuchungen iiber Wasser- und Sumpfgewachse. III. DieUferflora. xxxiv + 644 pp., 8 pis., 105 text-figs. Jena, 1911. [A detailed study of the manner of life of those plants which grow on the margin of fresh waters and have adopted an amphibious habit. As in his previous work, the author com- bines cultural experiments with observations in the field. He shows that a large number of shore plants have aquatic forms which have remained hitherto undescribed. Like Gliick, H. (1905) and (1906) the book is beautifully illustrated and pro- vided with a useful index.] Contributions to our Knowledge of the Species of Utricularia of Great Britain with Special Regard to the Morphology and Geographical Distribution of Utricularia ochroleuca. Ann. Bot. Vol. xxvu. 1913, pp. 607-620, 2 pis., 7 text-figs. (The author records Utricularia ochroleuca from a number of stations in Great Britain and discusses the morphology, biology and distribution of this species.) BIBLIOGRAPHY Goebel, K. (1879) [P- 225] Goebel, K. (1880) [p. 12] Goebel, K. (I8891) Goebel, K. (18892) [PP- 93, 99] Goebel, K. (i8893) [P- Goebel, K. (1891) [pp. 40, 100, 103,104, 106, and Fig. 68, p. 100] 371 Bot. Zeit. Ueber Sprossbildung auf Isoetesblattern. Jahrg. 37, 1879, pp. 1-6, 4 text-figs. (A record of the replacement of sporangia by young plants in the case of certain examples of Isoetes lacustris and /. echinospora from the Vosges.) Beitrage zur Morphologic und Physiologic des Blattes. (Schluss.) Bot. Zeit. Jahrg. 38, 1880, pp. 833-845, i pl- (On pp. 833-836 the heterophylly of Sagittaria sagittifolia is described. In opposition to de Candolle, Goebel takes the view that the band-shaped leaf of Sagittaria represents the entire leaf, not merely a modified petiole.) Ueber die Jugendzustande der Pflanzen. Flora, Neue Reihe, Jahrg. 47, 1889, pp. 1-45, 6 text-figs., 2 pis. (Pp. 40-43 contain an account of the germination of Utricularia montana.) Der Aufbau von Utricularia. Flora, Neue Reihe, Jahrg. 47 (G. R. Jahrg. 72), 1889, pp. 291-297, i pi. (This paper forms a continuation of the author's previous work on Utricularia', U. affinis, U. longifolia, and U. bryophila are figured.) Pflanzenbiologische Schilderungen. Teil i. 239 pp., 9 plates, 98 text-figs. Marburg, 1889. (Pp. 166-169 deal with one of the Podostemaceae, a species of Terniola.) Morphologische und Biologische Studien. V. Utricu- laria. VI. Limnanthemum. Ann. du Jardin Bot. de Buitenzorg, Vol. ix. 1891, pp. 41-126, n pis. (In these papers certain extra-European species of Utricularia and Limnanthemum are dealt with; the vexed question of the morphology of the Utricularia shoot receives special considera- tion.) Goebel, K. (1891-1893) Pflanzenbiologische Schilderungen. Teilu. iv + 386 [Passim and Figs. 1 4, pp., 31 pis., 121 text-figs. Marburg, Lief, i, 1891, Lief. 2, 1893. [This work contains sections dealing with Utricularia (pp. 127- 160, 173-181, pis. XIV, XV) and the Podostemaceae (pp. 331-354, pis. XXVI-XXX). There is also a very important general discussion of water plants from the biological stand- point (pp. 217-373, pis. XXIV, XXV, etc.).] p. 29, 20, p. 38, 60, p. 92, 65, p. 98, 92, p. 144, 103, p. 154, 143, p. 220, 150, p. 229, 160, p. 247] Goebel, K. (1895) Ueber die Einwirkung des Lichtes auf die Gestaltung der Kakteen und anderer Pflanzen. Flora, Bd. 80, 1895, PP- 96-116, 5 text-figs. [This paper includes a short account (pp. no, in) of certain experiments upon Sagittaria which show that want of light induces this plant to return to the 'youth form' in which only band-shaped leaves are developed. Its behaviour is thus analogous to that of Phyllocactus which, under similar con- ditions, also reverts to the youth form.] 24—2 372 Goebel, K. (1896) [p- 156] Goebel, K. (1904) [p. 104 and Fig. 70, p. 104] Goebel, K. (1908) [pp. 161, 281] Goebel, K. (1913) [PP. 234, 344] Goppert, H. R. (1847) Goppert, H. R. (1848) [p. 86] Graebner, P. (1901) [p. 290] Graebner, P. Gratiolet, P. Gray, A. (1848) [P- 309] BIBLIOGRAPHY Ueber Jugendformen von Pflanzen und deren kiinstliche Wiederhervorrufung. Sitzungsber. d. math.-phys. Classe d. k. b Akademie d. Wissensch. zu Miinchen, Bd. xxvi. 1897 (f°r I^96), pp. 447-497, 1 6 text-figs. (Pp. 487-491 are devoted to heterophylly in water plants. The author regards the band-shaped submerged leaves of many Monocotyledons, not as representing a direct adaptation to the medium, but as a juvenile form of leaf which may also be produced at later stages in the life of the plant, if the external conditions are unfavourable.) Morphologische und biologische Bemerkungen. 15. Regeneration bei Utricularia. Flora, Bd. 93, 1904, pp. 98-126, 17 text-figs. (Includes an account of the formation of adventitious shoots from the leaves of the water Utricularias.) Einleitung in die experimentelle Morphologic der Pflanzen. viii + 260 pp., 135 text-figs. Leipzig and Berlin, 1908. (In this book heterophylly in amphibious plants is dealt with at some length, with special reference to Myriophyllum pro- serpinacoides and Limnophila heterophylla.) Morphologische und biologische Bemerkungen. 22. Hydrothrix Gardneri. Flora, N.F. Bd. 5 (Ganze Reihe, Bd. 105), 1913, pp. 88—100, 9 text-figs. (An investigation of a peculiar submerged member of the Pontederiaceae with 'long' and 'short' shoots and cleisto- gamic flowers.) Ueber die Schlauche von Utriculavia vulgaris und einen Farbestoff in denselben. Bot. Zeit. Jahrg. 5, 1847, pp. 721-726. (An account of the structure and development of the bladder, which the author regards as a metamorphosed "Fiederblatt- chen." He records the occurrence of blue pigment in the cells of the bladder.) Ueber den rothen Farbestoff in den Ceratophylleen. Bot. Zeit. Jahrg. vi. 1848, pp. 147, 148. (A record of the occurrence of a violet colouring matter, turning brown with age, in the cellular processes at the tips of the leaf segments in Ceratophyllum.) Die Heide Norddeutschlands. (Engler, A. und Drude, O. Die Vegetation der Erde, V.) xii + 320 pages, i map. Leipzig, 1901. (This book contains some information about the flora of low- land heath pools.) See Ascherson, P. and Graebner, P. (1907). See Cloez, S. and Gratiolet, P. (1850). Remarks on the Structure and Affinities of the Order Ceratophyllaceae. Annals of the Lyceum of Nat. Hist., New York, Vol. iv. 1848, pp. 41-50 (read Feb. 20, 1837). (The author regards Ceratophyllum as allied to the Cabombaceae and Nelumbiaceae and supports this conclusion by a com- parison of the seed characters.) BIBLIOGRAPHY 373 Greene, E. L. (1909) [P- 285] Grew, Nehemiah (1682) [P- 154] Griset, H. E. (1894) Gronland, J. (1851) [p. 127] Guppy, H. B. (1893) [pp. 35, 220, 243, 244, 297, 301, 302] Guppy, H. B. (I8941) [pp. 85, 88, 273, 274, 275, 301 and Fig. 55, p. 86] Guppy, H. B. (18942) [PP- 75, 77, 275] Guppy, H. B. (i8943) [P- 274] Guppy, H. B. (1896) [P- 274] Landmarks of Botanical History. Part I. Prior to 1562 A.D. Smithsonian Misc. Coll. Vol. 54, 1909, pp. 1-329. (On pp. 126, 127, attention is drawn to the opinions of Theo- phrastus upon the ecology of water plants.) The Anatomy of Plants. 1682. 304 pp., 83 pis. (This classic account of structural botany contains occasional references to aquatics or to subjects bearing on their study.) Circulatory Movements of Protoplasm. Science- Gossip, Vol. i. New Series, 1894, PP- 132-133, 2 text- figs. (The author draws attention to the stipules of Hydrocharis Morsus-ranae and the diaphragms of the petiole and peduncle of Alisma Plantago as affording excellent material for the observation of intracellular protoplasmic movements.) Beitrag zur Kenntniss der Zostera marina L. Bot. Zeit. Jahrg. ix. 1851, pp. 185-192, i pi. [This account of the ovules and anthers of Zostera is supple- mented and corrected by Hofmeister, W. (1852).] The River Thames as an Agent in Plant Dispersal. Journ. Linn. Soc. Bot. Vol. xxix. 1893, pp. 333-346. (An account of observations upon river drift in the Thames, Lea and Roding, with a discussion of the part played by birds in the dispersal of aquatic plants.) Water-Plants and their Ways. Science-Gossip, Vol. i. New Series, 1894. Their Dispersal and its Observa- tion, pp. 145-147. Their Thermal Conditions, pp. 178-180. Ceratophyllum demersum, pp. 195-199, i text-fig. (These short papers, though published in a popular journal, contain original observations of great importance.) On the Habits of Lemna minor, L. gibba, and L. polyrrhiza. Journ. Linn. Soc. Lond. Bot. Vol. xxx, 1895 (for 1894), pp. 323-330. [Observations on the life-history of these forms, including a detailed study of the temperature conditions necessary for germination, flowering, etc. The paper may be regarded as supplementary to Hegelmaier, F. (1868).] River Temperature. Part I. Its Daily Changes and Method of Observation. Proc. Roy, Phys. Soc. Edinburgh, Vol. xn. 1892-1894, pp. 286-312. [A more detailed consideration of the subject than in Guppy, H. B. (I8941)-] River Temperature. Part III. Comparison of the Thermal Conditions of Rivers and Ponds in the South of England. Proc. Roy. Phys. Soc. Edinb. Vol. xin. 1894-1897, pp. 204-211. [The comparison of the temperatures of ponds with that of the Thames is treated more fully in this paper than in Guppy, H. B. (I8941)-] BIBLIOGRAPHY On the Postponement of the Germination of the Seeds of Aquatic Plants. Proc. Roy. Phys. Soc. Edinburgh, Vol. xm. 1894-1897, pp. 344-359. (An account of experimental work on delayed germination of the seeds of water plants kept in water, with notes on the effect of drying, freezing and exposure to light or darkness.) Observations of a Naturalist in the Pacific between 1896 and 1899. Vol. n. Plant-dispersal, xxviii +627 pp., i pi. London, 1906. (The water-side plants of the British flora are considered in Chapters in. and iv. Note 10, pp. 535-538, records the degree of buoyancy of the seeds and seed vessels of more than 300 British plants, including a large number of aquatics. The book also contains numerous other notes on water plants, e.g. dis- tribution of Naias, p. 367.) Plants, Seeds, and Currents in the West Indies and Azores, x + 531 pages, 3 maps, i pi. London, 1917. (This book contains further developments of the author's "differentiation" hypothesis. A number of references to water plants are included.) See Ascherson, P. andGiirke, M. (1889). On some Points in the Morphology and Anatomy of the Nymphaeaceae. Trans. Linn. Soc. Lond. Ser. II. Vol. v. 1895-1901, Part 7, 1897, pp. 287-299, 2 pis. (The most important discovery recorded in this paper is that of the occurrence of clear cases of polystely in certain stem structures of the Nymphaeaceae.) Haberlandt, G. (1914) Physiological Plant Anatomy, translated from the [pp. 45, 183] fourth German edition by Montagu Drummond. xv + 777 pages, 291 text-figs., 1914. (This standard work contains many references to the structure of water plants and its interpretation.) 374 Guppy, H. B. (1897) [pp. 243, 244, 280, 301] Guppy, H. B. (1906) [pp. 88, 162, 241, 296, 297. 301, 303, 304, 305] Guppy, H. B. (1917) [PP. 303, 304, 333] Giirke, M. Gwynne-Vaughan, D. T. (1897) [PP- 33, 37. 38, 182] Hall, J. G. (1902) Hallier, E. (1859) [p. 192] Hannig, £. (1912) [pp. 260, 266] An Embryological Study of Limnocharis emarginata. Bot. Gaz. Vol. xxxin. 1902, pp. 214-219, i pi. (An account of the embryo-sac and embryo in this species.) Aedemone mirabilis Kotschy. Ein neues Schwimm- holz vom weissen Nil, anatomisch bearbeitet. Bot. Zeit. Jahrg. 17, 1859, pp. 153-156, i pi. [The anatomy of Aedemone mirabilis, Kotschy (=Herminiera Elaphroxylon, Guill. et Perr.) is described and its close resem- blance is pointed out to that of Aeschynomenepaludosa, Roxb. (=Sesbania aculeata, Poir.).] Untersuchungen iiber die Verteilung des osmotischen Drucks in der Pflanze in Hinsicht auf der Wasser- leitung. Ber. d. deutschen bot. Gesellsch. Jahrg. xxx. 1912, pp. 194-204. [On p. 200 the author gives an account of the differences between the osmotic pressure in leaf and root in certain water plants. For a criticism of his interpretation of his results see Snell, K. (1912).] BIBLIOGRAPHY 375 Hansgirg, A. (1903) Phyllobiologie. xiv + 486 pp., 40 text-figs. Leipzig, [pp. 143, 151, 154] 1903. [This book includes (pp. 52-84) a summarised account, chiefly based upon previous work, of the various types of leaf met with among aquatic plants.] Hauman-Merck, L. Sur un cas de geotropisme hydrocarpique chez (19131) Pontederia rotundifolia L. Recueil de 1'Institut Bot. [p. 239 and Fig. 155, Leo Errera, T. ix. 1913, pp. 28-32, i text-fig. P- 240] (The author shows that after fertilisation the inflorescences of this plant, which have been previously held erect above the water, bend down through an angle of 180° and dip into the water where the fruits ripen.) Hauman-Merck, L. Observations ethologiques et syst6matiques sur deux (19132) especes argentines du genre Elodea. Recueil de [PP- 55. 57> 236] 1'Instit. Bot. Leo Errera, T. ix. 1913, pp. 33-39. (An account of the morphology and mode of pollination of Elodea densa and E. callitrichoides .) Hauman, L. (1915) Note sur Hydromystria stolonifera Mey. Anales del (formerly Hauman- Museo Nac. de Hist. Nat. de Buenos Aires, T. 27, Merck) 1915, pp. 325-331. [p- 57] (The author draws attention to root dimorphism and hydro- anemophily in this species.) Hausleutner, (I85O1) Ueber Aldrovanda vesiculosa. Bot. Zeit. Jahrg. vm. [pp. in, 289] 1850, p. 600. Nachtrag zu Aldrovandia. Bot. Zeit. Jahrg. vm. 1850, pp. 831, 832. (These notes describe certain occurrences of this plant in the wild state, and give directions for its cultivation. The author shows that, in nature, it grows among reeds or protected by the leaves of Waterlilies, and that it can only be cultivated successfully if these shade conditions are reproduced.) Hausleutner, (i85O2) Ueber eine neue Nymphaea aus Schlesien. Bot. Zeit. Jahrg. vm. 1850, pp. 905-908. (This is the record of the occurrence of a new species of Castalia, called by the author Nymphaea neglecta.) Hausleutner, (1851) Ueber die Aldrovanda in Schlesien. Bot. Zeit. Jahrg. [p. in] ix. 1851, pp. 301-304. (A discussion of the anomalous distribution of Aldrovandia. It has been found in Schlesia in two lakes alone. The author thinks that it is improbable that it is distributed by water- fowl, since it perishes so rapidly on being removed from the water.) Hegelmaier, F. (1864) Monographie der Gattung Callitriche. 64 pp., 4 pis. [pp. 169, 175, 216, Stuttgart, 1864. 236* 31 J] (In this memoir the anatomy and floral structure of the genus are fully treated and all the species are described. The geo- graphical distribution and affinities are also discussed. The author returns to Robert Brown's opinion that this genus belongs to the Halorrhagideae, and he does not accept the newer view which relates it to the Euphorbiaceae.) 376 Hegelmaier, F. (1868) [PP- 73, 74, 75, 77, 80, 314 and Figs. 48, p. 76, 50, P- 79, 52, Ml] Hegelmaier, F. (1870) [P- 70] Hegelmaier, F. (1871) [PP- 73, 74, 80 and Fig. 47, p. 74] Hegelmaier, F. (1885) [P. 73] Henfrey, A. (1852) [P- 309] Henslow, G. (1893) [pp. 142, 322] Henslow, G. (1911) [pp. 322, 339] Hentze, W. (1848) Hiern, W. P. (1872) [P- 30] BIBLIOGRAPHY Die Lemnaceen. Eine Monographische Untersuch- ung. 169 pp., 16 pis. Leipzig, 1868. (This monograph deals with the family systematically and also discusses its affinities and distribution. The vegetative and floral morphology of the different genera and species, and their anatomy and biology, are also treated in detail.) Ueber die Entwicklung der Bliithentheile von Pota- mogeton. Bot. Zeit. Jahrg. 28, 1870, pp. 281-289, 297-305, 313-319, i pi. (An account of the morphology and development of the flowers and fruit of this genus, P. crispus being studied in the greatest detail.) Ueber die Fructifikationstheile von Spirodela. Bot. Zeit. Jahrg. 29, 1871, pp. 621-629, 645-666, i pi. (After writing his monograph of the Lemnaceae, the author obtained some of the very rare flowers of Spirodela polyrrhiza from N. America, on which the present illustrated account is based.) Wolffia microscopica. Bot. Zeit. Jahrg. 43, 1885, pp. 241-249. (A description of some material of this species from India.) On the Anatomy of the Stem of Victoria Regia. Phil. Trans. Roy. Soc. Lond. 1852, pp. 289-294, 2 pis. (An early account of the structure of this plant, which suffers from the fact that the only specimen available for study was partially decayed.) A Theoretical Origin of Endogens from Exogens, through Self -Adaptation to an Aquatic Habit. Journ. Linn. Soc. Bot. Vol. xxix. 1893, pp. 485-528. [An exposition of the author's theory of the aquatic origin of Monocotyledons. For a criticism of this paper see Sargant, E. (1908).] The Origin of Monocotyledons from Dicotyledons, through Self-Adaptation to a Moist or Aquatic Habit. Ann. Bot. Vol. xxv. 1911, pp. 717-744. [A further development of the views put forward in Henslow, G. (1893) with a reply to the criticisms contained in Sargant, E. (1908).] Beschreibung einer neuen Nymphaea. Bot. Zeit. Jahrg. 6, 1848, pp. 601-603. Weitere Mittheilung iiber die Untersuchung deutscher Seerosen. Bot. Zeit. Jahrg. 6, pp. 697-702, 1848. (These papers deal with several distinct forms of Castalia alba. The author leaves open the question whether these are, or are not, true species.) A Theory of the Floating Leaves in certain Plants. Proc. Camb. Phil. Soc. Vol. n. 1876 (for 1864-1876), Part XIII, read March 13, 1871, pp. 227-236. (A mathematical paper, demonstrating the advantages con- ferred on a floating leaf by a circular form.) BIBLIOGRAPHY 377 Hildebrand, F. (1861) [P- 67] Hildebrand, F. (1870) Hildebrand, F. (1885) [pp. 207, 228] Hiltner, L. (1886) [P- 233] Hochreutiner, G. (1896) [pp. 174, 204, 205, 245, 261, 281, 282 and Fig. 137, p. 206] Hochreutiner, G. (1897) Hoffmann, J.F. (1840) [P- 78] Hofmeister,W.(i852) Einige Beobachtungen aus dem Gebiete der Pflanzen- Anatomie. Herrn Professor L. C. Treviranus zur Feier seines... Doctor- Jubilaums...dargebracht. 30 pp., 2 pis. Bonn, 1861. (These miscellaneous notes include an account of the winter- buds of Potamogeton crispus, pp 24-26, with i figure.) Ueber die Schwimmblatter von Marsilia und einigen anderen amphibischen Pflanzen. Bot. Zeit. Jahrg. 28, 1870, pp. 1-8, 17-23, i pi. (A record of the occurrence of floating leaves in Marsilia quadrifolia, M. data and M. pubescens, and a comparison of the anatomy of the floating and aerial leaves in these cases, and also in Sagittaria sagittifolia and Polygonum amphibium.) t)ber Heteranthera zosterifolia. Engler's Bot. Jahr- biich. Bd. vi. 1885, pp. 137-145, i pi. [Observations on living plants of this species grown from Brazilian seed. The author draws attention to the floating leaves, which were not noticed in Solms-Laubach, H. Graf zu (1883).] Untersuchungen iiber die Gattung Subularia. Engler's Bot. Jahrbuch. Bd. 7, 1886, pp. 264-272, i pi., i text-fig. (The author concludes, from a study of their morphology and anatomy, that Subularia monticola and the forms of S. aquatica are not true species, but owe their differences to their varying environments.) fitudes sur les Phanerogames aquatiques du Rhone et du Port de Geneve. Rev. gen. de Bot. T. vm. 1896, pp. 90-110, 158-167, 188-200, 249-265, i pi., 15 text-figs. [The first part of these studies consists of a detailed account of the morphology, anatomy and development of Zannichellia palustris (pp. 90—110). The remaining instalments deal with the following branches of the physiology of submerged plants: — the ascent of water (pp. 158-167), geotropism (pp. 188-200, 249-258), hydrotropism (pp. 258-263), rheotropism (pp. 263- 264) and heliotropism (pp. 264-265).] Notice sur la Repartition des Phanerogames dans le Rhone et dans le Port de Geneve. Bull, de 1'Herbier Boissier, Annee v. No. i, 1897, pp. 1—14, i pi. (A study of the distribution and ecology of the water plants of this region.) Beitrage zur naheren Kenntniss von Lemna arrhiza nebst einigen Bemerkungen iiber L. polyrrhiza, gibba, minor und trisulca. Wiegmann's Archiv fur Natur- geschichte, Berlin, Jahrg. 6, 1840, pp. 138-163, 2 pis. (A translation of this memoir by Buchinger appeared in Ann. d. Sci. nat. Ser. n. T. xiv. Bot. pp. 223-242.) Zur Entwickelungsgeschichte der Zostera. Bot. Zeit. Jahrg. x. 1852, pp. 121-131, 137-149, i pi. [An account of the development of the pollen, ovule and embryo of Zostera, which supplements and corrects Gronland, J . ( 1 85 1 ) . Some account of Ruppia is also given. For criticism see Engler, A. (1879).] 378 BIBLIOGRAPHY Hofmeister, W. (1858) Neuere Beobachtungen iiber Embryobildung der [p. 82] Phanerogamen. Pringsheim's Jahrbiich. f. wissen- schaft. Bot. Bd. I. 1858, pp. 82-188, 4 pis. [This memoir contains some account of the ovule and embryo of the following water plants: — Alisma, p. 147, Lemna, p. 152, Naias, p. 145, Nelumbium, p. 85, Nuphar (Nymphaea), p. 83, Pistia, p. 152, Pontederia, p. 166, Trapa, p. 105, Zannichellia, p. 147.] Holm, T. (1885) [p. 129] Recherches anatomiques et morphologiques sur deux monocotyledones submergees (Halophila ' Baillonii Asch. et Elodea densa Casp.). Bihang till k. Svenska Vet.-Akad. Handlingar, Bd. 9, No. 13, 1885, 24 pp., 4 pis. [These two species are described in some detail. In the case of Halophila, this paper may be regarded as supplementary to Balfour, I. B. (1879).] The Botany of the Antarctic Voyage of H.M. Dis- covery Ships Erebus and Terror. I. Flora Antarctica. Part II. 364 pp., 198 pis. London, 1847. (On p. 334 there is an account of cleistogamy in Limosella.) Hooker, J. D. (1887) On Hydrothrix, a new genus of Pontederiaceae. Ann. Bot. Vol. i. 1887-1888, pp. 89-94, i Pi- [This paper is chiefly of systematic interest. Hydrothrix is a reduced and aberrant member of the family. See also Goebel, K. (1913).] Hooker, J. D. (1847) [PP- 23 Hope, C. W. (1902) [pp. 192, 214] The ' Sadd ' of the Upper Nile : its Botany compared with that of similar Obstructions in Bengal and American waters. Ann. Bot. Vol. xvi. 1902, pp. 495-516. (An account of the plants which play a part in the great vegetable accumulations that form barriers on the Nile, the floating vegetation of Bengal, etc.) Horen, F. van (1869) Observations sur la physiologic des Lemnacees. [pp. 74, 75, 76] Bull, de la Soc. Roy. de Bot. de Belgique, T. vni. 1869, pp. 15-88, i pi. [These observations, which deal largely with the hibernation of the Lemnaceae, are intended by the author to supplement Hegelmaier, F. (1868). For an English version of this paper see Horen, F. van (1870).] Horen, F. van (1870) On the Hibernation of Lemnaceae. Journ. Bot. Vol. vin. 1870, pp. 36-40. [This is an abridged translation by A. W. Bennett of Horen, F. van (1869).] Hovelacque, M. (1888) Recherches sur 1'appareil veg6tatif des Bignoniacees, [pp. 104, 107] Rhmanthacees, Orobanchees et Utriculari6es. Paris, 1888. (The fourth part of this book — pp. 635-745, 126 text-figs. — deals with the anatomy of the Utriculariaceae.) BIBLIOGRAPHY 379 Humboldt, A.\ Plantae Aequinoctiales. T. i. vii + 232 pp., 65 pis. de, and I (1808) Paris, 1808. Bonpland, A. ) [The aerenchyma of Mimosa lacustris (Neptunia oleracea, Lour.) [p. 189] is noticed on p. 56, but it is mistakenly described as a foreign body.] Hutchinson, J. (1916) Aquatic Compositae. Card. Chron. Vol. 59, 1916, [PP- 3!3> 321, 324] P- 3°5, 4 text-figs. (An account of Bidens Beckii, Cotula myriophylloides, Pectis aquatica and Erigeron heteromorphus.) Im Thurn, E. F. (1883) Among the Indians of Guiana, xvi + 445 pp., 10 pis., [pp. 118, 120, 300] 43 text-figs., i map. London, 1883. (This book contains references to the Podostemaceae and Victoria regia living in Guiana waters.) Im Thurn, E. F. and Oliver, D. [p. 109] (1887) The Botany of the Roraima Expedition of 1884: being Notes on the Plants observed, by Everard F. im Thurn; with a list of the Species collected, and Determinations of those that are new, by Prof. Oliver, F.R.S., F.L.S., and others. Trans. Linn. Soc. Lond. Ser. n. Vol. n. Bot. 1881-1887, Part XIII. 1887, pp. 249-300, 10 pis. (This memoir contains an account of the curious mode of life of Ulricularia Humboldtii, which lives in the water collected in the leaf axils of a Bromeliad.) Kurze botanische Mittheilungen. 6. Nymphaea alba und Nuphar luteum, 7. Potamogetondensus. 8. Dauer der Ceratophyllum-Arten. Flora, N.R. Jahrg. xi. (G.R. Jahrg. xxxvi.) 1853, pp. 527-528, i pi. Fin these notes attention is called to the stipula axillaris of Nymphaea (Cistalia) alba — the fact that the apparently opposite leaves of Potamogeton densus are really alternate — and the fact that the shoots of Ceratophyllum may vegetate through the winter.] Irmisch, T. (1854) Bemerkung iiber Hippuris vulgaris L. Bot. Zeit. [Fig. 112, p. 173] Jahrg. 12, 1854, pp. 281-287, J P1- (A detailed account of the mode of branching of the sympodial stems.) Irmisch, T. (1853) [pp. 26, 87] Irmisch, T. Irmisch, T. (i8582) [pp. 52, 271] Botanische Mittheilungen. i. Ueber Utricularia minor. Flora, Neue Reihe, Jahrg. xvi. (Ganz. Reihe, Jahrg. XLI.) 1858, pp. 33-37, i pi. (An account of the morphology of this species; the author interprets the branching in connexion with the inflorescence axis as sympodial.) Ueber das Vorkommen von schuppen- oder haar- formigen Gebilden innerhalb der Blattscheiden bei monokotylischen Gewachsen. Bot. Zeit. Jahrg. 16, 1858, pp. 177-179. (This paper records the occurrence of "squamulae intra- vaginales" in a number of Helobieae.) 380 Irmisch, T. (i8s83) [P- 59] Irmisch, T. (iSS [pp. 169, 245] Irmisch, T. (i8592) Irmisch, T. (1861) [P- 2°5] Irmisch, T. (1865) [P- 244] Ito, T. (1899) [P- 83] Jaensch, T. (I8841) [p. 192] Jaensch, T. (i8842) [p. 192] Jaggi, J. (1883) [p. 302] BIBLIOGRAPHY Ueber einige Arten aus der natiirlichen Pflanzen- familie der Potameen. Abhandl. d. naturwiss. Vereines f. Sachsen und Thiiringen in Halle, Bd. n. Berlin, 1858 (Vol. published 1861), pp. 1-56, 3 pis. (The morphology and life-history of Potamogeton natans L., P. lucens L., P. crispus L., P. obtusifolius M. et K., P. pectinatus L., and also of Zannichellia and Ruppia, are described and illustrated with the thoroughness characteristic of the author.) Bemerkungen iiber einige Wassergewach.se. Bot. Zeit. Jahrg. 17, 1859, pp. 353-356. (Notes on the morphology of Myriophyllum, Callitriche, Potamogeton trichoides, Hydrocharis and Stratiotes.) Zur Naturgeschichte des Potamogeton densus L. Flora, N.R. Jahrg. xvn. (G.R. Jahrg. XLII.) 1859, pp. 129-139, i pi. [This paper is supplementary to Irmisch, T. (i8583).] Ueber Polygonum amphibium, Lysimachia vulgaris, Comarum palustre und Menyanthes trifoliata. Bot. Zeit. Jahrg. 19, 1861, pp. 105-109, 113-115, 121-123, i pi. (A description of the seedlings and of the development and morphology of the mature plant in the four species named.) Beitrag zur Naturgeschichte des Stratiotes A hides. Flora, N.R. Jahrg. xxm. (G.R. Jahrg. XLVIII.) 1865, pp. 81-91, i pi. (The fruit, seed and seedling of Stratiotes aloides, the seedling of Naias major and the axillary shoots of Hydrocharis Morsus- ranae, and Vallisneria spiralis are described and figured in this paper.) Floating-apparatus of the Leaves of Pistia Stratiotes, L. Ann. Bot. Vol. xm. 1899, p. 466. (Notes on the structure and mode of flotation of the leaves of this plant which was studied in its native habitat.) Nachtrag zur Kenntniss von Herminiera Elaphroxy- lon G.P.R. Ber. d. deutsch. bot. Gesellsch. Bd. n. 1884, pp. 233-234. (A note on the occurrence of Aedemone mirabilis, Kotschy, in Senegambia.) Zur Anatomic einiger Leguminosenholzer. Ber. d. deutsch. bot. Gesellsch. Bd. n. 1884, pp. 268-292, i pi. [This memoir includes an account of the structure of the wood in Herminiera Elaphroxylon (Aedemone), Aeschynomene and Sesbania. For a criticism see Klebahn, H. (1891).] Die Wassernuss, Trapa natans L., und der Tribulus der Alten. Zurich, 34 pp., i pi., 1883. (This paper deals mainly with the history of Trapa, its distri- bution and uses, and the causes which are leading to its extinction in Switzerland and elsewhere. The author regards it as a plant of Southern Europe introduced into other localities at a very early period as a food plant.) BIBLIOGRAPHY t)ber Schwimmblatter. Funfstiick's Beitrage zur Wissens. Botanik, Bd. i. 1897, PP- 281-294, i pi. (A general consideration of floating leaves, especially of the manner in which they are supposed to be adapted to their mechanical needs.) The Morphology of the Central Cylinder in the Angiosperms. Trans. Canad. Inst. Vol. vi. 1899, pp. 599-636, 5 Pis- [The section of this paper relating to polystely should be read in connexion with Scott, D. H, (1891).] The Botany of the Eastern Borders, London, 1853. xii + 336 pp., 13 pis. (Pp. 191-192 give an early account of the spread of Elodea canadensis in this country.) Der richtende Einnuss stromenden Wassers auf wachsende Pflanzen und Pflanzentheile (Rheotro- pismus). Ber. d. deutsch. Bot. Gesellsch. Bd. i. 1883, pp. 512-521. (The author proposes the term "Rheotropismus" for the directive influence exerted upon plants by a water current.) Om befruktningen hos slagtet Najas samt hos Callitriche autumnalis. Lunds Univ. Ars-skrift, Tom. xx. Part IV. 1883-1884, 26 pages, i pi. (A Swedish paper with a resume in German dealing with the pollination of Naias and Callitriche.) Cynomorium und Hippuris. Svensk. Bot. Tidskrift, Bd. 4, 1910, pp. 151-159, 6 text-figs. (A comparison of these two genera leads the author to the conclusion that there is little ground for assuming a relation- ship between them. He shows that it is not even certain that Hippuris belongs to the Choripetalae, but if placed in this group it is best treated as representing a distinct family allied to Halorrhagideae.) Studien iiber die Entwicklungsgeschichte von Hip- puris vulgaris. Nova Acta Regiae Societatis Scien- tiarum Upsaliensis, Ser. iv. Vol. 2, N. n, 1907-1911, 26 pp., 3 pis. (The author's study of the development of the ovule and embryo leads him to the conclusion that the systematic position of the genus is still uncertain.) Jussieu, A.L.de (1789) Genera Plantarum. Ixxii + 499 pp. Paris, 1789. [P- 3T5l (In this work Zoster a is included among the Aroids, see p. 24.) Das Bliihen der Wasserlinsen. Zeitschrift. f. Natur- wissenschaften, Bd. 68 (Folge v. Bd. 6), 1895, pp. 136-138. (The author's observations suggest certain minor corrections in the accounts hitherto published of the flowering of Lemna minor.) Jahn, E. (1897) [pp. 30, 31] Jeffrey, E. C. (1899) [p. 1 80] Johnston, G. (1853) [p. 210] Jonsson, B. (1883) [p. 282] Jonsson, B. (1883-1884) [P- 236] Juel, O. (1910) [P- 312] Juel, O. (1911) [P- 312] Kalberlah, A. (1895) [p. 76] 382 BIBLIOGRAPHY Kamienski, F. (1877) Vergleichende Untersuchungen iiber die Entwickel- [pp. 100, 103 and ungsgeschichte der Utricularien. Bot. Zeit. Jahrg. Fig. 67, p. 100] 35, 1877, pp. 761-776, i pi. (This paper is concerned with the embryology, germination and anatomy of Utricularia vulgaris.) Karsten, G. (1888) [pp. 146, 284] Keller, I. A. (1893) [p. 272] Kerner, A. and Oliver, F. W. (1894-1895) [P- 301] Ueber die Entwickelung der Schwimmblatter bei einigen Wasserpflanzen. Bot. Zeit. Jahrg. 46, 1888, PP- 565-578, 581-589- [The author has repeated and extended the experiments on the regulation of growth of the petiole in floating leaves recorded by Frank, A. B. (1872) and he comes to conclusions differing from those of the latter author. He employed Hydrocharis, Marsilea and Ranunculus sceleratus.} The Glandular Hairs of Brasenia peltata Pursch. Proc. Acad. Nat. Sci. Philadelphia, 1894 (for 1893), pp. 188-193, J pl- (The author shows that the mucilaginous coat covering the younger parts of this member of the Nymphaeaceae is due to the secretory activity of ephemeral hairs.) The Natural History of Plants. 2 vols., 1760 pp., 1000 figs., 16 pis. (This well-known book includes a good deal of information about water plants.) Kingsley, M. H. (1897) Travels in West Africa, xvi + 743 pp., 47 illustra- [p. 213] tions. London, 1897. (Pp. 378-380 contain an account of the rapid multiplication of Pistia Stratiotes in the river Ogowe.) Kirchner, O. von, Lebensgeschichte der Bliitenpflanzen Mitteleuropas. Loew, E. and Bd. i. Abth. i. and in., and Bd. 11. Abth. in. Stutt- Schroter,C. (1908, etc.) gart, 1908, 1909 and 1917. [PP. 5°. 59, 74» 8l, (The life-history of the following aquatic groups is dealt with: 123, 205, 206, 276 Helobieae, Abth. i. pp. 394-714, 195 text-figs.; Lemnaceae, and Figs. 49, p. 79, Abth> m- PP- 57-8o, 23 text-figs.; Ceratophyllaceae, Bd. n. 136, p. 205] Abth. in. pp. 51-73, 16 text-figs.) Kirchner, O. See Schroter, C. and Kirchner, O. (1902). Kirschleger, F. (1856) Etwas iiber fluthende Pflanzen (Plantae fluitantes) [p. 12] und sonstige Notizen in Bezug auf die rheinische Flora. Flora, N.R. Jahrg. xiv. (G.R. Jahrg. xxxix.) 1856, pp. 529-536- (Observations on the forms of Sagittaria sagittifolia, Scirpus lacustris and Sparganium simplex with floating leaves.) Kirschleger, F. (1857) Nachtrag zu der Notiz iiber fluthende Pflanzen. [p. 287] Flora, N.R. Jahrg. xv. (G.R. Jahrg. XL.) 1857, pp. 193-194. [A continuation of Kirschleger, F. (1856) giving further references, and an account of the dependence of Scirpus lacustris upon the nature of the soil.] BIBLIOGRAPHY 383 Klebahn, H. (1891) [p. 192] Klebs, G. (1884) [p. 245 and Fig. 158, P- 245] Klinge, J. (1881) [pp. 15, 1 8] Klinsmann, F. (1860) [P- 54] Knoch, E. (1899) [p- 34] Knupp, N. D. (1911) [p. 232] Koch, K. (1852) [pp. 82, 316] Koehne, E. (1884) Ueber Wurzelanlagen unter Lenticellen bei Her- miniera Elaphroxylon und Solanum Dulcamara. Nebst einem Anhang iiber die Wurzelknollchen der ersteren. Flora, N.R. Jahrg. 49 (G.R. Jahrg. 74), 1891, pp. 125-139, i pi. [The author shows, in opposition to Jaensch, T. (i8842) that the lenticels of Herminiera Elaphroxylon, G.P.R. (Aedemone mirabilis, Kotschy) are not " Markstrahlrindenporen," but are lenticels of normal structure. He also describes, both in this plant and in Solanum Dulcamara, the occurrence beneath the lenticels of rudimentary adventitious roots, which may develop under favourable circumstances.] Beitrage zur Morphologic und Biologic der Keimung. Unters. bot. Inst. Tubingen, Bd. i. Heft 4, 1884 PP- 536-635> 24 text-figs. (In this paper the seedlings of certain water plants come under consideration.) Ueber Sagittaria sagittaefolia L. Sitzungsber. d. Naturforscher-Gesellsch. bei d. Univ. Dorpat, Bd. v. Heft in. 1881 (for 1880), pp. 379-408. (A description of the morphology and anatomy of this species and of the different forms in which it occurs.) Ein Beitrag zur Entwickelungsgeschichte von Stra- tiotes aloides. Bot. Zeit. Jahrg. 18, 1860, pp. 81-82, i pi. (The author succeeded in germinating the seeds of this plant and describes and figures the seedling.) Untersuchungen iiber die Morphologic, Biologic und Physiologic der Bliite von Victoria Regia. Bibliotheca Botanica, Bd. ix. 1899, Heft 47, 60 pp., 6 pis. (In this memoir the anatomy and morphology of the flower of Victoria regia are dealt with, and special attention is paid to the development of heat at the flowering period.) The Flowers of Myriophyllum spicatum L. Proc. Iowa Acad. Sci. (Des Moines), Vol. xvm. 1911, pp. 61-73, 4 pis. (A study of the development and general structure of the flowers of this species.) Ueber Pistia in Allgemeinen und Pistia Turpini Blume insbesondere. Bot. Zeit. Jahrg. 10, 1852, PP- 577-585. i pl- (The author describes the germination of Pistia, which he regards as differing from that of Lemna. He also emphasizes the dissimilarity of Pistia and the Aroids. He describes the flowers of Pistia Turpini which he observed in the living state.) Ueber Zellhautfalten in der Epidermis von Blumen- blattern und deren mechanische Function. Ber. d. deutsch. bot. Gesells. Bd. n. 1884, pp. 24-29, i pi. (The author shows that the folds in the lateral walls of epidermal cells of petals serve a mechanical purpose in strengthening the organ. This conclusion may have some bearing on the loss of folding in the epidermal cells of water leaves.) 384 BIBLIOGRAPHY Kotschy, T. (1858) [p. 192] [pp. 62,142,143,154, 239, 305, 314] Kurz, S. (1867) [PP- 73, 291] Lamarck, J. B. P. A. (1809) [P- 1.55] Land, W. J. G. Lebel, E. (1863) [pp. 134, 195, 216, Korzchinsky, S. (1886) Ueber die Samen der Aldrovandia vesiculosa L. Bot. [p. 244] Centralbl. Bd. xxvu. 1886, pp. 302-304, 334-335, i pi. (An account of the structure of the ripe seed and the germina- tion of this plant.) Eine neue Leguminose vom weissen Nil. Oester- reichische Bot. Zeitschrift, Jahrg. vm. 1858, No. 4, pp. 113-116, i pi. (The first scientific description of Aedemone mirabilis, Kotschy — Herminiera Elaphroxylon, G.P.R.) Aponogetonaceae, in Das Pflanzenreich, iv. 13 (herausgegeben von A. Engler), 24 pp., 71 text-figs. Leipzig, 1906. (A monograph of this family of hydrophytes.) Enumeration of Indian Lemnaceae. Journ. Linn. Soc. Bot. Vol. ix. 1867, pp. 264-268, i pi. (A systematic paper with some general notes on distribution of the group.) Philosophic Zoologique. 2 vols. Paris, 1809. (The heterophj'lly of the Water Crowfoot is discussed in Vol. i. Chapter vn. p. 230.) See Coulter, J. M. and Land, W. J. G. (1914). Callitriche. Esquisse Monographique. Mem. de la Soc. Imp. des Sci. Nat. de Cherbourg, T. xi. 1863, pp. 129-176. (A systematic monograph of the genus, dealing also with its anatomy, relationships, etc. — a most vividly written and interesting memoir.) Ueber den Einfluss des Wassers auf das Wachsthum der Stengel und Wurzeln einiger Pflanzen. (Gelehrte Schriften der k. Universitat in Kasan, 1873.) Abstracted in Just's Bot. Jahresbericht, Jahrg. i. 1873, P- 594- (According to the abstract, this Russian paper deals with the development of aerenchyma in the stems and roots of Epilobiunt hirsutum, Lycopus europaeus and two species of Lythrum, when grown in water.) Zur Frage iiber den Einfluss des Mediums auf die Form der Pflanzen. (Gelehrte Schriften der k. Universitat in Kasan, 1873.) Abstracted in Just's Bot. Jahresbericht, Jahrg. i. 1873, pp. 594, 595. (According to the abstract this Russian paper deals with the effect of an aquatic life on Rubus fruticosus .) Lewakoffski,N. (1877) Ueber den Einfluss des Wassers auf die Entwickelung [p. 200] einiger Arten von Salix (Beilage zu dem Protocolle der 91. Sitzung der Naturforsch. an der Universitat zu Kazan). Abstracted in Just's Bot. Jahresber. Jahrg. v. 1879 (for 1877), pp. 575, 576. (According to the abstract, this Russian paper deals chiefly with the effect of submergence on Salix shoots and demon- strates that very little effect is produced on their anatomy by water life.) Lewakoffski, N. (I8731) [p. 188] Lewakoffski, N. (i8732) [p. 200] BIBLIOGRAPHY 385 Lindberg, S. 0. (1873) [P- 46] Lister, G. (1903) [P- Loeselius, J. (1703) [pp. II, 20] Loew, E. Loew, O. (1893) Ludwig, F. (1881) [pp. 80, 84] Ludwig, F. (1886) [P- 243] Luetzelburg, P. von (1910) [pp. 91, 96, 102, 105 and Fig. 71. p. 105] Lundstrom, A. N. (1888) [p. 62] Lyte, H. A. W. P. Is Hydrocharis really Dioecious? Trans, and Proc. Bot. Society, Edinburgh, Vol. xi. 1873, p. 389. (The author suggests that Hydrocharis, instead of being dioecious as is commonly supposed, is really monoecious or monoico-female.) On the Occurrence of Tristicha alternifolia, Tul., in Egypt. New Phyt. Vol. n. 1903, pp. 15-18, I pi. (The author discovered Tristicha alternifolia Tul. var. pulchella Warmg. in rushing water below the first cataract on the Nile ; this is the first record of the family Podostemaceae from Egypt.) Flora Prussia, sive Plantae in Regno Prussiae sponte nascentes. . .Curantejohanne Gottsched. . .Regiomonti ...Sumptibus Typographiae Georgianae, 1703. [Plantago aquatica (—Alisma graminifolium, Mich.), PI. 62 and p. 199, and Sagittaria aquatica foliis variis (=Sagittaria sagittifolia, L.), PI. 74 and p. 234, represented in both cases with the ribbon type of leaf and bearing an inflorescence.] See Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.). Worauf beruht die alkalische Reaction, welche bei Assimilationsthatigkeit von Wasserpflanzen beo- bachtet wird? Flora, Bd. 77, 1893, pp. 419-422. (The red coloration obtained, when phenolphthalein is added to the water in which aquatic plants are living and assimilating, is attributed by the author to calcium carbonate held in a colloidal state by the presence of organic matter.) Ueber die Bestaubungsverhaltnisse einiger Siiss- wasserpflanzen und ihre Anpassungen an das Wasser und gewisse wasserbewohnende Insekten. Kosmos (Stuttgart), Jahrg. v. Bd. x. 1881, pp. 7-12, 17 text- figs. (Observations on the pollination of Lemna, Callitriche, Myrio- phyllum and Ceratophyllum.) Ueber durch Austrocknen bedingte Keimfahigkeit der Samen einiger Wasserpflanzen. Biol. Centralbl. Bd. vi. No. 10, 1887 (for 1886), pp. 299, 300. (A note on the effect of drying on the seeds of Mayaca fluvia- tilis.) Beitrage zur Kenntnis der Utricularien. Flora, Bd. 100, 1910, pp. 145-212, 48 text-figs. (This paper is concerned, in part, with the aquatic Utricularias. The secretions of the bladders are investigated, and a number of cultural experiments are described.) Ueber farblose Oelplastiden und die biologische Bedeutung der Oeltropfen gewisser Potamogeton- Arten. Bot. Centralbl. Bd. xxxv. 1888, pp. 177-181. (A discussion of the cause and significance of the oily surface possessed by the leaves of some submerged Potamogetons.) See Dodoens, R. (1578). 25 386 McCallum, W. B. (1902) [p. 160] MacCaughey, V. (1917) [p. 182] MacDougal, D. T. [p. 162] MacLeod, J. (1893 and 1894) [pp. 9, 230] Magnin, A. (1893) [pp. 274, 279, 287, 290, 323] Magnus, P. Magnus, P. (i [P- 135] Magnus, P. (1871) [p. 169] BIBLIOGRAPHY On the nature of the stimulus causing the change of form and structure in Proserpinaca palustris. Bot. Gaz. Vol. xxxiv. 1902, pp. 93-108, 10 text-figs. [Experimental work on the "land type" and "water type" of leaf in Proserpinaca palustris. This paper should be read in conjunction with Burns, G. P. (1904).] Gunner a petaloidea Gaud., a remarkable plant of the Hawaian Islands. American Journ. Bot. Vol. iv. 1917, pp. 33-39. (A "titanic herbaceous-perennial" belonging to a genus whose anatomy is of interest in relation to that of certain water plants.) The Determinative Action of Environic Factors upon Neobeckia acquatica Greene. Flora, N.F. Bd. vi. ^G.R. Bd. 106), 1914, pp. 264-280, 14 text-figs. (A study of the heterophylly of this plant under a variety of conditions.) Over de bevruchting der bloemen in het kempisch gedeelte van Vlaanderen. Bot. Jaarboek, Vol. v. 1893, pp. 156-452, 58 text-figs.; Vol. vi. 1894, pp. 119-511, 65 text-figs. (The second part of this elaborate memoir on the pollination of the plants of Flanders, concludes with an index and a summary in French.) Recherches sur la vegetation des lacs du Jura. Rev. g6n. de Bot. T. v. 1893, pp. 241-257, 303-316, 8 text-figs. (An ecological survey of 62 out of the 66 lakes which occur in the Jura region.) Beitrage zur Kenntniss der Gattung Najas L. viii + 64 pages, 8 pis. Berlin, 1870. (This monograph of the genus contains an historical account of the literature, a description of the germination, general morphology, apical development and anatomy, and a discussion of the interpretation of the peculiar floral structure.) Ueber die Anatomic der Meeresphanerogamen. Sitzungs-Berichte d. Gesellsch. naturforsch. Freunde zu Berlin, Dec. 20, 1870, pp. 85-90. [An anatomical account of some marine Phanerogams, which should be read in connexion with Ascherson, P. (1870).] Einige Bemerkungen zu dem Aufsatze des Herrn J. Borodin "Ueber den Bau der Blattspitze einiger Wasserpflanzen." Bot. Zeit. Jahrg. 29, 1871, pp. 479-484. [A criticism of Borodin, J. (1870). The author points out the analogy of the ephemeral stomates described by Borodin at the leaf apex of Callitriche with the stomates found at the nerve endings of the leaves of such land plants as Crassula. He shows, on the other hand, that the analogy, suggested by Borodin, with the outgrowths at the leaf apices of Myriophyllum and Ceratophyllum does not hold.] BIBLIOGRAPHY 387 Magnus, P. (1872) [P- 135] Magnus, P. (1883) [P- 332] Magnus, P. (1894) Magnus, W. and Werner, E. [p. 121] (1913) Maisonneuve, D. de (1859) [p. no] Marloth, R. (1883) [p. 241] Marshall, W. (1852) [pp. 55, 210] Untersuchungen iiber die Anatomic der Cymodoceen. Sitzungs-Ber. d. Gesellschaft naturforsch. Freunde zu Berlin, 1872, pp. 30-33. (These notes are chiefly devoted to the occurrence of " Schlauch- gefasse" in Cymodocea.) Ueber eine besondere geographische Varietat der Najas graminea Del. und deren Auftreten in England. Ber. d. deutsch. bot. Gesetlsch. Bd. i. 1883, pp. 521-524- [This paper on a form of Naias graminea which grows in the Egyptian rice fields should be read hi connexion with Ascherson, P. (1874) and Bailey, C. (1884).] Ueber die Gattung Najas. Ber. d. deutsch. bot. Gesellsch. Bd. xn. 1894, pp. 214-224, i pi., 3 text- figs. [A reply to the criticisms on Magnus, P. (I87O1) contained in Schumann, K. (1892).] Die atypische Embryonalentwicklung der Podoste- maceen. Flora, N.F. Bd. 5 (G.R. Bd. 105), 1913, pp. 275-336, 4 pis., 41 text-figs. (A detailed comparative account of the embryo-sac and embryo in the Podostemaceae, with a general discussion of the ecological and morphological significance of the peculiarities observed.) Aldrovandia. Bull, de la Soc. bot. de France, T. vi. l859, PP- 399-401- (The author of this note shows that many plants of Aldrovandia may remain at the bottom of the water even in June, weighted down by the remnant of the turion.) tfber mechanische Schutzmittel der Samen gegen schadliche Einfliisse von aussen. Engler's Bot. Jahrb. Bd. iv. 1883, pp. 225-265, i pi. (A detailed account of the protective layers in seed coats, including references to certain water plants. The paper con- cludes with an index of the species studied.) Excessive and noxious Increase of Udora Canadensis (Anacharis Alsinastrum). Phytologist, Vol. iv. 1852, PP- 705-7I5- (An historical account of the introduction of this plant.) Marshall, W. (1857) The American Water-weed. Anacharis Alsinastrum. [pp. 55, 210] Phytologist, Vol. n. N.S. 1857-8, pp. 194-197. [An additional note on the nuisance caused by this weed; see Marshall, W. (1852).] Martens, G. von (1824) Reise nach Venedig. Ulm, 1824. [?• I35] [This book contains (p. 623) an early reference to the hetero- phylly of Sagittaria sagittifolia. There is also a mention (p. 550) of the part played by Zostera marina in the Venetian lagoons, and its use from time immemorial in packing Venetian glass.] 25—2 388 BIBLIOGRAPHY Martins, C. (1866) (i) Sur les racines aeriferes ou vessies natatoires des [pp. 189, 192] especes aquatiques du genre Jussiaea L. (2) Sur la synonymie et la distribution geographique du Jussiaea repens de Linne. Memoires de la section d. sci. Acad. des Sci. et Lettres de Montpellier, Vol. vi. 1866, pp. 353-381, 4 pis. (An account of the air roots of Jussiaea. Habit drawings of three species are given. The same papers appeared without illustrations in Bull. Soc. bot. de France, T. xui. pp. 160-189, 1866.) Massart, J. (1910) Esquisse de la Geographic botanique de la Belgique. [pp. 198, 283,291 and Recueil de 1'Inst. bot. Leo Errera, Tome supp!6men- Figs. 13, p. 28, 99 taire vn. bis. xi + 332 pp., 101 text-figs. Brussels, and 100, p. 152]- 1910. (This work, which deals exhaustively with the ecology of Belgium, contains a certain amount of information about aquatics — see especially pp. 115-123. There is also a separate "annexe" with numerous photographs of the vegetation, including a number of pictures of water plants.) Matthews, J. R. (1914) The White Moss Loch : A Study in Biotic Succession. [p. 289 and Fig. 165, New Phyt. Vol. xin. 1914, pp. 134-148, 2 text-figs. p. 288] [An ecological study in which the aquatic formation of the Loch is dealt with (pp. 137-140).] Matthiesen, F. (1908) Beitrage zur Kenntnis der Podostemaceen. Bibl. [pp. 112, 114, 117, Bot. Bd. xv. Heft 68, 1908, 55 pp., 9 pis., i text-fig. 122, 255 and Fig. 8l, (This memoir is chiefly occupied with a description of certain p no] species of Podostemaceae from Venezuela, but it also includes a general account of the morphology and anatomy of the group.) Meierhofer, H. (1902) Beitrage zur Anatomic und Entwickelungsgeschichte [p. 103 and Figs. 61, der Utricularia-Blasen. Flora, Bd. 90, 1902, pp. 84- p. 93, 62, p. 95, 73, 114, 9 pis. p. 107] (The author describes the structure and development of the bladders of the European aquatic Utricularias and comes to the conclusion that these organs are foliar in nature.) Meister, F. (1900) Beitrage zur Kenntnis der europaischen Arten von [pp. 100, 1 01, 299] Utricularia. Memoires de 1'Herbier Boissier, No. 12, 1900, 40 pp., 4 pis. (A systematic account with biological notes.) Mellink,J.F.A.(i886) Zur Thyllenfrage. Bot. Zeit. Jahrg. 44, 1886, pp. [p. 258] 745-753, i pl- [An account of a petiole of Nymphaea (Castalia) alba which had at some time been wounded at various points. It was found that, in the neighbourhood of the wounds, the air canals were choked by hairs, which had grown out from the surrounding parenchyma cells in a thylose-like manner into the canals.] Mer, £. (iSSo1) Des modifications de forme et de structure que [pp. 163, 165, 279] subissent les plantes, suivant qu'elles vegetent a 1'air ou sous 1'eau. Bull, de la Soc. bot. de France, T. xxvu. (Ser. n. T. n.) 1880, pp. 50-55. (An analysis of the differences in morphology and structure exhibited by the land and water forms of Ranunculus aquatilist R. Flammula, Littorella lacustris, etc. The author suggests a. comparison between etiolated and submerged plants.) BIBLIOGRAPHY 389 Mer, £. (i88o2) Mer, £. (1881) Mer, E. (I8821) [pp. 32, 42, 165, 195] Mer, £. (1882*) Merz, M. (1897) [p. i oo] Micheli, M. (1881) Micheli, P. A. (1729) [pp. 76, 235] Milde, (1853) Miller, G. S. \ and [-(1912) Standley, P. C.) Des causes qui modifient la structure de certaines plantes aquatiques vegetant dans 1'eau. Bull, de la Soc. bot. de France, T. xxvu. (Se"r. n. T. n.) 1880, pp. 194-200. (This paper is concerned with the differences between the forms of Littorella and Isoetes growing under different conditions.) Observations sur les variations des plantes suivant les milieux. Bull, de la Soc. bot. de France, T. xxvm. (Se"r. n. T. m.) 1881, pp. 87-90. (Brief notes on the submerged and aerial forms of Callitriche Littorella, etc.) De la vegetation a 1'air des plantes aquatiques. Comptes rendus de 1'acad. des sciences, Paris, T. 94, 1882, pp. 175-178. (An account of the experimental production of land forms in the case of certain aquatic plants.) De quelques nouveaux exemples relatifs a 1'influence de 1'heredite et du milieu sur la forme et la structure des plantes. Bull. Soc. bot. de France, T. xxix. 1882, pp. 81-87. (A study of the leaf characters of Potamogeton rufescens growing in deep or shallow water.) Untersuchungen iiber die Samenentwickelung der Utricularieen. Flora, Bd. 84 (Erganzungsband zum Jahrg. 1897), 1897, pp. 69-87. 34 text-figs. (A detailed account of the embryo-sac and seed in ten species of the genus, which is characterised by the early disappearance of the nucellus and the development of endospermic haustoria at both ends of the sac.) Alismaceae, Butomaceae, Juncagineae. A. and C. de Candolle's Monographiae Phanerogamarum, Vol. in. 1881, pp. 7-112. (A systematic account of these families with a discussion of their affinities, etc.) Nova Plantarum Genera.... Florentiae, 1729. (The flowers of Lemna gibba are figured on PI. u under the name of " Lenticula," and the sterile plants of L. minor, L. trisulca, Wolffia arrhiza and Spirodela polyrrhiza under the name of " Lenticularia." Vallisneria with its floating $ flowers and spiral peduncles is shown on PI. 10.) Wolffia Michelii Hork. (Lemna arrhiza L.). Bot. Zeit. Jahrg. xi. 1853, pp. 896, 897. (A note on the occurrence of this plant in Germany.) The North American Species of Nymphaea. Con- tributions from the U.S. National Herbarium, Vol. 16, Pt 3, 1912. Smithsonian Institution. U.S. National Museum, viii + 109 pp., 39 text-figs., 13 pis. (This systematic monograph is fully illustrated, especially with photographs of fruits and with maps showing the distribution, of the various species in N. America.) 390 BIBLIOGRAPHY Minden,M. von (1899) Beitrage zur anatomischen und physiologischen [pp. 83, 266, 268, 269 Kenntnis Wasser-secernierender Organe. Bibliotheca and Fig. 53, p. 82] Botanica, Bd. ix. Heft 46, 1899, 76 pp., 7 pis. (Pp. 1-30 deal with the secretion of watery solutions from water pores and apical openings in the leaves of water plants, and form an exhaustive account of the subject so far as it had been worked out by the end of the nineteenth century.) Mo bius, M. (1895) Ueber einige an Wasserpflanzen beobachtete Reizer- [p. 281] scheimmgen. Biol. Centralbl. Bd. 15, 1895, pp. 1-14, 33-44, 8 text-figs. (Observations on the effect of light and darkness on water plants.) Moeller, J. (1879) Aeschynomene aspera Willd. (Papilionaceen) . Bot. [p. 191] Zeit. Jahrg. 37, 1879, pp. 720-724, i text-fig. (An account of the anatomy of the floating wood of this plant.) Monkemeyer, W. Die Sumpf- und Wasserpflanzen. Ihre Beschreibung, (1897) Kultur und Verwendung. iv + 189 pp., 126 text- [p. 291] figs. Berlin, 1897. (A useful compendium of water and marsh plants arranged on Engler and Prantl's system, with notes on their cultivation. It is by the Inspector of the Leipzig Botanic Garden, and is primarily intended to help those who wish to grow water plants in an aquarium or water garden.) Montesantos,N.(i9i3) Morphologische und biologische Untersuchungen [pp. 50, 51, 52, 157, iiber einige Hydrocharideen. Flora, N.F. Bd. v. 239, 282] (Ganze Reihe, Bd. 105), 1913, pp. 1-32, 5 pis. (This paper deals with the genera Limnobium, Blyxa, Ottelia and Stratiotes. Certain experiments are described showing that the heterophylly of Limnobium and Stratiotes is not due to the direct action of the medium, and that the sinking of Stratiotes in the autumn is due to the increase of weight brought about by a deposition of chalk on the leaves.) Monti, Gaetano(i747) De Aldrovandia novo herbae palustris genere. De [p. 109] Bononiensi Scientiarum et Artium Institute atque Academia Commentarii. Tomi secundi Pars tertia, i747> PP- 404-4H. i Pi- [This old and rare memoir on Aldrovandia is analysed in Aug6 de Lassu (1861).] Mori, A. (1876) Nota sull' irritabilita delle foglie dell' Aldrovandia [p. no] vesiculosa. Nuovo Giornale Botanico Italiano, Vol. vm. 1876, p. 62. . (The author shows that the irritability of the Aldrovandia leaf is confined to the central glandular region.) Moss, C. E. (1913) Vegetation of the Peak District, x + 235 pp., 36 figs., [p. 291] 2 maps. Cambridge, 1913. (Chapter vi. contains an account of the marsh and aquatic associations of the district.) Muller, F. (1877) Untersuchungen iiber die Struktur efhiger Arten von [p. 311] Elatine. Flora, N.R. Jahrg. xxxv. (G.R. Jahrg. LX.) 1877, pp. 481-496, 519-526, i pi. (A description of the anatomy and flower structure of Elatine and a discussion of the affinities of the genus.) Miiller, F. (1883) [pp. 239, 282] Miinter, J. (1845) [P- 15] Murray, H. Nakano, H. (1911) [P- 291] Nolte, E. F. (1825) [pp. 15,50/52,54 and Fig. 32, p. 53] Ohno, N. (1910) [P- 258] Oliver, D. Oliver, F. W. (1888) [pp. 151, 234, 266] Oliver, F. W. (1889) Oliver, F. W. (1894) Onslow, The Hon. Mrs Huia Osbeck, P. (1771) [P- i?] BIBLIOGRAPHY Einige Eigenthiimlichkeiten der Eichhornia crassipes. Kosmos, Jahrg. vn. Heft iv. 1883, pp. 297-300. (Notes on the floral biology of this species.) Beobachtungen iiber besondere Eigenthiimlichkeiten in der Fortpflanzungsweise der Pflanzen durch Knospen. III. Ueber die Knospen der Sagittaria sagittaefolia L. Bot. Zeit. Jahrg. 3, 1845, pp. 689-697. [An account of the tuber formation in this species, with a mention of certain early references to the subject. The author discusses the statement in Nolte, E. F. (1825), that tuber formation was once found by him in Alisma Plantago, and decides that this is undoubtedly an error.] See Weiss, F. E. and Murray, H. (1909). The Vegetation of Lakes and Swamps in Japan. I. Teganuma (Tega-Swamp) . Bot. Mag. Tokyo, Vol. xxv. 1911, pp. 35-51, 6 text-figs. (The first ecological survey of a Japanese lake and swamp.) Botanische Bemerkungen iiber Stratiotes und Sagit- taria. Kopenhagen, 44 pp., 2 pis., 1825. [An admirable account of the life-history of these two genera, in which special attention is paid to their methods of vegetative reproduction, and, in the case of Stratiotes, to the distribution of the sexes and the structure of the fruit. For a criticism see Miinter, J. (1845).] Ueber lebhafte Gasausscheidung aus den Blattern von Nelumbo nucifera Gaertn. Zeitschrift f. Bot. Jahrg. ii. 1910, pp. 641-644, 4 text-figs. (In this paper — from a Japanese laboratory — the author draws the conclusion that the pressure which produces the streaming of gas bubbles from the leaves of Nelumbo can be explained on purely physical grounds, but that it also has a physiological significance.) See Im Thurn, E. F. and Oliver, D. (1887). On the Structure, Development, and Affinities of Trapella, Oliv., a new Genus of Pedalineae. Ann. Bot. Vol. n. 1888-1889, PP- 75-H5, 5 pis., i text-fig. (A monograph of Trapella sinensis, Oliv., a Chinese water plant discovered by Dr Henry.) On a new form of Trapella sinensis. Ann. Bot. Vol. in. 1889-1890, p. 134. [A brief account of a land form of this species. This note is supplementary to Oliver, F. W. (1888).] See Kerner, A. and Oliver, F. W. (1894). See Wheldale, M. (1916). A Voyage to China.... Translated... by John Reinhold Forster. Vol. i. xx + 367 pp., 4 pis. London, 1771. (The author mentions on pp. 334, 335 that a species of Sagittaria is cultivated by the Chinese as a food plant.) 392 BIBLIOGRAPHY Osborn, T. G. B. (1914) Botany and Plant Pathology. Reprinted from Hand- [p. 127] book of South Australia. British Association Visit. Adelaide, 1914, 27 pp., 7 figs. (On p. n there is a brief reference to the marine Angiosperms of S. Australia, and a mention of the curious seedlings of Cymodocea antarctica.) OstenfeldjC. H. (1908) On the Ecology and Distribution of the Grass-Wrack [pp. 123, 134] Otis, C. H. (1914) [p. 261] Overton, E. (1899) [pp. 228, 280, 290] Paillieux, A.) and V (1888) Bois, D. ) [pp. 17, 24] Pallis, M. (1916) [pp. 207, 211] Parkin, J. Parmentier, P. (1897) [P- 312] Payne-Gallwey, R. (Zoster a marina) in Danish Waters. Report of the Danish Biological Station to the Board of Agriculture. XVI. Translated from Fiskeri-Beretning for 1907, Copenhagen, 1908, 62 pp., 9 text-figs. (The distribution of this plant is dealt with in detail, and special attention is paid to the algae and marine animals with which it is associated.) The transpiration of emersed water plants: its measurement and its relationships. Bot. Gaz. Vol. 58, 1914, pp. 457-494, 3 text-figs, and 14 charts. (An extremely elaborate study of the transpiration of water plants whose leaves are in contact with the atmosphere, e.g. Castalia, Sagittaria and Pontederia.) Notizen iiber die Wassergewachse des Oberengadins. Vierteljahrsschrift der Naturforschenden Gesellschaft in Zurich, Jahrg. 44, 1899, pp. 211-228. (A record of certain experiments on the effect of light and temperature on the flowering of Hydrocharis and Elodea,', and of the vertical distribution of the water plants of the Upper Engadine.) Les plantes aquatiques alimentaires. Bull, de la Soc. nat. d'acclimatation de France, Ser. iv. T. 5, Annee 35, 1888, pp. 782-793, 924-929, 1028-1035, 1 102- 1108. (An account of a number of aquatic plants which are used for food, including Aponogeton, Trapa, Eleocharis, Sagittaria and various Nymphaeaceae.) The Structure and History of Plav: the Floating Fen of the Delta of the Danube. Journ. Linn. Soc. Bot. Vol. 43, 1916, pp. 233-290, 15 pis., i text-fig. (This paper deals with Phragmites communis, Trin., fiflavescens, Gren. & Godr.) See Arber, E. A. N. and Parkin, J. (1907). Recherches anatomiques et taxinomiques sur les Onotheracees et les Haloragacees. Ann. des sci. nat. Ser. vin. Bot. T. in. 1897, PP- 65-149, 6 Pls- (A discussion, based on the anatomical characters of stem and leaf, of the relation of the Onothereae, Ludwigieae, Halorageae and Gunnereae.) See Walsingham, Lord, and Payne-Gallwey, R. (1886). BIBLIOGRAPHY 393 Pearsall, W. H. The Aquatic and Marsh Vegetation of Esthwaite (1917-1918) Water. Journ. of Ecology, Vol. v. 1917, pp. 180-202 [p. 288] arid Vol. vi. 1918, pp. 53-74, 12 text-figs. (A detailed ecological survey of a Lancashire lake.) Pearsall, W. H. (1918) On the classification of Aquatic Plant Communities, [p. 288] Journ. of Ecology, Vol. vi. 1918, pp. 75-83. (The author regards aquatic, fen and moor successions as together forming a unit and leading up to the one formation moor.) Perrot, £. (1900) Sur les organes appendiculaires des feuilles de [p. 169 and Fig. no, certains Myriophyllum. Journ. de Bot. T. xiv. 1900, p. 170] pp. 198-202, 5 text-figs. (An account of the peculiar processes borne by the leaf of Myriophyllum verticillatum and M. spicatum, which the author regards as pluricellular caducous trichomes.) Ueber einige deutsche Nymphaen. Bot. Zeit. Jahrg. 12, 1854, pp. 172-175. (A critical article in which special stress is laid on the import- ance for systematic purposes of ascertaining the characters of the ripe fruit.) Pieters, A. J. (1894) The Plants of Lake St Clair. Bull. Michigan Fish [p. 288] Commission, No. 2, 1894, 12 pp., i map. [An ecological study in which the zonation of the plants observed in Lake St Clair is compared with that recorded by Magnin, A. (1893) for the Jura lakes.] Pfeiffer, L. (1854) Pieters, A. J. (1902) [p. 291] Contributions to the Biology of the Great Lakes. The Plants of Western Lake Erie, with Observations on their Distribution. Bull. United States Fish Commission, Vol. xxi. 1902 (for 1901), pp. 57-79, 10 pis., 9 text-figs. (An ecological study.) Planchon, J. E. (1844) Observations sur le genre Aponogeton et sur ses [p. 314] afnnites naturelles. Ann. d. sci. nat. Ser. in. Bot. T. i. 1844, pp. 107-120, i pi. (The author describes Aponogeton distachyus and brings forward evidence for removing it from the neighbourhood of Saururus and placing it in a sub-order between the Alismaceae and Juncaginaceae.) Planchon, J. E. (1853) fitudes sur les Nympheacees. Ann. des sci. nat. Ser. in. T. xix. 1853, pp. 17-63. (A systematic account of this family.) Pond, R. H. (1905) [p. 264] Contributions to the Biology of the Great Lakes. The Biological Relation of Aquatic Plants to the Substratum. University of Michigan. Inaug. Diss. Ann Arbor, 1905, 43 pp., 6 text-figs. (This paper contains important experimental work relating to the function of the roots in aquatic plants.) 394 Porsch, O. (1903) [pp. 165, 1 66] Porsch, O. (1905) [p. 1 66 and Fig. 107, P- Praeger, R. L. (1913) [p. 297] Prankerd,T. L. (1911) [pp. 181, 197, 216, 228, 233 and Fig. 127, p. 197] Preston, T. A. (1895) [pp. 232, 291] Prillieux, E. (1864) [pp. 63, 173] Pringsheim, N. (1869) [pp. 97, 1 06 and Fig. 72, p. 106] Pringsheim, N. (1888) Queva, C. (1910) [p. 244] BIBLIOGRAPHY Zur Kenntnis des Spaltofmungsapparates submerser Pflanzenteile. Sitzungsber. d. Math.-naturwiss. Klasse d. k. Akad. d. Wissens. Wien, Bd. cxn. Abt. i. 1903, pp. 97~I38> 3 PJs. (An account of the means by which the flooding of the inter- cellular spaces through the stomata of submerged organs is prevented.) Der Spaltoffnungsapparat im Lichte der Phylogenie. xv + 196 pp., 4 pis. and 4 text-figs. Jena, 1905. [Pp. 83-87 deal with the stomates of water plants and form a resume of Porsch, 0. (1903).] On the Buoyancy of the Seeds of some Britannic Plants. Sci. Proc. Royal Dublin Soc., N.S., Vol. xiv. I9i3~19i5, PP- 13-62. [This memoir is supplementary to the parts of Guppy, H. B. (1906) which relate to seed buoyancy.] On the Structure and Biology of the Genus Hottonia. Annals of Bot. Vol. xxv. 1911, pp. 253-267, 2 pis. and 7 text-figs. (A general account of H. palustris and H. inflata. The author shows that the mature plant of H. palustris is not rootless as generally supposed. Traces of polystely occur at the base of the inflorescence axis in both species.) The Flora of the Cropstone Reservoir. Trans. Leic. Lit. and Phil. Soc. Vol. in. 1895, pp. 430-442. (An account of the flora of a reservoir which had been in existence twenty-four years.) Recherches sur la vegetation et la structure de I'Althenia filiformis Petit. Ann. d. sci. nat. Se"r. v. Bot. T. ii. 1864, pp. 169-190, 2 pis. (A general account of this Mediterranean member of the Zannichellieae.) Uber die Bildungsvorgange am Vegetationskegel von Utricularia vulgaris. Monatsber. d. k. preuss. Akad. d. Wissens. Berlin, 1869, pp. 92-116, i pi. (The classic account of the apical development of Utricularia and its bearing on the morphological perplexities presented by the genus.) Ueber die Entstehung der Kalkincrustationen an Siisswasserpflanzen. Pringsheim's Jahrb. f . wiss. Bot. Bd. xix. 1888, pp. 138-154. (The author shows experimentally that the chalk incrustation on the surface of so many fresh-water plants is due to the abstraction, during the process of assimilation, of CC>2 which has held the calcium carbonate in solution.) Observations anatomiques sur le " Trapa natans L." Association Fran£aise pour Tavancement des sciences. Compte rendu de la 38e session, Lille, 1909 (1910), pp. 512-517, 2 text-figs. (The author's anatomical study of the seedling leads him to the conclusion that the primary root is entirely unrepresented. The anatomy of the hypocotyl is modified by the insertion of numerous adventitious roots which are localised on the same side of the axis as the large cotyledon.) BIBLIOGRAPHY 395 Raciborski, M. (1893) Raciborski,M. (1894^ Raciborski, M. (i8942) [PP. 29, 35, 272, 338] Raciborski, M. (1895) Raffeneau-Delile, A. (1841) [pp. 38, 258] Raunkiaer, C. (1896) [Figs. 34, p. 55, 159, p. 246, 161, p. 248, 166, p. 319, 167, p. 339, 168, p. 339] Ueber die Inhaltskorper der Myriophyllumtrichome. Ber. d. deutsch. bot. Gesellsch. Bd. xi. 1893, pp. 348-35I- (The highly refractive bodies present in the trichomes of Myriophyllum are considered to be of the nature of a glucoside, and to be related to substances found in the trichomes of the leaves of Ceratophyllum, Elatine, etc.) Die Morphologic der Cabombeen und Nymphaeaceen. Flora, Bd. 78, 1894, pp. 244-279, 9 text-figs. (In this memoir special attention is paid to the ontogeny of the flower and the vegetative shoots.) Beitrage zur Kenntniss der Cabombeen und Nym- phaeaceen. Flora, Bd. 79 (Erganzungsband), 1894, pp. 92-108, i pi. [This paper is supplementary to Raciborski, M. (I8Q41) and includes a reply to the criticisms on the latter contained in Schumann, K. (1894).] Die Schutzvorrichtungen derBluthenknospen. Flora, Bd. 8 1 (Erganzungsband), 1895, pp. 151-194, 30 text-figs. (This paper contains a section, pp. 190—192, dealing with the protection of the flower-bud among water plants.) Evidence du mode respiratoire des feuilles de Nelumbium. Ann. d. sci. nat. Ser. n. T. xvi. Bot. 1841, pp. 328-332. (This paper on the emission of air from the leaves of the Lotus is followed by nine pages of controversy on the subject with Dutrochet.) De Danske Blomsterplanters Naturhistorie. Bd. i. Enkimbladede i. Helobieae, 1896, 138 pp., 240 text- figs. (This fully illustrated account of the biology of the Helobieae native to Denmark is in Danish.) Raunkiaer, C. (1903) [pp. 62, 65, 331 and Fig. 38, p. 61] Ravn, F. K. (1894) Regnard, P. (1891) [pp. 253/255, 278] Anatomical Potamogeton-Studies and Potamogeton fluitans. Botanisk Tidsskrift, Vol. 25, 1903, pp. 253- 280, 9 text-figs. (In this paper, which is written in English, the author shows the value of anatomical characters of the leaf and stem in classifying the genus Potagometon.) Om Flydeevnen hos Fr^ene af vore Vandog Sump- planter. Botanisk Tidsskrift, Vol. 19, 1894, pp. 143-188, 26 text-figs. (This Danish paper on the floating power of the seeds of aquatic and marsh plants concludes with a French resume.) Recherches experimentales sur les conditions phy- siques de la vie dans les eaux. vii + 500 pp., 236 text-figs., 4 pis. Paris, 1891. (This book consists of a series of lectures on aquatic biology. The physical aspect is fully treated; the applications relate chiefly to animals, but plants are not excluded.) Reid, C. (1892) [pp. 296, 298] Reid, C. (1893) [P- 54] Reid, C. (1899) [P- 303] Reinsch, P. (1860) Rendle, A. B. (1899) [P- 315] Rendle, A. B. (1900) Rendle, A. B. (1901) [P- 304] Rendle, A. B. (1904) IP- Richard, L. C. (1808) [P- BIBLIOGRAPHY On the Natural History of Isolated Ponds. Trans. Norfolk and Norwich Nat. Soc. Vol. v. 1894 (for 1889-1894), Part 3, 1892, pp. 272-286. (This paper is chiefly based on a study of the dew ponds of the South Downs. It forms an important contribution to the subject of the methods of dispersal of water plants.) On Paradoxocarpus carinatus, Nehring, an extinct fossil plant from the Cromer Forest-bed. Trans. Norfolk and Norwich Nat. Soc. Vol. v. 1894 (for 1889-1894), Part 4, 1893, pp. 382-386, i text-fig. [An account of a fossil fruit which was eventually discovered to belong to Stratictes aloid.es L. (vide note by same author in Trans. Norfolk and Norwich Nat. Soc. Vol. vi. Pt 3, 1897, P. 328).] The Origin of the British Flora, vi + 191 pp. London, 1899- (This classical study, based on the flora of the Newer Tertiary beds, contains many references to water plants.) Morphologische Mittheilungen. 5. Ueber die dreierlei Arten der Blatter der Sagittaria sagittaefolia L. Flora, N.R. Jahrg. xvm. (G.R. Jahrg. XLIII.) 1860, pp. 740-742, i fig. (An account of the heterophylly of Sagittaria including a mention of the distribution of the stomates in the different types of leaf. The arrow-head leaves are distinguished as " Bluthezeitblatter.") A Systematic Revision of the Genus Najas. Trans. Linn. Soc. Lond. Ser. n. Vol. v. 1895-1901, Part xn. 1899, pp. 379-436. 4 Pls- (This monograph includes a general introduction dealing with the morphology, structure and distribution of the genus.) Supplementary Notes on the Genus Najas, Trans. Linn. Soc. Lond. Ser. n. Vol. v. 1895-1901, Part xm. 1900, pp. 437-444- [This paper supplements Rendle, A. B. (1899).] Naiadaceae, in Das Pflanzenreich, iv. 12 (heraus- gegeben von A. Engler). 21 pp., 71 text-figs. Leipzig, 1901. (An authoritative account of all the species of the genus Naias ; the general description of the group is in English.) The Classification of Flowering Plants. Vol. i. Gymnosperms and Monocotyledons, xiv + 403 pp., 187 text-figs. Cambridge, 1904. (This instalment of a text book of systematic botany gives a very useful account of the Helobieae and other Monocoty- ledonous aquatics.) D6monstrations Botaniques ou Analyse du Fruit. Paris, 1808. xii + in pp. (On p. 33 the author makes the suggestion that Callitriche is related to the Euphorbiaceae by its seed structure.) BIBLIOGRAPHY 397 Rodier, £. (1877*) [p. 90] Rodier, £. (18772) [p. 90] Rohrbach, P. (1873) Roper, F. C. S. (1885) [p. 146] Rosanoff, S. (1871) [p. 189 and Fig. 123, p. 191] Rossmann, J. (1854) [p. 144] Roux, M. le (1907) [p. 291] Roxburgh, W. (1832) [p. no] Royer, C. (1881-1883) [pp. 24, 27, 87, 216, 234, 236] Sur les mouvements spontanes et re"guliers (Tune plante aquatique submergee, le Ceratophyllum de- mersum. Comptes rendus de 1'acad. des sciences, Paris, T. 84, 1877, pp. 961-963. [For an English account of this work see Rodier, 6. (i8772).] The Movements of a Submerged Aquatic Plant. Nature, Vol. xvi. 1877, pp. 554-555, i text-fig. (This brief paper is translated from an article by the author in "La Nature" and contains substantially the facts recorded in Rodier, £. (1877*) with the addition of a text-figure showing the successive positions assumed in the course of two days by a branch of Ceratophyllum demersum.) Beitrage zur Kenntniss einiger Hydrocharideen. Abhandl. d. naturforsch. Gesellschaft zu Halle, Bd. xii. 1873, pp. 53-114, 3 pis. (This memoir deals chiefly with the morphology and anatomy of Hydrocharis Morsus-ranae, Stratiotes aloides and ValUsncria spiralis. Special attention is paid to the shoot and inflorescence systems and to the development of the flower.) Note on Ranunculus Lingua, Linn. Journ. Linn. Soc. Bot. Vol. xxi. 1886, pp. 380-384, 2 pis. (An account of the submerged leaves of this species. The two types of leaf are clearly figured, and there is an historical account from the literature of the records of their occurrence.) Ueber den Bau der Schwimmorgane von Desmanthus natans Willd. Bot. Zeit. Jahrg. 29, 1871, pp. 829-838 i pi. [A study of the aerenchyma of Desmanthus natans, Willd. (Neptunia oleracea, Lour.).] Beitrage zur Kenntniss der Wasserhahnenfusse, Ranunculus sect. Batrachium. vi + 62 pp. Giessen, 1854. (This memoir is divided into two parts; the first deals generally with the Water Buttercups, and discusses their heterophylly, while the second consists of descriptions of the species recognised at this date.) Recherches biologiques sur le lac d'Annecy. Annales de Biologic Lacustre, T. n. Fasc. i and 2, 1907, pp. 220-387, 6 pis., 14 text-figs. (This memoir includes an ecological study of the flora of the lake.) Flora Indica. Vol. n. vi + 691 pp. Serampore, 1832. (On p. 112 the author mentions that Aldrovandia verticillata is " Found swimming on ponds of water over Bengal during the cold and hot season.") Flore de la Cote-d'Or avec determinations par les parties souterraines. 2 vols., 693 pp. (2 vols. paged as one). Paris, 1881-1883. (This flora, of that Departement of France which includes Dijon, is unusual in paying special attention to the biology and life- history of the plants enumerated. It contains a good many useful notes on water plants.) 398 Roze, E. (1887) [P- 7i] Roze, E. (1892) [P- 85] Russow, E. (1875) [pp. 107, 1 80] Sanio, C. (1865) [pp. 65, 86, 175, 176, 179] Sargant, E. (1903) [p. 320] Sargant, E, (1908) [pp. 308, 320, 323] BIBLIOGRAPHY Le mode de fecondation du Zannichellia palustris L. Journ. de Bot. T. i. 1887, pp. 296-299, i text-fig. (Observations on the submerged pollination of this species.) Sur le mode de fecondation du Najas major Roth et du Ceratophyllum demersum L. Bull, de la Soc. bot. de France, T. xxxix. (Ser. n. T. xiv.) 1892, pp. 361-364. [The pollination of Naias is described, and, in the case of Ceratophyllum, the observations of Dutailly, G. (1892) are confirmed.] Betrachtungen iiber das Leitbiindel- und Grundge- webe (Jubilaumschrift Dr Alexander von Bunge). 78 pp. Dorpat, 1875. (The anatomy of water plants is dealt with in this memoir in some detail.) Einige Bemerkungen in Betreff meiner iiber Gefass- biindelbildung geausserten Ansichten. Bot. Zeit. Jahrg. 23, 1865, pp. 165-172, 174-180, 184-187, 191-193, 197-200. [This paper forms a reply to the criticism of the author's anatomical views by R. Caspary in Prings. Jahrb. Bd. 4, 1865-6, pp. 101-124. It includes an account of the anatomy of certain water plants — Hippuris (pp. 184-186), Myriophyllum (p. 186), Elodea (pp. 186, 187 and 191-192), Ceratophyllum (pp. 192, 193), Trapa (p. 193), and Potamogeton (p. 193).] A Theory of the Origin of Monocotyledons, founded on the Structure of their Seedlings. Ann. Bot. Vol. *7» 1903, PP- 1-92, 7 pis., 10 text-figs. [This paper does not deal with water plants, but should be read in connexion with the theory of the aquatic origin of Monocotyledons proposed in Henslow, G. (1893).] The Reconstruction of a Race of Primitive Angio- sperms. Ann. Bot. Vol. xxn. 1908, pp. 121-186, 21 text-figs. (This memoir contains a criticism (pp. 175-178) of Henslow's theory of the aquatic origin of Monocotyledons.) Sauvageau, C. (I8891) Sur la racine du Najas. Journ. de Bot. Vol. in. 1889, [p. 208 and Fig. 140, pp. 3-11, 7 text-figs. p. 209] (A detailed account of the extremely reduced anatomy of the roots of Naias.) Sauvageau, C. (i8892) Contribution a 1' etude du systeme mecanique dans [p. 66] la racine des plantes aquatiques; les Potamogeton. Journ. de Bot. Vol. in. 1889, pp. 61-72, 9 text-figs. (A full comparative study of the root anatomy of the genus, bringing out the interesting point that lignin is as abundant in the roots of Potamogeton as in those of many land plants.) BIBLIOGRAPHY 399 Sauvageau, C. (i8893) [P- 135] Sauvageau, C. [pp. 124, 131, 164] Sauvageau, C. [p. Sauvageau, C. [pp. 124, 131] Sauvageau, C. [pp. 123, 131, 254, 261, 264, 266, 331 and Figs. 84, p. 125, 85 and 86, p. 128, 88 and 89, p. 132, 108, p. 167, 162, p. 262] Sauvageau, C. (i89i2) Sauvageau, C. [PP- 135, 33i] Sauvageau, C. (1893) [p. 269 and Fig. 164, p. 270] Contribution a 1'etude du systeme mecanique dans la racine des plantes aquatiques; les Zostera, Cymo- docea et Posidonia. Journ. de Bot. Vol. in. 1889, pp. 169-181, 5 text-figs. (A continuation of the author's detailed study of the roots of submerged plants. In Zostera and Cymodocea the mechanical tissue is of the nature of collenchyma, while in Posidonia it is sclerised.) Observations sur la structure des feuilles des plantes aquatiques; Zostera, Cymodocea et Posidonia. Journ. de Bot. T. iv. 1890, pp. 41-50, 68-76, 117-126, 128-135, 173-178, 181-192, 221-229, 237-245, 38 text-figs. (The author's elaborate study of these three genera leads to the conclusion that anatomical data serve here to distinguish species.) Sur la feuille des Hydrocharidees marines. Journ. de Bot. T. iv. 1890, pp. 269-275, 289-295, 3 text-figs. (This memoir deals with the leaf structure of Enhalus, Thalassia and Halophila.) Sur la structure de la feuille des genres Halodule et Phyllospadix : Journ. de Bot, Vol. iv. 1890, pp. 321- 332, 7 text-figs. (This paper forms the conclusion of the author's study of the leaves of marine Angiosperms, and includes a summary of his results.) Sur les feuilles de quelques monocoty!6dones aquatiques. Ann. d. sci. nat. Ser. vii. Bot. T. xm. 1891, pp. 103-296, 64 text-figs. (Also published as Theses presentees a la faculte" des sciences de Paris, Se"r. A, No. 158, No. d'ordre 720, 1891.) [An exhaustive account of the leaf structure of forty-eight species of the Potamogetonaceae (as denned by Ascherson), incorporating the results published in Sauvageau, C. (1890*) and (iSQO3). The memoir contains some experimental work on the transpiration current in submerged plants.] Sur la tige des Zostera. Journ. de Bot. T. v. 1891, PP. 33-45. 59-68, 9 text-figs. (A description of the anatomy and morphology of the stems of the five species of Zostera, showing that the stem anatomy gives even better criteria for distinguishing the species than those deduced from the author's study of the leaves.) Sur la tige des Cymodocees Aschs. Journ. de Bot. T. v. 1891, pp. 205-211, 235-243, 6 text-figs. (The author shows that the different species of Cymodocea and Halodule can be distinguished by the anatomy of their stems just as they can by that of their leaves.) Sur la feuille des Butomees. Ann. des sci. nat. Ser. 7, Bot. T. 17, 1893, pp. 295-326, 9 text-figs. (Certain of the plants dealt with in this paper are aquatic, e.g. Hydrocleis nymphoides, whose leaf anatomy is fully described.) 400 Sauvageau, C. (1894) [PP. 59, 63, 71, 243 and Figs. 37, p. 60, 43, P. 68] Schaffner,J.H.(i896) [P- 19] Schaffner,J.H.(i897) [P- 9] Schaffner,J.H.(i904) [PP- 309, 314] Schenck, H. (1884) [p. 202 and Fig. 133, p. 202] Schenck, H. (1885) [Passim] Schenck, H. (1886) [Passim and Figs. 40, p. 64, 41, p. 65, 51, P- 79, 56, p. 87, 74, p. 108, 106, p. 165, 109, p. 168, in, p. 170, 114, p. 176, 138 and 139, p. 209] Schenck, H. (1887) BIBLIOGRAPHY Notes biologiques sur les Potamogeton. Journ. de Bot. T. vni. 1894, pp. 1-9, 21-43, 45-58, 98-106, 112-123, 140-148, 165-172, 31 text-figs. [An account of the anatomy and life-history of Potamogeton crispus L., P. trichoides Ch. et Schl., P. pusillus L., P. gemnti- parus (Robbins) Morong, P. acutifolius Link, P. perfoliatus L., P. polygonifolius Pourr., P. lucens L., P. pectinatus L., P. natans L., P. densus L.] The embryo-sac of Alisma Plantago. Bot. Gaz. Vol. xxi. pp. 123-132, 2 pis., 1896. [An account of fertilisation and embryo development in this species.) Contribution to the Life History of Sagittaria varia- bilis. Bot. Gaz. Vol. xxm. 1897, pp. 252-273, 7 pis. (This paper is confined to an account of the gametophytes, fertilisation and embryology of this species.) Some Morphological Peculiarities of the Nym- phaeaceae and Helobiae. The Ohio Naturalist, Vol. iv. 1904, pp. 83-92, 3 pis., 2 text-figs. (The author attempts to show that the Nymphaeaceae are Monocotyledons . ) Ueber Structuranderung submers vegetirender Land- pflanzen. Ber. d. deutsch. bot. Gesellsch. Bd. n. 1884, pp. 481-486, i pi. (An account of the differences observed between the structure of the normal terrestrial form of Cardamine pratensis, and of the same species when growing submerged.) Die Biologic der Wassergewachse. Verhandl. des naturhist. Vereines d. preuss. Rheinlande, Westfalens und des Reg.-Bezirks Osnabriick, Jahrg. 42 (Folge v. Jahrg. 2), 1885, pp. 217-380, 2 pis. [This memoir, in conjunction with Schenck, H. (1886), forms one of the most important general contributions ever made to the study of water plants ; it summarises the state of knowledge of a generation ago. Many of the more recent accounts of this biological group are based on Schenck's work.] Vergleichende Anatomic der submersen Gewachse. Bibliotheca Botanica, Bd. i. Heft i. 1886, 67 pp., 10 pis. (A detailed and fully illustrated account of the anatomy of those water plants which are most completely specialised for an aquatic life.) Beitrage zur Kenntniss der Utricularien. Utricularia montana Jacq. und Utr. Schimperi nov. spec. Pringsheim's Jahrb. Bd. xviu. 1887, pp. 218-235, 3 Pis. (The two species described in this paper are terrestrial, but they are compared with the aquatic members of the genus.) BIBLIOGRAPHY 401 Schenck, H. (1889) Ueber das Aerenchym, ein dem Kork homologes [pp. 188,189, 192 and Gewebe bei Sumpfpflanzen. Pringsheim's Jahrb. f. Fig. 122, p. 190] wissen. Bot. Bd. xx. 1889, pp. 526-574, 6 pis. (A detailed account of the occurrence of aerenchyma in Onagraceae, Lythraceae, Melastomaceae, Hypericaceae, Cap- paridaceae, Euphorbiaceae, Labiatae and Leguminosae. The aerenchyma is regarded as primarily a breathing tissue.) Scheuchzerus, J. Agrostographia sive Graminum, Juncorum, Cypero- (1719) rum, Cyperoidum, iisque affinium Historia. . . .Tiguri, [p. 154] Typis et Sumptibus Bodmerianis, 1719. (This book contains an early reference to the floating leaves of Scirpus lacustris; see "Scirpus paniculatus," p. 354.) Schiller, K. See Schorler, B., Thallwitz, J. and Schiller, K. (1906) . Schilling, A. J. (1894) Anatomisch-biologische Untersuchungen iiber die [p. 271] Schleimbildung der Wasserpflanzen. Flora, Bd. 78, 1894, pp. 280-360, 17 text-figs. (A full account of the mucilage organs of water plants; the author shows that they are all of the morphological nature of hairs. He believes that the function of the mucilage, which is formed in all cases at the expense of the cell wall, is to prevent excess of water from passing into the young tissues.) Schindler, A. K. (1904) Die Abtrennung der Hippuridaceen von den Halor- [pp. 181, 312] rhagaceen. Beiblatt zu den Bot. Jahrb. (Engler) Bd. xxxiv. Heft 3, 1904, pp. 1-77. [A detailed study of the anatomy and morphology of these families from which the author concludes that Halorrhagaceae (Halorrhagideae + Gunnereae) and Hippuridaceae are entirely unrelated.] Schlechtendal, D. F. L. Einige Worte iiber Nymphaea neglecta und biradiata. von (1852) Bot. Zeit. Jahrg. 10, 1852, pp. 557-559. (The author shows that these two species of Castalia tend to approach one another, but he leaves open the question as to whether transitional forms exist.) Schlechtendal, D. F. L. Betrachtungen iiber die Limosella-Arten. Bot. Zeit. von (1854) Jahrg. 12, 1854, pp. 909-918. (A critical account of the species and varieties.) Schleiden, M. J. (1837) Beitrage zur Kenntniss der Ceratophylleen. Linnaea, [pp. 63, 84, 86] Bd. n, 1837, pp. 513-542, i pi. (A very thorough account of the family ; the author includes all the known forms under the single species Ceratophyllum vulgare, Schl.) Schleiden, M. J. ( 1 8381) Bemerkungen iiber die Species von Pistia. Allge- [p. 316] meine Gartenzeitung, Jahrg. 6, No. 3, 1838, pp. 17-20. (A systematic account of the genus, with a discussion of its affinities.) Schleiden, M.J.( 1 8382) Berichtigungen und Nachtrage zur Kenntniss der Ceratophylleen. Linnaea, Bd. 12, 1838, pp. 344-346, i pi. [Supplementary to Schleiden, M. J. (1837). The germination of the seeds is described, and attention is drawn to the suppres- sion of the primary root and the absence of adventitious roots.] A. W. P. 26 4O2 Schleiden,M.J.(i839) [P- 73] Schoenefeld, W. de (1860) [p. no] Schorler, B., Schiller, K. ] [p. 291] Schrenk, J. (1888) [pp. 30, 205, 258, 266, 267, 272] Schrenk, J. (1889) [p. 193 and Fig. 124, p. 193] Schroter, C. \ and I (1902) Kirchner, 0. J [pp. 291, 322] Schroter, C. Schuchardt, T. (1853) Schultz, F. (1873) [p. 101] Schumann, K. (1892) [P- 70] BIBLIOGRAPHY Prodromus Monographiae Lemnacearum oder Con- spectus Generum atque Specierum. Linnaea, Bd. xin. 1839, pp. 385-392. (A systematic account of the Lemnaceae which are treated as a tribe of the Aroideae.) Sur le mode de vegetation de I' A Idrovanda vcsiculosa en hiver et au printemps. Bull, de la soc. bot. de France, T. vii. 1860, pp. 389-392. (The author shows that when kept indoors the turions of this plant may float all the winter and then germinate in the spring.) Pflanzen- und Tierwelt des Moritzburger Grossteiches bei Dresden. Annales de Biologic Lacustre, T. i. Fasc. 2, 1906, pp. 193-310. (This work includes an ecological study of the vegetation of this lake by B. Schorler.) On the Histology of the Vegetative Organs of Brasenia peltata, Pursch. Bull. Torr. Bot. Club, Vol. xv. 1888, pp. 29-47, 2 pis. (The points to which special attention is paid in this paper are the nature and origin of the surface layer of mucilage, the internal hairs, and the submerged leaves.) On the Floating-tissue of Nesaea verticillata (L.), H.B.K. Bull. Torr. Bot. Club, Vol. xvi. 1889, pp. 315-323, 3 Pis. (An account of the biology and anatomy of the "aerenchyma" in a member of the Lythraceae. The author regards it primarily as a floating tissue which serves only secondarily, if at all, for purposes of aeration.) Die Vegetation des Bodensees. T. n. Der " Bodensee- Forschungen"neunter Abschnitt. 86 pp., 3 pis., i map, 15 text-figs. Lindau i. B. 1902. [In this book the water and marsh vegetation (higher plants) of the Bodensee is discussed from an ecological standpoint.] See Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.). Beitrage zur Kenntniss der deutschen Nymphaeen. Bot. Zeit. Jahrg. xi. 1853, pp. 497-510. [A critical account of the species and varieties of Nymphaea (Castalia) native to Germany.] Beitrage zur Flora der Pfalz (Schluss). Flora, Neue Reihe, Jahrg. xxxi. (Ganz. Reihe, Jahrg. LVI.) 1873, pp. 247-251. (The author mentions that Utricularia intermedia, had at that time existed in Pfalz for forty years without flowering.) Morphologische Studien. Heft i. x + 206 pp., 6 pis. Leipzig, 1892. [Pp. 119-186 contain an account of the ontogeny of the flowers of the Potamogetonaceae, Zannichelliaceae and Naiadaceae. The author criticises the views on the flower ofNaias expressed by Magnus, P. (iSyo1).] BIBLIOGRAPHY 4°3 Schumann, K. (1894) Die Untersuchungen des Herrn Raciborski iiber die Nymphaeaceae und meine Beobachtungen iiber diese Familie. Ber. d. deutsch. bot. Gesellsch. Bd. xn. 1894, pp. I73-I78- [A criticism of Raciborski, M. (I8941).] Scott, D. H. (1891) [p. 180] Scott, D. H.) and > (1888) Wager, H. J Scott, J. (1869) [P- 235] Seehaus, C. (1860) [p. 286] Seidel, C. F. (1869) [PP- 34, 309] Sergueeff, M. (1907) [pp. 142, 244, 281, 314 and Fig. 91, p. 142] Shull, G. H. (1905) [P- 162] Origin of Polystely in Dicotyledons. Annals of Bot. Vol. v. 1890-1891, pp. 514-517. (In this paper the hypothesis is put forward that the cases of polystely known to occur among Angiosperms maybe associated with an aquatic ancestry.) On the Floating-Roots of Sesbania aculeata, Pers. Ann. Bot. Vol. i. 1887-1888, pp. 307-314, i pi. (An account of the aerenchyma developed on the roots of this Leguminous plant. The spongy tissue is produced by a cortical phellogen.) Note on the Isoetes capsularis, Roxb. Journ. Linn. Soc. Bot. Vol. x. 1869, pp. 206-209, i pi. (In this note, the curator of the Calcutta Botanic Garden shows that Roxburgh's so-called "Isoetes capsularis" is the detached male flower of Vallisneria spiralis, L.) Hydrilla verticillata (L. fil.) Casp. var. pomeranica (Rchb.) Casp. Verhandlung. d. bot. Vereins f. d. Provinz Brandenburg, Heft n. 1860, pp. 95—102. (Observations on the life-history of this species.) Zur Entwickelungsgeschichte der Victoria regia Lindl. Nov. Act. Acad. Caes. Leopoldino-Carolinae Germanicae Naturae Curiosorum (Verhandl. d. Kais. Leop.-Car. deutschen Akad. d. Naturforscher), T. 35, 1870 (for 1869), No. 6, 26 pp., i table, 2 pis. (A general account of Victoria regia with a discussion of its affinities. The author regards the Nymphaeaceae as Mono- cotyledons related to the Hydrocharitaceae.) Contribution a la morphologic et la biologic des Aponogetonacees. Universite" de Geneve. Thdse... docteur es sciences, Institut de Botanique. Prof. Dr Chodat, 7me serie, vmme fasc. 1907, 132 pp., 5 plsM 78 text-figs. [A detailed study of Aponogeton (Ouvirandra) fenestralis, and A. distachyus. A general account of the family and a discussion of its affinities are included.] Stages in the Development of Slum cicutae folium. Carnegie Institution of Washington, Publication No. 30. Papers of Station for Experimental Evolution at Cold Spring Harbor, New York, No. 3, 1905, 28 pp., 7 pis., ii text-figs. (A study of heterophylly in this species. 26 — 2 404 Siddall, J. D. (1885) [pp. 55, 210, 211] Snell, K. (1908) [pp. 208, 260, 265] BIBLIOGRAPHY The American Water Weed, Anacharis Alsinastrum, Bab. : Its Structure and Habit ; with some Notes on its introduction into Great Britain, and the causes affecting its rapid spread at first, and apparent present diminution. Proc. Chester Soc. Nat. Sci. No. 3, 1885, pp. 125-134, i pi. (This paper gives the early history of Elodea canadensis, Michx. in this country.) Untersuchungen iiber die Nahrungsaufnahme der Wasserpflanzen. Flora, Bd. 98, 1908, pp. 213-249, 2 text-figs. (The author's main conclusion is that, in the case of rooted submerged plants, the greater part of the water supply is taken in by the roots, but that the leaves may also absorb water.) Der Transpirationsstrom der Wasserpflanzen. Ber. d. deutschen bot. Gesellsch. Jahrg. xxx. 1912, pp. 361, 362. [A note which should be read in connexion with Snell, K. (1908) and Hannig, E. (1912).] Contributions to the knowledge of the diaphragms of water plants. I. Scirpus validus. Bot. Gaz. ¥01.58, 1914, pp. 495-51?. l6 text-figs. (This paper contains a comprehensive review of the records in the literature relating to the occurrence of diaphragms in various groups of the higher plants.) Systematisch-anatomische Untersuchung des Blattes der Hydrocharitaceen. Beihefte zum Bot. Central bl. Bd. xxx. Abth. i. 1913, pp. 24-104, 53 text-figs. (A highly detailed comparative study of the leaves of the Hydrocharitaceae. The author has examined all the genera belonging to this family.) Zur Anatomie und Biologic der neuen Hydrocharis- Arten aus Neuguinea. Mededeelingen van's Rijks Herbarium Leiden, No. 21, 1914, 2 pp. (A description of the leaf structure of H. parnassifolia and H. parvula — the former has typical air leaves like H. asiatica, and the latter, swimming leaves like H. Morsus-ranae.) Solms-Laubach, H. Pontederiaceae. A. andC.de Candolle'sMonographiae Graf zu (1883) Phanerogamarum, Vol. iv. 1883, pp. 501-535. [p. 31?] (A systematic account of this group with a discussion of the geographical distribution, etc.) Spenner, F. C. L. (1827) Ueber Nuphar minima Smith. Flora, Jahrg. x. Bd. i. [p. 28] 1827, pp. 113-119, 2 pis. [In his account of this plant, the author describes the submerged leaves and figures them (PI. I). He suggests that leaves of this type probably occur in other Nymphaeaceae, but that they have been overlooked.] Snell, K. (1912) [p. 266] Snow, L. M. (1914) [P- 183] Solereder, H. (1913) [pp. 42, 46, 52, 135, 165, 169, 340] Solereder, H. (1914) [p. 42] BIBLIOGRAPHY 405 Spruce, R. (1908) [pp. 3i, 99, I54> 190, 229, 290, 291, 311] Stahl, E. (1900) [p. 164] Standley, P. C. Stein, B. (1874) [p. in] Stohr, A. (1879) [pp. 165, 171, 279] Strasburger, E. (1884) [P- 37] Strasburger, E. (1902) [pp. 85, 86, 272, 309] Sykes, M. G. Sylven, N. (1903) Tackholm, G. (1914) [P- Notes of a Botanist on the Amazon and Andes... during the years 1849-1864, edited by A. R. Wallace. 2 vols. London, 1908. (These volumes contain a number of notes on the aquatic plants observed by Spruce in S. America.) Der Sinn der Mycorhizenbildung. Pringsheim's Jahrbuch. Bd. 34, 1900, pp. 539-668, 2 text-figs. (The author does not deal with aquatics, but his classification of plants into "starch leaved" strong transputers and "sugar leaved" weak transpirers seems to have a bearing upon the nature of the epidermis of submerged leaves.) See Miller, G. S. and Standley, P. C. (1912). t)ber Reizbarkeit der Blatter von Aldrovanda vesicu- losa. Zweiundfiinfzigster Jahres-Ber. d. Schlesischen Gesellsch. 1875 (1874), pp. 83-84. (The author records the sensitiveness of the Aldrovandia leaf to contact.) Cfber Vorkommen von Chlorophyll in der Epidermis der Phanerogamen-Laubblatter. Sitzungsberichte der math.-naturwissens. Classe d. k. Akad. d. Wissens. Wien, Bd. LXXIX. Abth. i. 1879, pp. 87-118, i pi. (In opposition to the current opinion, the author shows that chlorophyll is frequently present in the epidermis of the lower side of the leaf in terrestrial Dicotyledons, while it is absent hi the case of terrestrial Monocotyledons.) Das Botanische Practicum. xxxvi + 664 pp., 182 figs. Jena, 1884. [This well-known text-book contains many references to the anatomy of water plants, e.g. Vallisneria, p. 54; Nymphaea (Castalia), p. 171; Potamogeton,p.i82; Hippuris,pp. 185 and 249; Elodea, p. 187.] Ein Beitrag zur Kenntniss von Ceratophyllum sub- mersum und phylogenetische Erorterungen. Prings. Jahrb. f. wiss. Bot. Bd. 37, pp. 477-526, 3 pis., 1902. (This investigation supports the view that Ceratophyllum is allied to Nymphaeaceae. The development of embryo-sac and pollen-grain are described in detail. There is a discussion of the use of mucilage in water plants.) See Thoday, D. and Sykes, M. G. (1909). Studier ofver organisationen och lefnadssattet hos Lobelia Dortmanna. Arkiv for Botanik utgifvet af K. Svenska Vetenskaps-Akad. Bd. 1. 1903-1904, i pi., PP- 377-388. (This Swedish paper is reviewed in the Bot. Centralbl. Bd. 93, 1903, pp. 613-614). Zur Kenntnis der Embryosackentwicklung von Lopezia coronata Andr. Svensk Bot. Tidsk. Vol. 8, 1914, pp. 223-234, 5 text-figs. (The author is in favour of removing Trapa from the Onagraceae on account of its embryo-sac structure.) 406 BIBLIOGRAPHY Tackholm, G. (1915) Beobachtungen iiber die Samenentwicklung einiger [p. 311] Onagraceen. Svensk Bot. Tidsk. Vol. 9, 1915, pp. 294-361, 1 6 text-figs. (It is pointed out on p. 354 that in the true Onagraceae and in Tvapa we have two highly differentiated and widely separated embryo-sac types.) Tansley, A. G. (1911) Types of British Vegetation, by Members of the [pp. 286, 287, 288, Central Committee for the Survey and Study of 290] British Vegetation, edited by A. G. Tansley. Cambridge, 1911. (The following sections of this book deal with the ecology of British water plants: pp. 187-203, Aquatic Vegetation; pp. 223-229, The Aquatic Formation of the River Valleys of East Norfolk.) Tapper, J. G. O. (1882) Some Observations on the Propagation of Cymodocea [pp. 127, 205] antarctica (Endl.). Trans, and Proc. and Rep. of the Royal Society of South Australia, Vol. iv. 1882 (for 1880-81), pp. 1-4, i pi. Further Observations on the Propagation of Cymodocea antarctica. Ibid. pp. 47- 49, i pi. (The first account of the viviparous growth of this plant.) Terras, J. A. (1900) [pp. 48, 280] Thallwitz, J. Theophrastus (Hort) (1916) [p. 208] Thiebaud, M. (1908) [P- 291] ay, D. ) md V J, M. G.) Thoday, D. and Sykes [pp. 262, 266] (1909) Thurn, E. F. Im Notes on the Germination of the Winter Buds of Hydrocharis Morsus-Ranae. Trans, and Proc. of the Bot. Soc. of Edinburgh, Vol. xxi. 1900 (for 1896- 1900), Part iv. 1900, pp. 318-329. (An account of experiments upon the germination of winter buds of this species, with special reference to conditions of illumination.) See Schorler, B., Thallwitz, J. and Schiller, K. (1906). Enquiry into Plants, with an English translation by Sir Arthur Hort. 2 vols. London, 1916. (Book iv. Chapter 9 contains an exceedingly clear description of Trapa natans.) Contribution a la Biologic du Lac de Saint-Blaise. Annales de Biologic Lacustre, T. in. Fasc. i, 1908, pp. 54-140, 5 pis. (This work contains a short section dealing with the plants of the Lake.) Preliminary Observations on the Transpiration Current in Submerged Water-plants. Ann. Bot. Vol. xxin. 1909, pp. 635-637. (The authors have demonstrated by experiments on plants of Potamogeton lucens, in situ in the River Cam, that an unex- pectedly rapid water current occurs in detached, rootless stems, and that this current is to a great extent dependent on the leaves.) See Im Thurn, E. F. BIBLIOGRAPHY 407 Tieghem,P.van(i866) [P- 256] Tieghem, P. van (1867) Tieghem,P.van(i868) [pp. 107, 108, 227] Tieghem, P. van (1869!) [p. 107] Tieghem, P. van (1869s) Tittmann,J.A.(i82i) [P- 34] Trecul, A. (1845) [PP- 37, 309] Trecul, A. (1854) [PP- 33, 37, 38, 309] Treviranus, L. C. (1821) [p. 164] Recherches sur la respiration des plantes submerges. Bull, de la Soc. bot. de France, T. xm. 1866, pp. 411-421. [An account of some experiments dealing with assimilation (not respiration in the modern sense). The author claims to show that if, in the case of certain submerged plants, the decomposition of CO2 is initiated in direct sunlight, it may continue actively for some hours after the plant has been placed in darkness; see, however, Tieghem, P. van (iS6gz).] Note sur la respiration des plantes aquatiques. Comptes rendus de Tacad. des sciences, Paris, T. 65, 1867, pp. 867-871. [A further communication dealing with the same results as Tieghem, P. van (1866).] Anatomic de I'Utriculaire commune. Bull, de la Soc. bot. de France, T. xv. 1868, pp. 158-162. (An account of the anatomy of the submerged and aerial parts of Utricularia vulgaris.) Anatomic de 1'Utriculaire commune. Ann. d. sci. nat. Ser. v. Bot. T. x. 1869, pp. 54-58. [See Tieghem, P. van (1868).] Sur la respiration des plantes submergees. Comptes rendus de 1'acad. des sciences, Paris, T. 69, 1869, PP- 531-535- [In this paper the author withdraws his previously expressed opinion (Tieghem, P. van (1866) and (1867) that assimilation in submerged plants continues after the removal of the light.] Die Keimung der Pflanzen. viii + 200 pp., 27 pis. Dresden, 1821. [The seedlings of a number of water plants are described and figured: Alisma Plantago, Nymphaea (Castalia) alba and Nuphar luieum (Nymphaea lutea), Potamogeton natans, Trapa natans.] Recherches sur la structure et le developpement du Nuphar luteum. Ann. des sci. nat. Ser. in. Bot. T. iv. 1845, pp. 286-345, 4 Pis. (The anatomy of the stem, roots and leaves, and the structure of the reproductive organs, are described in detail. Attention is drawn to the points in the anatomy and mode of germination which recall the Monocotyledons.) fitudes anatomiques et organogeniques sur la Victoria regia, et anatomic compared du Nelumbium, du Nuphar et de la Victoria. Ann. d. sci. nat. Ser. iv. Bot. T. i. 1854, pp. 145-172, 3 pis. [From his study of Victoria regia and other Waterlilies the author concludes that the Nelumbiaceae differ widely from. the Nymphaeaceae. Among the points to which he draws attention are the operculum of the seed of Victoria and Nuphar ( Nymphaea) and the succession of leaf types in the seedling of Victoria. For a criticism see Blake, J. H. (1887).] Vermischte Schriften. Bd. 4. ii + 242 pp., 6 pis. Bremen, 1821. (The "absence of an epidermis" on the lower side of the iea of Potamogeton crispus is alluded to on p. 76.) 408 Treviranus, L. C. (1848^) [PP. 93, 99, 154] Treviranus, L. C. (1848*) Treviranus, L. C. (i853) Treviranus, L. C. (1857) [P- 67] Tulasne, L. R. (1852) [p. 112] Unger, F. (1849) Unger, F. (1854!) Unger, F. (1854*) [P- 256] BIBLIOGRAPHY Noch etwas iiber die Schlauche der Utricularien. Bot. Zeit. Jahrg. 6, 1848, pp. 444-448. (Notes on the bladders of Utricularia which the author regards as of foliar nature.) Observationes circa germinationem in Nymphaea et Euryale. Abhandl. d. Math.-Phys. Classe d. konig. bay. Akad. d. Wiss. Bd. v. Abt. n. 1848, pp. 397-403, i pi. [A description in Latin of the germination of Nymphaea (Castalia) caerulea and Euryale ferox. In the latter case the author figures the four outgrowths which were later described by Goebel as breathing organs.] De germinatione seminum Euryales. Bot. Zeit. Jahrg. xi. pp. 372-374, 1853. [This short paper should be read in connexion with Treviranus, L. C. (l8482) since it consists of corrections of the latter, based on a better supply of material of Euryale ferox.] Vermischte Bemerkungen. i . Hybernacula des Pota- mogeton crispus. 2. Hybernacula der Hydrocharis Morsus Ranae L. Bot. Zeit. Jahrg. 15, 1857, pp. 697-702, i pi. [As regards Potatnogeton crispus Treviranus confirms the observations recorded by Clos, D. (1856). He also gives a short description of the winter buds of Hydrocharis.] Podostemacearum Monographia. Archives du Mu- seum d'hist. nat. T. vi. 1852, 208 pp., 13 pis. (This highly important Latin monograph is illustrated with a series of exquisite plates, giving a clear idea of the peculiarities of this anomalous family.) Die Entwickelung des Embryo's von Hippuris vulgaris. Bot. Zeit. Jahrg. 7, 1849, pp. 329-339, 2 pis. (A description of the embryology of Hippuris. The spherical multicellular embryo becomes sunk in the endosperm by means of a long suspensor.) Einiges iiber die Organisation der Blatter der Victoria regia Lindl. Sitzungsber. d. k. Akad. d. Wissenschaften, Math.-Naturwissens. Classe, Bd. xi. Wien, 1854 (for 1853), pp. 1006-1014, i pi. (The author describes the minute perforations which are characteristic of the leaves of Victoria regia.) Beitrage zur Physiologic der Pflanzen. I. Bestim- mung der in den Intercellulargangen der Pflanzen enthaltenen Luftmenge. Sitzungsber. d. k. Akad. d. Wissenschaften, Math.-Naturwissens. Classe, Bd. xii. Wien, 1854, pp. 367-378. (The author shows experimentally how much air is contained in various plant tissues. One of the organs investigated was the leaf of Pistia.) BIBLIOGRAPHY 409 linger, F. (1862) [p. 260] Ursprung, A. (1912) CP. 258] Uspenskij,E.E.(i9i3) [pp. 139, 195] Vaucher, J. P. (1841) [pp. 216, 219] Vochting, H. (1872) [p. 178 and Figs. 1 16 and 1 17, p. 179] Volkens, G. (1883) [p. 267] Vries, H. de (1873) [P- 284] Wachter, W. (1897!) [pp. 12, 117, 156,266] Beitrage zur Anatomic und Physiologic der Pflanzen. XII. Neue Untersuchungen iiber die Transspiration der Gewachse. Sitzungsberichte der math. -naturwiss. Classe der k. Akad. der Wissens. Wien, Bd. XLIV. Abth. n. 1862, pp. 327-368, i text-fig. (Pp. 364-367 contain an account of experiments on water plants demonstrating the existence of a definite transpiration stream even in submerged plants.) Zur Kenntnis der Gasdiffusion in Pflanzen. Flora, N.F. Bd. 4 (G.R. Bd. 104), 1912, pp. 129-156. (The greater part of this memoir is occupied with a critical account of the literature dealing with the bubbling of gas which takes place, under certain conditions, from the leaves of the Nymphaeaceae. The writer also brings forward some fresh experimental evidence.) Zur Phylogenie und Ekologie der Gattung Potamo- geton. I. Luft-, Schwimm- und Wasserblatter von Potamogeton perfoliatus L. Bull, des Naturalistes de Moscou, N.S., Vol. 27, 1913, pp. 253-262, 3 text-figs. (An account of the land form of this species, followed by a general comparison between dissected and thin flat laminae, regarded as adaptations to aquatic life.) Histoire physiologique des plantes d'Europe. T. n. 743 PP- Paris, 1841. (On p. 358 the winter buds of Myriophyllum are described. Observations on other water plants are also included.) Zur Histologie und Entwickelungsgeschichte von Myriophyllum. Nova Acta Acad. Caesareae Leo- poldino-Carolinae Germanicae Naturae Curiosorum. T. xxxvi. 1873, 18 pp., 4 pis. (An account of the anatomy and apical development of this genus.) Ueber Wasserausscheidung in liquider Form an den Blattern hoherer Pflanzen. Jahrb. d. k. bot. Gartens und d. bot. Museums zu Berlin, Bd. n. 1883, pp. 166-209, 3 pis. (This paper is the earliest general account of the excretion of water in liquid form from the leaves of the higher plants. The structure and development of the apical opening in the leaf of Alisma Plantago are described and figured, p. 206 and PL VI, figs. 5 and 6.) Die vitalistische Theorie und der Transversal- Geotropismus. Flora, N.R. Jahrg. xxxi. (G.R. Jahrg. LVI.) 1873, pp. 305-315- [A criticism of Frank, A. B. (1872).] Beitrage zur Kenntniss einiger Wasserpflanzen. I. and II. Flora, Bd. 83, 1897, pp. 367-397, 21 text-figs. (The first part of this paper deals with the results of experi- mental work on the production of the different forms of leaves in Sagittaria natans, Michx., S. chinensis, Sims, Eichhornia azurea, Kth., Heteranthera reniformis, R. et P., Hydrocleis nymphoides, Buchenau. The second part deals with the morphology and anatomy of Weddellina squamulosa, Tul., one of the Podostemaceae.) 4io Wachter, W. (18972) [P- 159] Wager, H. Wagner, R. (1895) [p. 39 and Fig. 23, p. 41] Walker, A. O. (1912) [p. 212] Walsingham, Lord, and Payne-Gallwey, R. (1886) [PP- I35, 302] Walter, F. (1842) [PP- 15, i?] Warming, E. (1871) IP- J35] Warming, E. (1874) [p. 100] Warming, E. (1881, 1882, 1888, 1891) [pp. 112, 118, 310] BIBLIOGRAPHY Beitrage zur Kenntniss einiger Wasserpflanzen. III. Flora, Bd. 84, Erganzungsband zum Jahrgang 1897, PP- 343-348. [This paper is a continuation of the first part of Wachter, W. (I8971). It contains an account of experiments upon the heterophylly of Castalia, showing that the production of the different forms of leaf in this genus is dependent upon external conditions, just as in the case of the Monocotyledons previously investigated;] See Scott, D. H. and Wager, H. (1888). Die Morphologic des Limnanthemum nymphaeoides (L.) Lk. Bot. Zeit. Jahrg. 53, Abt. i. 1895, pp. 189-205, i pi., 1895. (A general descriptive paper dealing with the development, branching, etc. of this species.) The Distribution of Elodea canadensis, Michaux, in the British Isles in 1909. Proc. Linn. Soc. London, I24th session, 1912, pp. 71-77. (This paper gives the result of enquiries made in 1909 among local natural history societies as to the degree of success attained by Elodea in establishing itself in this country.) Shooting (Moor and Marsh). Badminton Library, xiii + 348 pp. London, 1886. (The authors mention, pp. 158 and 165, that Brent Geese feed on Zostera, and that these birds are almost confined to those parts of the coast where Zostera occurs.) Bemerkungen liber die Lebensweise einiger deutschen Pflanzen. Flora, Jahrg. xxv. Bd. n. 1842, pp. 737- 748, i pi. (A picturesque account of Walter's discovery of tuber-formation and heterophylly in Sanitaria sagittifolia.) Forgreningen hos Pontederiaceae og Zostera. Viden- skab. Meddel. fra den naturhist. Forening i Kjoben- havn for Aaret 1871, pp. 342-346, i text-fig. (This Danish paper deals with the nature of the shoot system in the plants mentioned.) Bidrag til Kundskaben om Lentibulariaceae. Viden- skab. Meddel. fra den naturhist. Forening i Kjoben- havn for Aaret 1874 (1874-5), PP- 33-58, 3 pis. (This paper, which is in Danish, deals with Genlisea and Utricularia. The germination of Utricularia is described.) Familien Podostemaceae. Kongel. Dansk. Videnskab. Selskabs Skrifter. Sjette Raekke. i. Vol. n. 1881, pp. 1-34, 6 pis. 2. Vol. ii. 1882, pp. 77-130, 9 pis. 3. Vol. iv. 1888, pp. 443-514, 12 pis. 4. Vol. vn. 1891, pp. 133-179, 185 text-figs. (This important monograph is in Danish, but each part is followed by a French resume.) BIBLIOGRAPHY 411 Warming, E. (1883!) [P- 245] Warming, E. (i88s2) [Figs. 76 and 77, p. 115, 79, p. 116] Warming, E. (1909) [p. 291] Webber, H. J. (1897) [p. 213] Botanische Notizen. Bot. Zeit. Jahrg. 41, 1883, pp. 193-204. (In section 2, "Zur Biologic der Keimpflanzen," pp. 200-203, the author refers to the development of long root-hairs, at the junction of root and hypocotyl, which attach the seedlings of certain water plants to the substratum.) Studien iiber die Familie der Podostemaceae. Engler's Bot. Jahrbiich. Bd. iv. 1883, pp. 217-223, 5 figs. (A German version of part of the author's work on this family.) (Ecology of Plants, xi + 422 pp. Oxford, 1909. (This English version of the author's well-known book contains sections dealing with aquatic and marsh plants; see especially pp. 97-100 and 149-190.) The Water Hyacinth, and its relation to navigation in Florida. U.S. Depart, of Agriculture. Division of Botany. Bulletin, No. 18, 1897, 20 pp., i pi., 4 text- figs. [An account of the excessive luxuriance of Piaropus crassipes, (Mart.) Britton, — Eichhor.nia speciosa, Kunth, = Eichhornia crassipes, (Mart.) Solms.] Observations sur une espece nouvelle du genre Wolffia (Lemnacees). Ann. des sci. nat. Ser. in. Bot. T. 12, 1849, pp. 155-173, i pi. (The author discovered in Brazil a minute species of Wolffia, which he named W. brasiliensis. Twelve of the flowering plants could be accommodated on one frond of Lemna minor.) Weddell, H. A. (1872) Sur les Podoste"macees en general, et leur distribution [pp. 113, 295] geographique en particulier. Bull, de la Soc. bot. de France, T. xix. 1872, pp. 50-57. (This paper is based upon the author's own observations in Brazil. Stress is laid upon the very local distribution of many of the Podostemaceae.) Weinrowsky , P. ( 1 899) Untersuchungen iiber die Scheiteloff nung bei Wasser- [pp. 261, 266, 269] pflanzen. Fiinfstiick's Beitrage zur Wissensch. Bot. Bd. in. 1899, pp. 205-247, 10 text-figs. (An extremely important account of the apical openings of the leaves of water plants.) Weddell, H. A. (1849) [pp. 80, 300] Weiss, F. E.) and Murray, H. ) [P- 303] On the Occurrence and Distribution of some Alien (1909) Aquatic Plants in the Reddish Canal. Mem. and Proc. of the Manchester Lit. and Phil. Soc. Vol. 53, 1909, No. 14, 8 pp., i map. [The authors show that Naias graminea (Del.) var. Delilei (Magnus), recorded in Bailey, C. (1884) as occurring in the warm water of this canal, has now disappeared. Certain alien Algae are also discussed, and the distribution of Vallisneria spiralis, which was planted here forty years ago.] 412 Went, F. A. F. C. (1910) [pp. 114, 122] Werner, E. West, G. (1905) [p. 287] West, G. (1908) [pp. 287, 290] West, G. (1910) [pp. 20, 87, 145, 200, 234, 287, 299, 325] Wettstein, R. von (1888) [p- 38] Wheldale, M. (The Hon. Mrs Huia Onslow) (1916) [p. 277] BIBLIOGRAPHY Untersuchungen ueber Podostemaceen. Verhande- lingen d. Konin. Akad. van Wetenschappen te Amsterdam, Tweede Sectie, Dl. xvi. No. i, 1910, 88 pp., 15 pis. [In this memoir, based upon the results of the author's travels in Surinam, the following members of the Podostemaceae are described: 6 sp. of Oenone of which 3 are new, 3 new species of Apinagia, Lophogyne (i sp.), Mourera (i sp.) and Tristicha (i sp.). The anatomy, and the development of the ovules, are treated, as well as the general morphology.] See Magnus, W. and Werner, E. (1913). A Comparative Study of the dominant Phanerogamic and Higher Cryptogamic Flora of Aquatic Habit, in Three Lake Areas of Scotland. Proc. Roy. Soc. Edinb. Vol. xxv. Part n. 1906 (for 1905), pp. 967- 1023, 55 pis. (A general ecological survey of certain Scottish Lakes.) Notes on the Aquatic Flora of the Ness Area. Bathymetrical Survey of the Fresh-water Lochs of Scotland. VIII. The Geogr. Journal, Vol. xxxi. 1908, pp. 67-72. [This brief paper, which is of a general nature, should be read in conjunction with the author's detailed studies of the Scottish lakes— West, G. (1905) and (1910).] A Further Contribution to a Comparative Study of the dominant Phanerogamic and Higher Crypto- gamic Flora of Aquatic Habit in Scottish Lakes. Proc. Roy. Soc. Edinb. Vol. xxx. 1910 (Session 1909-10), pp. 65-181, 62 pis. [A continuation of West, G. (1905).] Beobachtungen iiber den Bau und die Keimung des Samens von Nelumbo nucifera Gartn. Verhandl. d. k. k. zool.-bot. Gesellsch. in Wien, Bd. 38, 1888, pp. 41-48, i pi. (The structure and germination of the seed of this plant, which has no endosperm or perispenn, is figured with great clearness.) The Anthocyanin Pigments of Plants, x + 318 pp. Cambridge, 1916. [Chapter vi. (Physiological Conditions and Factors Influencing the Formation of Anthocyanins) and Chapter vm. (The Significance of Anthocyanins), may be consulted in connexion with the red coloration so prevalent in water plants.] Wheldon, J. A.J The Flora of West Lancashire. 51 1 pp., 15 pis., i map. and ["(1907) Eastbourne, 1907. Wilson, A. ) (On p. 339 a reference is made to a pond which was dug [p. 299] experimentally in order to see what water plants would colonise it.) BIBLIOGRAPHY 413 Wigand, A. (1871) [P- 37] Wight, R. (1849) [P- 99] Willdenow, C. L. (1806) [P- 85] Willis, J. C. (1902) [Passim and Figs. 78, p. 115, 80, p, 118, 82, p. 121] Willis, J. C. (I9I41) [pp. 112, 286, 327, 329] Willis, J. C. (I9I42) [P- 305] Willis, J. C. (I9I51) [p. 112] Willis, J. C. (I9i52) [pp. 112, 327] Willis, J. C. (1917) [p. 306] Nelumbium sped osum, W. Bot. Zeit. Jahrg. 29, 1871, pp. 813-826, i text-fig. (An account of the development, morphology, anatomy and starch distribution in this member of the Nymphaeaceae.) Conspectus of Indian Utriculariae. Hooker's Journal of Botany and Kew Garden Miscellany, Vol. i. 1849, PP- 372-374- (The author records the occurrence of a whorl of floats below the flower in U. stellaris.) Determination of a new aquatic vegetable Genus, called Caulinia, with general Observations on Water- plants. Annals of Botany (edited by C. Konig and J. Sims), Vol. ii. 1806, pp. 39-51. (A translation of a paper by this author who was the first to suggest that the pollination of Ceratophyllum was hydro- philous.) Studies in the Morphology and Ecology of the Podostemaceae of Ceylon and India. Ann. Roy. Bot. Gard. Peradeniya, Vol. i. 1902, pp. 267-465, 34 pis. (An important general work dealing with the structure and biology of this group.) On the Lack of Adaptation in the Tristichaceae and Podostemaceae. Proc. Roy. Soc. Vol. 87, B. 1914, PP- 532-550. (The detailed development of a thesis, to which the author has been led in the course of seventeen years' study of these families in India, Ceylon and Brazil — namely, that the natural selection of infinitesimal variations is quite incompetent to explain their evolution.) The Endemic Flora of Ceylon, with Reference to Geographical Distribution and Evolution in General. Phil. Trans. Roy. Soc. London, Ser. B, Vol. 206, 1914, pp. 307-342. (This paper does not deal with water plants, but is quoted here because it is the first of the series of contributions in which the author has developed his " Age and Area " hypothesis, which has an important bearing on the study of aquatics.) A New Natural Family of Flowering Plants — Tristichaceae. Linn. Soc. Journ. Bot. Vol. 43, 1915, pp. 49-54- (A proposal to separate the Podostemaceae into two families — Tristichaceae = Chlamydatae, and Podostemaceae = Achlamy- datae.) The Origin of the Tristichaceae and Podostemaceae. Ann. Bot. Vol. xxix. 1915, pp. 299-306. (A reconstruction of the type of ancestor from which these groups are probably derived.) The Relative Age of Endemic Species and other Controversial Points. Ann. Bot. Vol. xxxi. 1917, pp. 189-208. (See pp. 201, 202 for a consideration of the Podostemaceae and Tristichaceae from the point of view of the author's "Age and Area" Law of plant distribution.) 414 Willis, J. C. ) and L (1895) Burkill, I. H.j [p. 230] Wilson, A. Wilson, W. (1830) [P- 76] Wydler, H. (1863) Wylie, R. B. (1904) [PP. 55, 57] Wylie, R. B. (1912) [PP- 55, 86 and Fig- 35, P- 56J Wylie, R. B. (I9I71) [p. 234 and Fig. 153, P- 234] Wylie, R. B. (I9I72) [P- 235] Zacharias, O. (1891) BIBLIOGRAPHY Flowers and Insects in Great Britain. Ann. Bot. Vol. ix. 1895, pp. 227-273. (This paper includes observations on the pollination of Peplis and Mentha aquatica.) See Wheldon, J. A. and Wilson, A. (1907) Lemna gibba. Remarks on the Structure and Ger- mination. Hooker's Botanical Miscellany, Vol. i. 1830, pp. 145-149, i pl. (A description, with clear figures, of the seedlings of this species.) Morphologische Mittheilungen. Alisma Plantago, L. Flora, N.R. Jahrg. xxi. (G.R. Jahrg. XLVI.) 1863, pp. 87-90, 97-100, 2 pis. (A detailed study of the shoot relations and the inflorescence of Alisma Plantago, L.) The Morphology of Elodea canadensis. Bot. Gaz. Vol. xxxvu. 1904, pp. 1-22, 4 pis. (An account of the gametophytes, pollination, etc. in this species.) A long-stalked Elodea flower. Bull, from the Labs. of Nat. Hist. State University Iowa, Vol. vi. 1912, PP- 43-52, 2 pis. (A description of a new species of Elodea, E. ioensis, in which the male flower reaches the surface through great elongation of its stalk.) Cleistogamy in Heteranthera dubia. Bull, from the Labs, of Nat. Hist. State University Iowa, Vol. vn. No. 3, 1917, PP- 48-58, i pl. (The cleistogamy of this species, which is very thoroughly described, is considered by the author to be ' largely accidental.') The Pollination of Vallisneria spiralis. Bot. Gaz. Vol. 63, 1917, pp. 135-145, i pl. and 6 text-figs. (The author corrects a number of errors in earlier accounts of this plant, and lays great stress upon the part played in pollination by the surface film.) Die Tier- und Pflanzenwelt des Siisswassers, Vol. i. x + 380 pp., 79 text-figs. Leipzig, 1891. (F. Ludwig contributes a section, pp. 65-134, dealing with the Phanerogams of fresh waters.) [ 415 ] INDEX TO BIBLIOGRAPHY GENERA AND FAMILIES NAMED IN THE BIBLIOGRAPHY, EITHER IN TITLES OR ABSTRACTS Aedemone. Hallier, E. (1859) ; Jaensch, T. (I8841) and (18842) > Klebahn, H. (1891); Kotschy, T. (1858). Aeschynomene. Ernst, A. (18722); Hallier, E. (1859); Jaensch T (1884*)' Moeller, J. (1879). Aldrovandia. Auge de Lassu (1861) ; Caspary, R. (1858*), (1859 and 1862) ; Chatin, A. (18582); Cohn, F. (1850) and (1875); Darwin, C. (1875) and (1888); Delpino, F. (1871); Fenner, C. A. (1904); Hausleutner, (1850!) and (1851); Korzchinsky, S. (1886); Maisonneuve, D. de (1859); Monti, G. (1747); Mori, A. (1876); Roxburgh, W. (1832); Schoenefeld, W. de (1860); Stein, B. (1874). Alisma. Bolle, C. (1861-1862); Buchenau, F. (1857); Crocker, W. and Davis, W. E. (1914) ; Fauth, A. (1903) ; Gliick, H. (1905) ; Griset, H. E. (1894); Hofmeister, W. (1858); Loeselius, J. (1703); Miinter, J. (1845); Nolte, E. F. (1825); Schaffner, J. H. (1896); Tittmann, J. A. (1821); Volkens, G. (1883); Wydler, H. (1863). ALISMACEAE. Arber, E. A. N. and Parkin, J. (1907) ; Bolle, C. (1861-1862) ; Buchenau, F. (1882) and (1903!); Gliick, H. (1905); Micheli, M. (1881); Planchon, J. E. (1844). Althenia. Prillieux, E. (1864). Amphibolis. Agardh, C. A. (1821). ANACHARIDEAE. (See HYDRILLEAE.) Anacharis. (See also Elodea.) Douglas, D. (1880); Marshall, W. (1852) and (1857); Siddall, J. D. (1885). Apinagia. Went, F. A. F. C. (1910). Aponogeton. Paillieux, A. and Bois, D. (1888); Planchon, J. E. (1844); Sergueeff, M. (1907). APONOGETONACEAE. Krause, K. and Engler, A. (1906); Sergueeff, M. (1907). ARACEAE. Engler, A. (1877); Jussieu, A. L. de (1789); Schleiden, M. J. (1839). Bergia. Cambessedes, J. (1829). Bidens. Hutchinson, J. (1916). BIGNONIACEAE. Hovelacque, M. (1888). Blyxa. Montesantos, N. (1913). Brasenia. Keller, I. A. (1893); Schrenk, J. (1888). Bulliarda. Caspary, R. (1860). BUTOMACEAE. Arber, E. A. N. and Parkin, J. (1907); Buchenau, F. (1882) and (19032); Micheli, M. (1881); Sauvageau, C. (1893). Butomus. Buchenau, F. (1857); Fauth, A. (1903). CABOMBEAE. Gray, A. (1848); Raciborski, M. (1894!) aQd (18942). Caldesia. Gluck, H. (1905). Callitviche. Baillon, H. (1858); Borodin, J. (1870); Brown, R. (1814); Fauth, A. (1903); Frank, A. B. (1872); Hegelmaier, F. (1864); Irmisch, T. (1859!); Jonsson, B. (1883-1884); Lebel, E. (1863); Ludwig, F. (1881); Magnus, P. (1871); Mer, £. (1881); Richard, L. C. (1808). 416 INDEX TO BIBLIOGRAPHY Caltha. CAPPARIDACEAE. Cardamine. CARYOPH YLLACEAE . Castalia. Caulinia. CERATOPH YLLACEAE . Ceratophyllum. Coleanthus. Comarunt. Cotula. Crassula. Cymodocea. Cynomorium. CYPERACEAE. Damasonium. Desmanthus. Diplanthera. Echinodorus. Eichhornia. ELATINACEAE. Elatine. Eleocharis. Elisma. Elodea. Enhalus. Epilobium. Equisetum. Erigeron. EUPHORBIACEAE. Euryale. Genlisea. Glyceria. Gunnera. GUNNEREAE. Geneau de Lamarliere, L. (1906). Schenck, H. (1889). Schenck, H. (1884). Cambessedes, J. (1829). (See Nymphaea.) Willdenow, C. L. (1806). (See Ceratophyllum.} Borodin, J. (1870); Brongniart, A. (1827); Darwin, C. and F. (1880); Delpino, F. and Ascherson, P. (1871); Dutailly, G. (1892); Gliick, H. (1906); Goppert, H. R. (1848); Gray, A. (1848); Guppy, H. B. (1894!); Irmisch, T. (1853); Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.) ; Ludwig, F. (1881); Magnus, P. (1871); Raciborski, M. (1893); Rodier, £. (1877!) and (18772); Roze, E. (1892); Sanio, C. (1865); Schleiden, M. J. (1837) and (18382); Strasburger, E. (1902); Willdenow, C. L. (1806). Duval-Jouve, J. (1864). Irmisch, T. (1861). Hutchinson, J. (1916). Magnus, P. (1871). Agardh, C. A. (1821) ; Bornet, E. (1864) ; Cavolini, F. (1792*) ; Chrysler, M. A. (1907) ; Delpino, F. and Ascherson, P. (1871) ; Duchartre, P. (1872); Gaudichaud, C. (1826); Magnus, P. (1872); Osborn, T. G. B. (1914); Sauvageau, C. (1889*), (1890*) and (1891 3); Tepper, J. G. O. (1882). Juel, O. (1910). Ascherson, P. (1883); Esenbeck, E. (1914). Gliick, H. (1905). Rosanoff, S. (1871). (See Halodule.) Gluck, H. (1905). Boresch, K. (1912); Miiller, F. (1883); Wachter, W. (1897!) • Webber, H. J. (1897). Cambessedes, J. (1829). Cambessedes, J. (1829); Caspary, R. (1847)- Muller, F. (1877); Raciborski, M. (1893). Paillieux, A. and Bois, D. (1888). Fauth, A. (1903); Gliick, H. (1905). Bolle, C. (1865) and (1867); Brown, W. H. (1913); Caspary, R. (1858!), (18582) and (1858"); Douglas, D. (1880); Geneau de Lamarliere, L. (1906); Hauman-Merck, L. (i9i32)» Holm, T. (1885); Johnston, G. (1853); Overton, E. (1899); Sanio, C. (1865); Siddall, J. D. (1885); Strasburger, E. (1884); Walker, A. O. (1912); Wylie, R. B. (1904) and (1912). Cunnington, H. M. (1912); Delpino, F. and Ascherson, P. (1871); Sauvageau, C. (iSgo2). Batten, L. (1918); Lewakofifski, N. (I8731). Geneau de Lamarliere, L. (1906). Hutchinson, J. (1916). Baillon, H. (1858); Hegelmaier, F. (1864); Richard, L. C. (1808); Schenck, H. (1889). Anon. (1895); Treviranus, L. C. (18482) and (1853). Warming, E. (1874). Geneau de Lamarliere, L. (1906). MacCaughey, V. (1917). Schindler, A. K. (1904). INDEX TO BIBLIOGRAPHY HAEMODORACEAE. Ascherson, P. (1883). Halodule (Diplanthera) . Delpino, F. and Ascherson, P. (1871); Sauvageau, C. (iSgo3) and (iSgi3). Halophila. Balfour, I. B. (1879); Delpino, F. and Ascherson, P. (1871); Gaudichaud, C. (1826); Holm, T. (1885); Sauvageau, C. (i89o2). HALORRHAGIDEAE (HALORAGEAE) . Brown, R. (1814); Hegelmaier, F. (1864); Juel, O. (1910); Parmentier, P. (1897); Schindler, A. K. (1904). Herminiera. Hallier, E. (1859) ; Jaensch, T. (1884 J) and (18842) '> Klebahn, H. (1891); Kotschy, T. (1858). Heteranthera. Hildebrand, F. (1885); Wachter, W. (1897*); Wylie, R. B. (I9I71)- HIPPURIDACEAE. Schindler, A. K. (1904). Hippuris. Barratt, K. (1916); Borodin, J. (1870); Chatin, A. (I8551); Fauth, A. (1903); Irmisch, T. (1854); Juel, O. (1910) and (1911); Sanio, C. (1865); Strasburger, E. (1884); Unger, F. (1849). Hottonia. Geneau de Lamarliere, L. (1906); Prankerd, T. L. (1911). Hydrilla. Bennett, A. (1914); Caspary, R. (18582); Seehaus, C. (1860). HYDRILLEAE. Caspary, R. (1858!) and (18582). Hydrocharis. Frank, A. B. (1872) ; Griset, H. E. (1894) ; Irmisch, T. (1859^ and (1865); Karsten, G. (1888); Lindberg, S. O. (1873); Overton, E. (1899); Rohrbach, P. (1873); Solereder, H. (1914); Terras, J. A. (1900); Treviranus, L. C. (1857). HYDROCHARITACEAE. Ascherson, P. (1867) and (1875); Ascherson, P. and Giirke, M. (1889); Caspary, R. (1857), (1858!) and (1858'); Gliick, H. (1901); Montesantos, N. (1913); Rohrbach, P. (1873); Sauvageau, C. (iSgo2); Solereder, H. (1913). Hydvocleis. Buchenau, F. (19032); Ernst, A. (I8721); Sauvageau, C. (1893); Wachter, W. (I8971). Hydromystria. Hauman, L. (1915). Hydrothrix. Goebel, K. (1913); Hooker, J. D. (1887). HYPERICACEAE. Cambessedes, J. (1829); Schenck, H. (1889). Isnardia. Chatin, A. (1855!). Isoetes. Goebel, K. (1879); Mer, E\ (i88o2); Scott, J. (1869). JUNCAGINACEAE. Buchenau, F. (1882); Micheli, M. (1881); Planchon, J. E. (1844). Jussiaea. Chatin, A. (1855!); Martins, C. (1866). LABIATAE. Schenck, H. (1889). Lacis. Brown, C. Barrington (1876). Lagarosiphon. Caspary, R. (18582). LEGUMINOSAE. Ernst, A. (18722); Schenck, H. (1889). Lemna. Arber, A. (1919*); Brongniart, A. (1833); Caldwell, O. W. (1899); Clavaud, A. (1876); Dutailly, G. (1878); Ehrhart, F. (1787); Guppy, H. B. (18942); Hoffmann, J. F. (1840); Hofmeister, W. (1858); Kalberlah, A. (1895); Koch, K. (1852); Kurz, S. (1867); Ludwig, F. (1881); Micheli, P. A. (1729); Milde, (1853); Weddell, H. A. (1849); Wilson, W. (1830). LEMNACEAE. Arber, A. (1919*); Engler, A. (1877); Hegelmaier, F. (1868) and (1871); Horen, F. van (1869) and (1870); Kirchner, O. von, Loew, E. and Schroter, C. (1908, etc.); Kurz, S. (1867); Schleiden, M. J. (1839); Weddell, H. A. (1849). LENTIBULARIACEAE. Buchenau, F. (1865). Limnanthemum. Fau^h, A. (1903); Goebel, K. (1891); Wagner, R. (1895). Limnobium. Montesantos, N. (1913). A. w. P. 27 4i8 INDEX TO BIBLIOGRAPHY Limnocharis. Ernst, A. (I8721); Hall, J. G. (1902). Limnophila. Goebel, K. (1908). Limosella. Hooker, J. D. (1847); Schlechtendal, D. F. L. von (1854). Littovella. Buchenau, F. (1859); Fauth, A. (1903); Mer, E. (iSSo1) (i88o2) and (1881). Lobelia. Armand, L. (1912); Buchenau, F. (1866); Sylven, N. (1903). Lopezia. Tackholm, G. (1914). Lophogyne. Went, F. A. F. C. (1910). LUDWIGIEAE. Parmentier, P. (1897). JLycopus. Lewakoffski, N. (I8731). Lysimachia. Irmisch, T. (1861). LYTHRACEAE. Gin, A. (1909); Schenck, H. (1889); Schrenk, J. (1889). Lythrum. Lewakoffski, N. (I8731). Marsilea. Hildebrand, F. (1870); Karsten, G. (1888). May oca. Ludwig, F. (1886). MELASTOMACEAE. Schenck, H. (1889). Mentha. Willis, J. C. and Burkill, I. H. (1895). Menyanthes. Fauth, A. (1903); Irmisch, T. (1861). Merimea. Cambessedes, J. (1829). Mimosa. Humboldt, A. de and Bonpland, A. (1808). Montia. Focke, W. O. (I8931). Mourera. Aublet, F. (1775); Went, F. A. F. C. (1910). Myriophyllum. Bokorny, T. (1890); Borodin, J. (1870); Fauth, A. (1903); G6neau de Lamarliere, L. (1906) ; Goebel, K. (1908) ; Irmisch, T. (I8591); Knupp, N. D. (1911); Ludwig, F. (1881); Magnus, P. (1871); Perrot, E. (1900); Raciborski, M. (1893); Sanio, C. (1865); Vaucher, J. P. (1841); Vochting, H. (1872). NAIADACEAE. Rendle, A. B. (1901); Schumann, K. (1892). Naias. Ascherson, P. (1874); Bailey, C. (1884); Bennett, A. (1914); Campbell, D. H. (1897); Guppy, H. B. (1906); Hofmeister, W. (1858); Irmisch, T. (1865); Jonsson, B. (1883-1884); Magnus, P. (I87O1), (1883) and (1894); Rendle, A. B. (1899), (1900) and (1901); Roze, E. (1892); Sauvageau, C. (iSSg1); Schumann, K. (1892); Weiss, F. E. and Murray, H. (1909). Nasturtium. Chatin, A. (1858*); Foerste, A. F. (1889). NELUMBIACEAE. Gray, A. (1848). Nelumbo (Nelumbium). Anon., (1895) ; Berry, E. W. (1917) ,* Brongniart, A. (1827) ; Hofmeister, W. (1858); Ohno, N. (1910); Raffeneau-Delile, A. (1841); Trecul, A. (1854); Wettstein, R. von (1888); Wigand, A. (1871). Neobeckia. MacDougal, D. T. (1914). Nepenthes. Gardner, G. (1847). Neptunia. Humboldt, A. de and Bonpland, A. (1808); Rosanoff, S. (1871). Nesaea. Schrenk, J. (1889). Nuphar(Nymphaea). Arcangeli, G. (1890); Brand, F. (1894); Caspary, R. (1861); Hofmeister, W. (1858); Irmisch, T. (1853); Spenner, F. C. L. (1827); Tittmann, J. A. (1821); Trecul, A. (1845) and (1854). Nymphaea(Castalia). Arcangeli, G. (1890); Bachmann, H. (1896); Barber, C. A. (1889); Bauhin, G. (1623); Brand, F. (1894); Caspary, R. (iSyo1); Desmoulins, C. (1849); Fries, E. (1858); Geneau de Lamarliere, L. (1906); Hausleutner, (18502); Hentze, W. (1848); Irmisch, T. (1853); Mellink, J. F. A. (1886); Miller, G. S. and Standley, P. C. (1912); Otis, C. H. (1914); Schlechtendal, D. F. L. von (1852); Schuchardt, T. (1853); Strasburger, E. (1884); Tittmann, J. A. (1821); Treviranus, L. C. (i8482); Wachter, W. (i8972). INDEX TO BIBLIOGRAPHY 419 NYMPHAEACEAE. Anon., (1828); Arber, E. A. N. and Parkin, J. (1907); Blenk, P. (1884); Brand, F. (1894); Caspary, R. (1856*) ; Cook, M. T. (1906); Gwynne-Vaughan, D. T. (1897); Keller, I. A. (1893); Paillieux, A. and Bois, D. (1888); Pfeiffer, L. (1854); Planchon, J. E. (1853); Raciborski, M. (1894!) and (1894 2); Schaffner, J. H. (1904); Schuchardt, T. (1853); Schumann, K. (1894); Spenner, F. C. L. (1827); Strasburger, E. (1902); Ursprung, A. (1912); Wigand, A. (1871). Coleman, W. H. (1844). Went, F. A. F. C. (1910). Parmentier, P. (1897). Schenck, H. (1889); Tackholm, G. (1914) and (1915). Hovelacque, M. (1888). Montesantos, N. (1913). Sergueeff, M. (1907). Reid, C. (1893). Hutchinson, J. (1916). Oliver, F. W. (1888). Chatin, A. (1855!) ; Willis, J. C. and Burkill, I. H. (1895). Pallis, M. (1916). Phucagrostis (see also Cymodocea). Ascherson, P. (1870); Bornet, E. (1864); Cavolini, F. (i7922). Goebel, K. (1895). Chrysler, M. A. (1907); Dudley, W. R. (1894); Sauvageau, C. (1890"). Webber, H. J. (1897). Dangeard, P. A. and Barbe, C. (1887). Arber, A. (1919*) ; Engler, A. (1877); Hofmeister, W. (1858); Ito, T. (1899); Kingsley, M. H. (1897); Koch, K. (1852); Schleiden, M. J. (1838!) ; Unger, F. (18542). Aublet, F. (1775); Brown, C. Barrington (1876); Gardner, G. (1847) ; Goebel, K. (i8893) and (1891-1893) ; Im Thurn, E. F. (1883) ; Lister, G. (1903) ; Magnus, W. and Werner, E. (1913) ; Matthiesen, F. (1908); Tulasne, L. R. (1852); Wachter, W. (1897*); Warming, E. (1881, 1882, 1888, 1891) and (18832); Weddell, H. A. (1872) ; Went, F. A. F. C. (1910) ; Willis, J. C. (1902), (I9I41). (I9I51), (i9i52) and (1917)- Hildebrand, F. (1870); Irmisch, T. (1861). Blanc, M. le (1912); Hauman-Merck, L. (I9I31); Hofmeister, W. (1858); Otis, C. H. (1914). Arber, A. (1918); Ascherson, P. (1883); Goebel, K. (1913); Hildebrand, F. (1885) ; Hooker, J. D. (1887) ; Solms-Laubach, H. Graf zu (1883); Warming, E. (1871). Focke, W. O. (1893!). Cavolini, F. (1792*) ; Delpino, F. and Ascherson, P. (1871); Sauvageau, C. (i8Sg9) and (iSgo1). Bennett, A. (1896); Blanc, M. le (1912); Brongniart, A. (1834); Chrysler, M. A. (1907); Clos, D. (1856); Coster, B. F. (1875); Esenbeck, E. (1914); Fryer, A. (1887); Fryer, A., Bennett, A. and Evans, A. H. (1898-1915); Geneau de Lamarliere, L. (1906) ; Hegelmaier, F. (1870) ; Hildebrand, F. (1861); Irmisch, T. (1853), (1858*), (1859!) and (i8592); Lundstrom, A. N. (1888); Her, E. (i8822); Raunkiaer, C. (1903); Sanio, C. (1865); Sauvageau, C. (i88g2) and (1894); Strasburger, E. (1884); Thoday, D. and Sykes, M. G. (1909); Tittmann, J. A. (1821); Treviranus, L. C. (1821) and (1857); Uspenskij, E. E. (1913). Oenanthe. Oenone. OENOTHERACEAE. ONAGRACEAE. OROBANCHACEAE. Ottelia. Ouvirandra. Paradoxocarpus. Pectis. PED ALINE AE. Peplis. Phragmites, Phyllocactus. Phyllospadix. Pi ar opus. Pinguicula. Pistia. PODOSTEMACEAE. Polygonum. Pontederia. PONTEDERIACEAE. PORTULACACEAE. Posidonia, Potamogeton. 27 — 2 420 INDEX TO BIBLIOGRAPHY POTAMOGETONACEAE. Ascherson, P. (1867) and (1875) ; Ascherson, P. and Graebner, P. (1907); Chrysler, M. A. (1907); Fischer, G. (1907); Cluck, H. (1901); Irmisch, T. (i8583); Sauvageau, C. (iSgi1); Schumann, K. (1892). Burns, G. P. (1904); McCallum, W. B. (1902). Ascherson, P. (1873) ; Askenasy, E. (1870) ; Bailey, C. (1887) ; Belhomme, (1862); Dodoens, R. (1578); Freyn, J. (1890); Geneau de Lamarliere, L. (1906); Karsten, G. (1888); Lamarck, J. P. B. A. (1809) ; Mer, E. (iSSo1) ; Roper, F. C. S. (1885); Rossmann, J. (1854). Hovelacque, M. (1888). Lewakoffski, N. (18732). Chrysler, M. A. (1907) ; Delpino, F. and Ascherson, P. (1871) ; , C. (1826); Hofmeister, W. (1852); Irmisch, T. Proserpinaca. Ranunculus. RHINANTHACEAE. Rubus. Ruppia. Sagittaria. Salix. Saururus. Schizotheca. Scirpus. Sesbania. Sisymbrium. Sium. Solanum. Sparganium. Spirodela. Stratiotes. Subularia. Terniola. Thalassia. Tillaea. Trapa. Trapella. Tristicha. TRISTICHACEAE. Udora. (See also Anon., (1895) ; Arber, A. (1918) ; Bauhin, G. (1596) and (1620) ; Blanc, M. le (1912); Bolle, C. (1861-1862); Buchenau, F. (1857); Costantin, J. (18852); Coulter, J. M. and Land, W. J. G. (1914); Fauth, A. (1903); Gliick, H. (1905); Goebel, K. (1880) and (1895); Hildebrand, F. (1870); Kirschleger, F. (1856); Klinge, J. (1881); Loeselius, J. (1703); Martens, G. von (1824); Hunter, J. (1845); Nolte, E. F. (1825); Osbeck, P. (1771); Otis, C. H. (1914); Paillieux, A. and Bois, D. (1888); Reinsch, P. (1860); Schaffner, J. H. (1897); Wachter, W. (I8971); Walter, F. (1842). Lewakoffski, N. (1877). Planchon, J. E. (1844). Ascherson, P. (1870). Anon., (1895); Desmoulins, C. (1849); Esenbeck, E. (1914); Kirschleger, F. (1856) and (1857); Scheuchzerus, J. (1719); Snow, L. M. (1914). Hallier, E. (1859); Jaensch, T. (18842); Scott, D. H. and Wager, H. (1888). Chatin, A. (iSsS1). Shull, G. H. (1905). Klebahn, H. (1891). Kirschleger, F. (1856). Hegelmaier, F. (1871); Micheli, P. A. (1729). Arber, A. (1914); Caspary, R. (1875); Davie, R. C. (1913): Geldart, A. M. (1906); Irmisch, T. (1859!) and (1865); Klinsmann, F. (1860); Montesantos, N. (1913); Nolte, E. F. (1825); Reid, C. (1893); Rohrbach, P. (1873). Hiltner, L. (1886). Goebel, K. Sauvageau, C. Caspary, R. (1860). Anon., (1828); Anon., (1895); Areschoug, F. W. C. (1873!) and (18732); Barneoud, F. M. (1848); Caspary, R. (1847); Chatin, A. (1855*); Frank, A. B. (1872); Gibelli, G. and Ferrero, F. (1891); Hofmeister, W. (1858); Jaggi, J. • (1883); Paillieux, A. and Bois, D. (1888); Queva, C. (1910); Sanio, C. (1865); Tackholm, G. (1914) and (1915); Theo- phrastus (Hort) (1916); Tittmann, J. A. (1821). Anon., (1828); Oliver, F. W. (1888) and (1889). Cario, R. (1881); Lister, G. (1903); Went, F. A. F. C. (1910). Willis, J. C. (I9I41), (igis1), (i9i52) and (1917). Elodea.) Marshall, W. (1852). INDEX TO BIBLIOGRAPHY 421 Utricularia. UTRICULARIACEAE. Vallisneria. Victoria. Weddellina. Wolffia, Zannichellia. ZANNICHELLI ACE AE . Zostera. Buchenau, F. (1865); Burrell, W. H. and Clarke, W. G. (1911); Biisgen, M. (1888); Cohn, F. (1875); Crouan (Freres) (1858); Darwin, C. (1875) and (1888); Focke, W. O. (18932); Gardner, G. (1846); Gliick, H. (1902), (1906) and (1913); Goebel, K. (1889*), (18892), (1891), (1891-1893) and (1904); Goppert, H. R. (1847); Im Thurn, E. F. and Oliver, D. (1887); Irmisch, T. (1858!) ; Kamienski, F. (1877); Luetzel- burg, P. von (1910); Meierhofer, H. (1902); Meister, F. (1900) ; Merz, M. (1897) '> Pringsheim, N. (1869) ; Schenck, H. (1887); Schultz, F. (1873); Tieghem, P. van (1868) and (I8691) ; Treviranus, L. C. (1848*); Warming, E. (1874); Wight, R. (1849). Benjamin, L. (1848); Hovelacque, M. (1888). Chatin, A. (18552); Delpino, F. and Ascherson, P. (1871); Duchartre, P. (1855); Irmisch, T. (1865); Micheli, P. A. (1729); Rohrbach, P. (1873); Scott, J. (1869); Strasburger, E. (1884); Weiss, F. E. and Murray, H. (1909); Wylie, R. B. (I9i72). Blake, J. H. (1887) ; Caspary, R. (18562) ; Henfrey, A. (1852) ; Im Thurn, E. F. (1883); Knoch, E. (1899); Seidel, C. F. (1869); Trecul, A. (1854); Unger, F. (1854!). Wachter, W. (1897!). Hegelmaier, F. (1885); Micheli, P. A. (1729); Milde, (1853); Weddell, H. A. (1849). Campbell, D. H. (1897) ; Chrysler, M. A. (1907) ; Hochreutiner, G. (1896); Hofmeister, W. (1858); Irmisch, T. (1858'); Roze, E. (1887). Fischer, G. (1907) ; Prillieux, E. (1864) ; Schumann, K. (1892). Cavolini, F. (I7921) and (17922); Chrysler, M. A. (1907); Clavaud, A. (1878); Delpino, F. and Ascherson, P. (1871); Duchartre, P. (1872) ; Engler, A. (1879) ; Grdnland, J. (1851) ; Hofmeister, W. (1852) ; Jussieu, A. L. de (1789) ; Martens, G. von (1824); Ostenfeld, C. H. (1908); Sauvageau, C. (iSSg3), (iSgo1), (18912); Walsingham, Lord, and Payne-Gallwey, R. (1886); Warming, E. (1871). [ 422 ] INDEX [The names of authors which occur in the bibliography are not included in the following index, since page references are given in connexion with the titles in the bibliography, which thus also serves as an index of authors' names] Acacia phyllode, 340 Achillea ptarmica, 5, 199 Acquired characters, inheritance of, 333. 334 Adaptation, 171, 332-335 Aedemone mirabilis. See Herminiera elaphroxylon Aerating system, in tissues of hydro- phytes, 183-194, 256-259; of root, 185-187; of stem, primary, 183-185; of stem, secondary, 187-194 Aerenchyma, 187-194; from cambium, 191-192; from phellogen, 187-191, 193-194 Aeschynomene, aerenchyma, 191, 192 Aeschynomene aspera, 191 Aeschynomene hispidula, 191-192 Affinities of hydrophytes, 308-321 Africa, 213, 295, 298, 305 "Age and Area" in plant distribution, 305-307 Air spaces, lysigenous, 184, 185 Air spaces, schizogenous, 184, 185 Aldrovandia, affinities, 310; carnivorous habit, iio-in, 270; embryo, no; fruit ripening under water, 239; roots, absence of, 109, no, 204, 244; seed, no; sensitive leaves, no, in (Fig. 75) ; shade plant, 289; stem anatomy, 175; turions, no, 219 Aldrovandia vesiculosa, 8, 109-111 (Fig. 75), 289, 310 Algae, 113, 114, 123, 124, 142, 155, 172 Aliens, 303 Alisma, effect of freezing on fruit, 243; fruit, 242; germination and rupture of seed coats, 244; heterophylly, 19, 20; land and water plants, 153 (Figs. 101 and 102) ; ranalean features, 320 Alisma graminifolium, 19, 20, 23, 157, 280 Alisma natans, 234 Alisma Plantago, 19, 20, 23, 151, 153 (Figs. 101, 102), 156, 169, 242, 243, 244, 289, 297 Alisma ranunculoides. See Echinodorus ranunculoides Alismaceae, 5, 9-23, 24, 33, 151, 156, 195, 224, 248, 297, 313, 314, 319, 337, 346 Alocasia, 303 Aloe, Water. See Stratiotes aloides Alps, 290 Althenia, bracts, 316; in brackish water, 134; perigonium, 316; reduced stem anatomy, 63, 173 A Ithenia filiformis, 1 73 Altitude above sea-level, 289-291 Amazons, 31, 99, 113, 229 "Ambatsch," 192 Ambulia, affinities, 313; heterophylly, 151 Ambulia hottonoides, 151 America, 61, 108, 120, 190, 193, 210, 216, 286, 290, 295, 298, 312, 313 American Indians, 17, 118 Ammania, 303 Amphibious plants, effect of water upon, 20 1, 202 "Amphibolis zosteraefolia," 123 "An Idea of a Phytological History," 230 Anacharis. See Elodea Andes, 291 Anemophily. See Pollination, anemo- philous Anemophytes, 143 Angiosperms, Marine. See Marine An- giosperms Anthocyanin, 15, 17, 113, 276-278 Apical openings in leaves, of Callitriche, 268 (Fig. 163); Heteranthera, 268; Littorella, 269; Pistia, 82 (Fig. 53); Potamogeton, 167 (Fig. 108), 268, 269- Potamogetonaceae, 133; Zostera, 269 Aponogeton, affinities, 314; disarticula- tion of primary root, 244; distribu- tion, 305; fenestration, 142 (Fig. 91), 143; geotropism, 281; heliotropism, 281; heterophylly, 154; undulated leaf, 62 Aponogeton angustifolius, 143 Aponogeton Bernerianus, 142 Aponogeton distachyus, 215, 244, 281 Aponogeton fenestralis, 142 (Fig. 91), 143, 281, 314 Aponogeton ulvaceus, 62 Aponogetonaceae, 239, 248, 305, 313, 314. 315 Aquilegia, 314 Araceae, 74, 82, 314, 315 INDEX 423 Araguay, River, 295 Argentine, 55 Aroideae, 316 Arrowgrass. See Aponogeton Arrowhead. See Sagittaria sagittifolia Asia, 295, 298 Astrakhan, 303 Auricula, polystely in, 180, 181, 182 Australia, 295, 305 Awlwort. See Subularia aquatica Azores, 295, 333 Bacteria, 142 Baltic, 123 Baltimore, 253 Bananas, 143 Band leaves, n (Fig. 3), 12, 13 (Fig. 4), 14 (Fig. 5), 19, 20, 22, 23, 140, 141 (Fig. 90), etc. Bar clay a, 33 Bateson, W., on evolution, 334 Batrachian Ranunculi. See Ranunculus sect. Batrachium, Ranunculus aqua- tilis, etc.. Batrachospermum, 155 Bean, 249 Beetles, as pollinators of Lemnaceae, 80; in utricle of Utricularia, 93 Begonia hydrocotylifolia, 256 Belgium, 303 Bellis perennis, 165 Bengal, no Bermudas, 298 Bidens Beckii, 151, 313 Biological classification of hydrophytes, 4-8, 42 Birds and dispersal, 35, 298-302 Bittersweet. See Solanum dulcamara "Bitter-sweet," Grew on heterophylly in, 155 Black Sea, 302 Bladderwort. See Utriculavia Bladderwort, Common. See Utricularia vulgaris Blue Nile, 192 Bodensee, 322 Boottia, 57 Bostrychia Moritziana, 114 Brasenia peltata. See B. Schreberi Brasenia Schreberi, 38 (Fig. 20), 205, 272 Brazil, 206, 207, 243, 295 Brent Geese, 302 Broads, 288 Brocchinia cordylinoides , 109 Bromeliaceae, 108, 109 Bruch-Eicheln, 17 Brunfels, Otto, 27 Bull Nut. See Trapa natans Bulliarda (Tillaea). affinities, 310; aqua- tic with xerophilous ancestry, 310; cleistogamy, 234 Bulliarda (Tillaea) aquatica, 234, 310 Burton-on-Trent, 211 Butler, Samuel, 347 Butomaceae, 157, 248, 513 Butomus, 314 Buttercup, Water. See Ranunculus aquatilis, Ranunculus sect. Batra- chium, etc. Cabomba, anatomy, 37,38; heterophylly, 29 (Fig. 14), 146; polystely, 37; re- duced leaves, 338 Cabomba caroliniana, 338 Cabomboideae, 38, 309 Caddice worms, 217 Calcareous substratum, 286, 287 Caldesia, heterophylly, 23; turions, 22, 225 (Figs. 148, 149) Caldesia parnassifolia, 22, 23, 224, 225 (Figs. 148, 149) California, 123 Calla palustris, 167 (Fig. 107) Callitrichaceae, 134, 311, 318 Callitriche, affinities, 311, 312; altitude, 290; annual and perennial forms, 215, 216; as coloniser, 299; chlorophyll, absence in epidermis, 164; distribu- tion, 306, 307; flowers, 237 (Fig. 154); fruit, 242, 243; germination, 280; heterophylly, 146, 147 (Fig. 94); land form, 170 (Fig. in), 195; leaf anatomy, 163, 169, 170 (Fig. in); local races, 330; mucilage trichomes, 271; pollination, 236, 237; roots, air spaces in, 187; root anatomy, 208, 209 (Fig. 138); seeds, 297; stomates, 166; vascular strand of axis, 175, 176 (Fig. 114); vegetative reproduction, 216; water pores, 267, 268 (Fig. 163) Callitriche autumnalis, 6, 134, 169, 237, 268 (Fig. 163), 307 Callitriche stagnalis, 176 (Fig. 114), 208, 209 (Fig. 138), 271 Callitriche verna, 6, 146, 147 (Fig. 94), 163, 1 66, 169, 170 (Fig. in), 187,236, 237 (Fig. 154), 306 Caltha palustris, 198, 199 (Fig. 129) Cam, River, 150, 211, 263 Cambridge, 150 Cambridge Botanic Garden, 211 "Camichi," 300 "Cammomill," 144 Campanulaceae, 313 . Canadian Waterweed. See Elodea canadensis Canary Islands, 295 Canna, 244 Carbon dioxide, derived from sub- stratum, 254; excess of, available for hydrophytes, 254; proportion of, in free and dissolved air, 253 Cardamine, adventitious budding from leaves, 216, 217 (Fig. 141); land and water forms, 201, 202 (Fig. 133) 424 INDEX Cardaminepratensis, 201, 202 (Fig. 133), 216, 217 (Fig. 141), 309 Carnation, 155 Carolina, 286 Carrot, 249 Caryophyllaceae, 234, 310, 311 Caspian Sea, 302 Castalia, air leaves, 32; effect of frost on seeds, 243; floating leaves, 30, 146, 159; geophytic habit, 217, 323; length of peduncle and petiole, 31 (Fig. 15), 40; pigmented variety, 276; rhizome, 24-26 (Fig. n), 39, 217; seedlings, 28 (Fig. 13), 29; seeds, 302; stipules, 25, 26 (Fig. n); submerged leaves, 29, 146, 159; terjrestrial form, 32 Castalia alba, 24, 25, 26 (Fig. 11), 27, 28 (Fig. 13), 29-31 (Fig. 15), 32, 243, 276, 302 Castalia flav a, 37 Castalia Lotus, 34, 36, 37 (Fig. 19), 225 Castalia pygmaea, 33 Castelnavia, 117, 295, 306 Caucasus, 302 Ceratophyllaceae, 84-90, 318, 320 Ceratophyllum demersum, 84-90; affini- ties, 84, 309, 312; cuticularisation, 86; distribution, 295, 297, 298; effect of strong illumination, 279; epiphytic fauna, 88; flowers, 84, 85 (Fig. 54); freezing, effect of, 88, 89; hairs con- taining mucilage, 86, 272; high temperatures necessary for fruiting, 88, 275; in biological classification, 8; in deep water, 86, 288; leaf, anatomy of, 1 68, dimensions of, 140, juvenile, 86 (Fig. 55); luxuriance, 87; monoecism, 84, 85; movements, 90, 281 ; mucilage hairs, 86, 272; perenniation, 215; pollination, hydrophilous, 84, 85, 134, 237, 238; rhizoid branches, 88, 89 (Figs. 57 and 58), 98, 336, 337; roots, absence of, 85, 204, 244; seedling, 85, 86 (Fig. 55); stem anatomy, 86, 87 (Fig. 56); vegetative reproduction, 87, 216, 219; water absorption, 269, 270; water content, 86 Ceylon, 112, 216 Chalky incrustation on leaves of aquatics, 51 Chantransia, 155 Chara, 288 Chili, 181 Chimborazo, 290 China, 17 Chlorophyll, in epidermis of aquatics, 164, 168 (Fig. 109), 169, 254; in epi- dermis of terrestrial plants, 164, 165 Chydorus sphaericus, as food of Utri- eularia, 94 Cirsium anglicum, 198, 199 (Fig. 130) Classification, biological, of hydrophytes, 4-8, 42 Cleistogamy, 233, 234; in Alisma, 234; Bulliarda (Tillaea), 234; Echinodorus, 234; Euryale, 34, 234; Heteranthera, 234 (Fig. 153); Hydrothrix, 234; Ille- cebrum, 234; Limosella, 233, 235; Nesaea, 234; Peplis, 234; Podostemon, 121 (Fig. 82), 234; Ranunculus, 233, 234; Rotala, 234; Tillaea, 234; Tra- pella, 234 Climate and life-cycles, 275 Cnicus arvensis, 200 Cnicus pratensis, 198, 199, 200 Codium tomentosum, 123 "Collet." development of root-hairs from, 245 Colocasia, 303 Colonisation of waters, 289, 298, 299 Commissioners' Pits, Upware, 217 Competition, aquatic life as a refuge from, 324, 325 Compositae, 151, 313, 320, 321 Conifers, "Youth forms" of, 155 Copepods, as food of Uiricularia, 94 Cotton, aliens accompanying, 303 Cotula myriophylloides, 313 Crassulaceae, 234, 310 Cruciferae, 216, 309 Crustacea, as food of Utriculana, 93, 94 Cuscuta alba, 5, 199 (Fig. 131) Cuticle, slight development of, in hydrophytes, 163, 254, 260 Cuyuni River, 119 Cyclamen, 240 Cymodocea, absence of apical openings in leaves, 269; anatomy, 125 (Fig. 84), 131, 331; chlorophyll in epidermis, 164 ; flowers, 126 ; grappling apparatus of fruit, 127, 245; habit, 124 (Fig. 83), 125; leaf form, 124; life-history, 124 (Fig. 83)-127; pollination, hydro- philous, 126, 237; regarded as Alga, 123; spiral roots, 205; squamulae intravaginales, 126; vivipary, 127, 246 Cymodocea aequorea, 124 (Fig. 83), 125 (Fig. 84), 126, 127, 246 Cymodocea antarciica, 123, 127, 205, 245 Cymodocea isoetifolia, 124 Cyperaceae, 154, 317, 326 Cypris, as food of Utricularia, 94 Damasonium stellatum, 23 ^Danube, 212 Daphnidae, as food of Utricularia, 94 Darwin, Erasmus, on evolution, 334 Delayed germination in aquatics, 36, 71, 72, 243-244 Delesseria Leprieurii, 114 "Dents nageoires," 133, 314, 315 Depth to which plants can grow, 86, 123, 275 Desmanthus natans. See Neptunia oleracea INDEX 425 Diaphragms, 18, 19 (Fig. 8), 183, 184 (Figs. 118, 119), 257 Dicotyledonous families, proportion of among aquatics, 322 Dicraea elongata, 115 (Fig. 77) Dicraea stylosa, 114, 115 (Fig. 78), 116 (Fig. 79), 118 Dictyota dichotoma, 123 "Differentiation Theory" of plant dis- tribution, 304-307 Dionaea, m Diplanthera, 123 Distribution, geographical, 73, 112, 295-307 Dock, 271 Dodder. See Cuscuta alba Dodoens' Hiswire des Plantes, 144 Dollo's "Law of Irreversibility," 336, 347 Droseraceae, 109, in, 310 Duckweed. See Lemna, Lemnaceae, Spirodela, Wolffia Duckweed, Rootless. See Wolffia East Anglia, flood of 1912 in, 296 Echinodorus ranunculoides, cleistogamy, 234; heterophylly, 23; inflorescence and vegetative shoots, 224 (Fig. 147); land and water forms, 21 (Fig. 9) Ecology, 285-292 Egypt, 332 Egyptian cotton, aliens accompanying, 303 Eichhornia, affinities, 317; air tissue in petioles, 154; heterophylly, 154, 160, 161; phyllodic interpretation of leaf, 341 (Fig. 169), 342 (Fig. 170), 343, 344; vegetative multiplication, 213 Eichhornia azurea, 160, 161 Eichhornia crassipes, 154 Eichhornia speciosa, 213,' 341 (Fig. 169), 342 (Fig. 170), 343, 344 Elatinaceae, 310, 311 Elatine, affinities, 311; annual species, 215; conveyance by birds, 301; root- hairs from "collet/' 245 (Fig. 158) Elatine hexandra, 245 (Fig. 158), 311 Elatine hydropiper, 301, 311 Eleocharis, 286 Elisma, heterophylly, 23; relation of inflorescence and vegetative shoot, 20 Elisma natans, 20, 23 Ellis, Lake, 286 Elodea, aerating system, 256; circula- tion of protoplasm, 212 ; experimental cultures, 265 ; history in Britain, 55, 210-213; leaf anatomy, 165 (Fig, 106), 169; leaf form, 141; pollination mechanism, 55, 56 (Fig. 35), 57, 134, 236; root-hairs, 208; vegetative re- production, 55, 210—213; wintering shoots, 55 (Fig. 34), 219 Elodea callitrichoides, 55, 56, 236 Elodea canadensis, 6, 7, 55 (Fig. 34), 57, 165 (Fig. 106), 169, 173, 210-213, 219, 253. 254, 265, 266 Elodea densa, 57, 236 Elodea ioensis, 55, 56 (Fig. 35) Embryo, macropodous, 246 (Fig. 159), 248 (Fig. 161), 249, 319 (Fig. 166), 326; protection of, 242, 243; vivi- parous, 127 Engadine, 290 Enhalus, 57, 123, 124, 131, 236 Enter omorpha, 123 Entomophily. See Pollination, ento- mophilous Epidermis of aquatics, chlorophyll in, 164, 168, 169, 254; form of cells of, 163, 164 Epilobium hirsutum, 188 Epithem, 267 Eranthis hiemalis, 319 Eriocaulon, 286 ' ' Eu-anthostrobilus, "315 Eu-callitriche, 236, 237, 306, 307, 330 Eucalyptus Preissiana, 256 Eupatorium cannabinum, 188 Euphorbiaceae, 311 Europe, 290, 295 Euryale, affinities, 38; cleistogamy, 34, 234 Euryale fer ox, 34, 234 Farinosae, 316, 317, 341 Farmeria metzgerioides, 114, 248 Fenestration of leaves, 142 (Fig. 91) Fijis, 298, 303 Flagellates, 142 "Floating-leaf association," 288 Floating leaves, 30-32, 44-46, et passim Floating Sensitive Plant. See Neptunia oleracea Floating wood of Herminiera, 192 Florida, 213 "Flossenzahne," 133, 314 Flowers of water plants, 227-238, etc Fluviales. See Helobieae Fly pollinating Sagittaria, 9 "Flying germinators," Water-fowl as, 301 Fontinalis, 225 Food plants, Alismaceae, 17; Alocasia, 303; Colocasia, 303; Nymphaeaceae, 24; Trapa, 302 Forest of Dean, 276 Freezing, effect of, 220, 243, 278 Frogbit. See Hydrocharis Morsus-ranae Fruits of water plants, 239-249, etc. Fruits, ripening under water, 239, 240 Fungi, 172 Gaseous exchange, 253-259 Geneva, Lake of, 278 Gentianaceae, 39, 205, 312 Geographical distribution, 295—307, etc. 27—5 426 INDEX Geophytes, aquatic, 217, 323 Geotropism, 281, 282 Germany, 17, 53, 290, 299 Germination, delayed, 36, 71, 72, 243, 244; in situ, So, 127, 246, 248; of various genera, see heading, Seedlings Giant Waterlily. See Victoria regia Gill-tuftsof00« 155 Lady's Smock. See Cardamine pratensis Lake dwellings, 302 Lake Ellis, North Carolina, 286 Lake George, Florida, 213 Lake St Clair, Michigan, 288 Lakenheath Lode, 216 Land forms, of water plants, 195-198; of Alismaceae, 20, 21 (Fig. 9), 153 (Fig. 101), 195; Cattitriche, 195; Cardamine, 202 (Fig. 133); Hottonia, 197 (Fig. 127); Hydrocharis, 42, 49, 195; Lemnaceae, 77, 78; Limnan- themum, 19*5 ; Limosella, 198 ; Littorella 198 (Fig. 128); Myriophyllum, 195, 223 (Fig. 146); Nymphaeaceae, 32, 195; Polygonum, 152 (Figs. 99 and i oo), 197, 198; Potamogeton, 195, 196 (Fig. 125); Ranunculus, 195, 196 (Fig. 126), 203 (Fig. 134) Land plants, effect of water upon, 200, 201 (Fig. 132) "Law of Age and Area," 305-307 "Law of Irreversibility, " 336, 347 "Law of Loss," 182, 3~36-347 Lawia, germination, 117; mucilaginous seeds, 300; shoot thallus, 117; special- 428 INDEX ised Podostemad, 306 ; starch storage, 120; vegetative reproduction, 216 Lawia foliosa, 117 Lawia zeylanica, 117, 120, 216, 300 Leaves, band or ribbon, n (Fig. 3), 12, 13 (Fig. 4). J4 (Fig. 5), *9, 20, 22, 23, 140, 141, 343, 344, etc. Leaves, floating, 30-32, 44-46, etpassim Leaves, submerged, 139-143, 163-171 ; "adaptation" in, 171; aerating sys- tem, 167; diaphragms, 167; epidermal cells, form of, 163, 164; epidermis, chlorophyll in, 164; fenestration, 142 (Fig. 91); mesophyll for storage, 168 (Fig. 109); non-radial anatomy, 165 (Fig. 1 06), 169; radial anatomy, 168 (Fig. 109); ratio of surface to volume, 1 40; reduction of cuticle, 1 63 : stomates and hairs, 165-167 (Fig. 107); un- differentiated mesophyll, 167, 168 (Fig. 109); water pores, 167 Leguminosae, 188, 189, 191, 192 Lemnagibba, 7, 76 (Fig. 48), 77, 78, 81, 275. 297 Lemna minor, 7, 76, 77, 78, 80, 291, 295, 297, 301, 307 Lemna trisulca, 8, 78, 79 (Figs. 49, 50, 51), 80, 81 (Fig. 52), 208, 215, 295 Lemnaceae,73— 82 ; aerenchyma, 76 (Fig. 48); affinities, 74, 82, 314, 316; alti- tude, 290, 291; anatomy, 78, 79 (Fig. 51); anthocyanin, 276, 277; cotyledon as float, 81 (Fig. 52), 248; dispersed by water birds, 300, 301 ; distribution, 73, 112, 295, 307; entirely aquatic, 318; entomophily, 80, 230; flowers, 74 (Fig. 47), 79 (Fig. 50), 80; land forms, 77, 78; life in impure water, 81, 287; number of genera, 84; pro- tandry, 80; range, 73, 112, 295, 307; rarity of seeds, 75; reduced inflores- cences, 73, 74 (Fig. 47); root-caps, 74, 76 (Fig. 48); roots for equilibrium, 74, 207; seedlings, 80, 81 (Fig. 52); seeds, 80, 297; turions, 74-77; vegeta- tive morphology, 73-74; vigour of vegetative growth, 77, 81, 83 Length, of axes, of Oenanthe, 150, of Polygonum, 215, of Ranunculus, 214, of Utricularia, 215; of petiole, ped- uncle, etc., in Nymphaeaceae, 28 (Fig. 13), 31 (Fig. 15), 283; of petiole in Hydrocharis, Marsilea, Ranunculus, 283, 284; of submerged leaves of Sagittaria, 12, of Vallisneria, 140 Lentibulariaceae, 91, 104, 313 Lenticels, effect of submergence on, 187 Lesser Water Plantain .See Echinodorus ranunculoides Lignification, poor, in aquatics, 260 Liliaceae, 326 Limnanthemum, 39—41; affinities, 313; anthocyanin, 276; dehiscence, 240 (Fig. 156), 241, 242 (Fig. 157); dis- tribution, 304, 305; effect of drying on seeds, 243 ; fruit ripening in water, 239; geophytic habit, 41 (Figs. 22 and 23), 217, 323; germination, 248; land form, 195; mucilage, 271; rhizome, 39, 41 (Figs. 22, 23), 217, 323; seeds, 240 (Fig. 156), 241, 297: support of inflorescence, 228 Limnanthemum Humboldtianum, 239 Limnanthemum indicum, 40, 304 Limnanthemum nymphoides,3Q— 41 (Figs. 22, 23), 195, 228, 240 (Fig. 156),' 241, 242 (Fig. 157), 243, 248, 271 Limnobium, geotropic curvature of fruit- stalk, 239, 282; heterophylly, 157 Limnobium Boscii, 157, 282 Limnocharis Humboldtii, 166 Limnophila, heterophylly, 151, 161; sleep movements, 281; systematic position, 313 Limnophila heterophytta, 161, 281 Limnophila hottonoides, 151 Limnosipanea, heterophylly, 151; af- finities, 313 Limnosipanea Spruceana, 151 Limosella, affinities, 313, 318; cleisto- gamy, 233, 235; distributed by birds, 301; flowers, 313; land and water forms, 198 Limosella aquatica, 198, 233, 301, 313 Linaria Cymbalaria, 240 "Little Bell," Grew on heterophylly in, 155 Littorella, aerating system of leaf, 167; affinities, 313; anemophily, 232; apical openings of leaves, 269; flowers and fruit, 313; fruit, 241, 242; funicular plug in fruit wall, 242; geophytic habit, 323 ; germination in situ, 246; in peaty water, 287, 290; land and water forms, 198 (Fig. 128); ousted by Potamogeton, 333; radial leaf, 1 68; runners, 217, 218 (Fig. 142) Littorella lacustris, 7, 141, 198 (Fig. 128), 217, 218 (Fig. 142), 232, 241, 242, 246 Liverworts, 327 Lobelia Dortmanna, aerating system of leaves, 167; affinities, 313; fruit ripening in air, 239; in biological classification, 7; in peaty water, 287; in sandy pools and mountain lochs, 290; leaves, 141; root, 245; sinking of seeds, 297; stomates, 166 Lobelia, Water. See Lobelia Dortmanna Loess alluvium, 287 Loosestrife, Water. See Lythrum Sali- caria Lotus, 1 88 Lotus, Sacred. See Nelumbo Nelumbo Low countries, 9 Ludwigia, 311, 318 Lupinus, 200 INDEX 429 Luxuriance of vegetative growth, 210- 215 Lycopus europaeus, 188 Lysimachia, 188 Lyte's Herbal, 144 Lythraceae, 175, 188, 193, 234, 303, 311 Ly thrum Salicaria, 188, 311 McLean, R. C., on treatment of her- barium material, 331 Macropodous embryo, 246 (Fig. 159), 248 (Fig. 161), 249, 319 (Fig. 166), 326 Madagascar, 305 Madeira, 295 "Major plant individual," 211-213 Manchester, 275, 303 Marathrum utile, 113 Marburg, 189 Mare's-tail. See Hippuris vulgaris Marine Angiosperms, 123-135; affini- ties, 123, 320; association with Algae, 123; flowers, 126, 127, 129, 130 (Fig. 87); fruits, 126, 127, 248 (Fig. 161); leaf anatomy, 125 (Fig. 84), 128 (Figs. 85, 86), 130, 131, 132 (Figs. 88, 89); leaves, 124 (Fig. 83), 130 (Fig. 87), 133; origin of the group, 133-135; pollen-grains, Conferva-like, 124, 125, 126; pollen-grains in strings, 130; pollination, hydrophilous, 124-127, 129; seed-coats, 130; vegetative habit, 124 (Fig. 83), 130 (Fig. 87); vivipary, 127 Marsilea, 284 Mauritius, 129, 295 Mayaca fluviatilis, 243 Mayacaceae, 317 Mediterranean, 123, 125 Melilotus Taurica, sleep habits of, 161 Memory, unconscious, 333 Mentha aquatica, 201 Menyanthes, affinities, 313; submerged form; 199; tendril roots, 205 Menyanthes irifoliata, 199, 313 Mercurialis, 311 Michigan, 288 Milfoil, Water. See Myriophyllum Mimosa lacustris. See Neptunia oleracea Mirabilis, 121 Monocotyledonous families, proportion of, among aquatics, 322 Monocotyledons, aquatic origin of, 322- 326 Monstera, 142, 314 Montia, affinities, 310; biennial and perennial forms, 216; submerged xerophyte, 310 Montia fontana, 216, 310 Mosses, 113 M our era, anthocyanin, 113; flowering, 120; haptera, 114 M our era fluviatilis, 113, 114, 120 Mucilage, 38 (Fig. 20), 47, 271-272, 300 Mucilage-secreting trichomes, 13, 15, 38 (Fig. 20) Myriophyllum, affinities, 311, 312; air spaces in roots, 187; in stem, 179 (Fig. 116), 256; altitude, 290; ane- mophily, 230, 232; effect of freezing on fruits, 243; on turions, 220; fruits, 242; germination of turions, 222 (Fig. 145) ; growth in still or moving water, 283; habit, 172, 221 (Fig. 144); in biological classification, 6; land form, 195, 223 (Fig. 146); on sandy sub- stratum, 286; relation of turions to inflorescence, 224; sleep movements, 281; stem anatomy, 178, 179 (Figs. 116, 117), 181; submerged leaves, form, 255, structure, 168 (Fig. 109); trichomes, 168-170 (Fig. no) ; turions, 219, 220, 221 (Fig. 144), 222 (Fig. 145), 223 (Fig. 146), 224; wave motion, 289 Myriophyllum alternifolium, 168 Myriophyllum proserpinacoides, 281 Myriophyllum spicatum, 134, 140, 168 (Fig. 109), 179 (Figs. 116, 117), 195, 197, 232, 242, 243 Myriophyllum verticillatum, 6, 168-170 (Fig. no), 219, 220, 221 (Fig. 144), 222 (Fig. 145), 223 (Fig. 146) Naiadaceae, 248, 313, 315 Naias, alien weed with rice and cotton, 303; annual, 215; distribution, 304, '305; effect of depth, 279; flower, 315, 316, 320, 346; in biological classifica- tion, 6; in deep water, 279, 288; pollination, hydrophilous, 237; re- duction of primary root, 244; root anatomy, 208, 209 (Fig. 140) ; specific differences, 331, 332; stem anatomy, 175 ; submerged vegetative organs, 134 Naias flexilis, 215 Naias graminea, 237, 303 Naias graminea, var. Delilei, 332 Naias major, 209 (Fig. 140) Naias marina, 304, 305 Naias minor, 209 (Fig. 140), 215 Nasturtium, affinities, 309; air and water shoots, 201; budding from leaves, 216 Nasturtium amphibium, 201, 309 Nasturtium lacustre, 216 Nelumbium. See Nelumbo Nelumbo, absence of mucilage, 257, 272; affinities and structure, 38, 39; geologic distribution, 38, 39 (Fig. 21); movements of gases, 257, 258 ; possible case of reversion from aquatic to terrestrial life, 39; stability of seed- ling, 245 Nelumbo Nelumbo, 38, 39 (Fig. 21) Nelumbonoideae, 38 430 INDEX Nepenthes, 93, 310 Neptunia oleracea, 189-191 (Fig. 123) Nesaea, aerenchyma, 193 (Fig. 124), 194; cleistogamy, 234; distribution, 295 Nesaea verticillata, 193 (Fig. 124), 194 Nile, 113 Nile, Blue, 192 Nitella, 288 Nitrogen, proportion in free and dis- solved air, 253 Nuphar luteum. See Nymphaea lute a Nuphar minima, 28 Nuphar pumilum, 32 Nymphaea, dehiscence, 35, 36; effect of frost on seeds, 243; etiolation with depth, 279; floating leaves, 30, 31, 146, 159; fruit, 34 (Fig. 17), 35, 36, 240; general habit, Frontispiece; geophytism, 217, 323; length of petiole and peduncle, 31 (Fig. 15), 40; operculum of seed, 35 (Fig. 18), 36; pigmented variety, 276; reduction of primary root, 244, 281; rhizome, 24, 25 (Fig. 10), 26, 27 (Fig. 12), 36, 39, 217; roots, 25 (Fig. 10), 204, 281; seedlings, 34, 35 (Fig. 18), 36, 280; submerged leaves, 27 (Fig. 12), 28, 29, 146, 159, 279; trichome dia- phragms, 272 Nymphaea alba. See Castalia alba Nymphaea lutea, 6, 24, 25 (Fig. 10), 26. 27 (Fig. 12), 28-31, 34 (Fig. 17), 35 (Fig. 18), 36, 159, 243, 244, 272, 279, 280, 281, 288, see also Frontispiece Nymphaea lutea, var. rubropetala, 276 Nymphaea pumila, 32 Nymphaeaceae, 24-39; affinities, 309, 314, 318-320; air system in petioles and peduncles, 37, 257; anatomy, 36-38, 182; ancient aquatic habit, 321; and water fowl, 299, 300; anthocyanin, 276, 277; cleistogamy, 234 '> geophytic habit, 217; hetero- phylly, 27 (Fig. 12), 28, 29 (Fig. 14), 146; in Lake Ellis, 286; land forms, 32, 195; leaf and flower, 40; leaf, floating, 30, 31; leaf, submerged, 27 (Fig. 12), 28 (Fig. 13), 29 (Fig. 14), 255; mucilage, 35, 36, 38 (Fig. 20), 272; wound effects, 258 Nymphaeoideae, 32, 38 Oenanthe, aerenchyma, 188; hetero- phylly, 150; perenniation, 215; root system, 204, 205, 229; stomates, 166; submergence, 312 Oenanthe Phellandrium, 150, 204, 205, 229 Oenanthe Phellandrium, var. fluviatilis, 150, 166, 215, 312 Oenone, gill-tufts, or Kiemenbiischel, 119 (Fig. 81), 255 Oenone multibranchiata, 119 (Fig. 81) Onagraceae, 188, 189, 311, 318 "Open reed-swamp," 288 Organ Mountains, 108 Origin of Species, The, 260 Osmotic pressure of sap in leaves and roots, 266 Ottelia, geotropism of peduncle, 239; heterophylly, 57 Ouvirandra. See Appnogeton fenestralis Oxygen, proportion in free and dissolved air, 253; scarcity in water life, 255 Pacu myletes, 118 Padina pavonia, 123 Palms, 143 "Fancy," 155 Pandanaceae, 317 Parallel veining of Monocotyledonous leaves, 338 Parra jacana, 300 Parsnip, Water. See Sium latifolium Pea, 249 Peat-bog lakes, 275 Peaty substratum, 287 Pedaliaceae, 151, 234 Peplis, aerating system, 185, 259; affinities, 311; anthocyanin, 276, 277; cleistogamy, 234; detached shoots, 216, 276, 277; flowers and fruit, 230, 232 (Fig. 152); pollination, 230; vas- cular anatomy, 175; vegetative re- production, 216; winter state, 216 Peplis Portula, 175, 185, 216, 230, 232 (Fig. 152), 259, 276, 277, 311 Perenniation among hydrophytes, 215 Perthshire, 288 Phaseolus, 207 Phelloderm, air-containing, 187-191 Phellogen producing aerenchyma, 187- 191 Philodendron, 206 Phragmiies, in Jura Lakes and White Moss Loch, 287, 288 (Fig. 165); "major plant unit," 212; root differ- entiation, 207 Phragmites communis, 207, 212 "Phragmitetum," 288 " Phucagrostis major," 125 " Phucagrostis minor," 125 Phyllanthus fluitans, 311 Phyllode theory of Monocotyledonous leaf, 52, 161, 162, 337-345 Phyllospadix, 123, 124 Pico, 333 Piliferous layer, cuticularised, 208; death of, before death of root-hairs, 264 Pilularia, 225 Pinguicula, insectivorous habit and relation to Utricularia, in; polystely, 181 Pinguicula vulgaris, 181 INDEX Pistia Stratiotes, air tissue of leaves, 82, 154, 256; comparison with Lemna- ceae, 74, 82, 316; hairs, 82, 83; vigour of vegetative growth, 83, 213, 214; water pores, 82 (Fig. 53), 83, 167, 267 "Pith" helmets, 191 Plantago, 233, 313 " Plantago aquatica," 20 Plantago major, 241 Plantain, Lesser Water. See Echino- dorus ranunculoides Plantain, Water. See Alisma Plantago Podostemaceae (including Tristicha- ceae), 112-122, 327-333; affinities, 310, 319; anatomy, 117, 118 (Fig. 80); ancient aquatics, 321; and natural selection, 327-333; and wading birds, 300; anemophily, 120, 121 ; anthocya- nin, 112, 113, 276, 277; cleistogamy, 121 (Fig. 82) ; dependence on aeration, 257; distribution, 295, 306; dorsi- ventrality, 121, 122, 327-329; en- tirely aquatic, 318; flowers, 120, 121 (Fig. 82); germination in situ, 248; "gill-tufts," 118, 119 (Fig. 81), 255; habit, 114, 115 (Figs. 76, 77), 116 (Fig. 79), 117; haptera, 113, 114. 121; in biological classification, 7; inhabit rapids, 112, 113, 119, 257; "Kiemen- biischel," 118, 119 (Fig. 81), 255; lack of adaptation, 328-333; lack of intercellular spaces, 118 (Fig. 80), 257; morphology, 73, 121, 122; mucilaginous seeds, 300; polymor- phism of thallus, 114-117; rarity outside tropics, 112, 113; reduction of primary root, 244; root thallus, 114, 115 (Figs. 76, 77, 78), 116 (Fig. 79), 117, 208; secondary shoots, 114; seedling, 114, 115 (Fig. 78), 117; seeds, 121, 300; shoot thallus, 117; silica, 117; simulation of lower plants, 114-117; vegetative reproduction, 216; water reservoir in nucellus, 121 Podostemon, Alga-like form, 114 ; cleisto- gamy, 121 (Fig. 82), 234; distribution, 306 Podostemon Barberi, 121 (Fig. 82), 234 Podostemon subulatus. 114 Pollination, anemophilous, 57, 120, 121, 230, 232, 233 Pollination, aquatic. See Pollination, hydrophilous Pollination, cleistogamic. See Cleisto- gamy Pollination, entomophilous, 9, 57, 80, 230 Pollination, hydrophilous, i, 6, 8, 55- 57, 70, 71, 84, 85, 124, 127, 129, 130, 134, 235-238, 345, 346 Polygonaceae, 311 Polygonum, affinities, 311, 318; colonis- ing new waters, 280; land and water forms, 150, 151, 152 (Figs. 99, 100), I97, 198; heterophylly, 150, 151, 152 (Figs. 99, 100); length of shoot system, 215; mucilage, 271; vegeta- tive reproduction, 225 Polygonum amphibium, 150-152 (Figs. 99, 100), 197, 198, 215, 271, 289, 311, 3i8 Polygonum viviparum, 225 Polypetalae, 309-312, 319 Polystely, 37, 180-182, 346 Pond weed. See Polamogeton Pond weed, Horned. See Zannichellia Pontederia, geotropic curvature of fruit stalk, 239, 240 (Fig. 155); phyllodic leaf anatomy, 341 (Fig. 169), 342 (Fig. 170). 343,344; specialised genus, 317; stomates, 166 Pontederia cordata, 166, 341 (Fig. 169), 342 (Fig. 170), 343, 344 Pontederia rotundifolia, 239, 240 Pontederiaceae, affinities, 316, 317; cleistogamy, 234 (Fig. 153); entirely aquatic, 318; geotropic curvature of fruit stalk, 239, 240 (Fig. 155), 282; heterophylly, 154, 1 60, 161; phyllodic leaf structure, 337, 341 (Fig. 169), 342 ( Fig. 170)-344 ; vegetative multi- plication, 213 Poplar, 187, 316 Portulacaceae, 310 Posidonia, apical openings absent, 269; chlorophyll in epidermis, 164; fibres in leaf sheath, 133; habit, 124; leaf anatomy, 132 (Fig. 89); marine Angiosperm, 123; pollen thread-like, 124, 125 Posidonia Caulini, 125, 132 (Fig. 89) Potamogeton, 58-72; air tissue in fruit wall, 71, 72 (Fig. 46); altitude, 290, 291; anatomy of inflorescence axis, 65; anatomy of root, 65 (Fig. 41), 66, 208; anatomy of stem, 62 (Fig. 39), 63, 64 (Fig. 40), 65, 175; apical openings of leaves, 167 (Fig. 108), 268; chlorophyll in epidermis, 164; choking mill sluices, 210; cortical bundles, 65; cuticle, waxy, 254; de- layed germination, 71, 72; dia- phragms, 65, 184 (Fig. 118); dispersal by ducks, 301, 302; distribution, 295, 297, 298; dwarfing due to heat, 275; exudation of water drops, 269; fibres in leaves, 61 (Fig. 38), 169; flowers, 69-71; fruits, 71, 72 (Fig. 46), 297; germination, 280; heterophylly, 151, 153, 154 (Fig. 103), 157, 158 (Fig. 104), 159 (Fig. 105), 339 (Figs. 167 and 1 68); in biological classification, 6; land forms, 61, 195, 196 (Fig. 125); leaf forms, 61,339 (Figs. 167 and 168); oil drops, 62 ; perenniation, 215 ; phyl- lodic interpretation of leaf, 339 (Figs. 167, 168), 340 43 INDEX Potamogeton crispus, 61, 62 (Fig. 39)-64 (Fig. 40), 67 (Fig. 42), 68 (Fig. 43), 69, 71, 164, 269, 275, 295 Potamogeton densus, 65 (Fig. 41), 71, 167 (Fig. 108), 206 (Fig. 137), 268, 298 Potamogeton fluitans, 69, 151, 157, 158 (Fig. 104) Potamogeton heterophyllus, 61, 195 Potamogeton lucens, 61-64 (Fig. 40), 164, 173, 262, 330, 339 (Fig. 167) Potamogeton natans, 6, 31, 32, 61, 62 (Fig. 39), 63, 65 (Fig. 41), 66, 70, 72, 151, 157, 159 (Fig. 105), 166, 167 (Fig. i07),i84(Fig.n8),i95,i96(Fig.i25), 272, 280, 289, 301, 339 (Fig. 168) Potamogeton obtusifolius, 206 Potamogeton pectinatus, 62-64 (Fig. 40), 65 (Fig. 41), 66, 70, 134, 262, 282, 291 Potamogeton pennsylvanicus, 303 Potamogeton perfoliatus, 58, 59 (Fig. 36), 61, 63, 69, 71, 140, 195 Potamogeton polygonifolius, 196 (Fig. 125), 333 Potamogeton praelongus, 62 Potamogeton pulcher, 61, 62 (Fig. 39), 63, 65 Potamogeton pusittus, 63, 64 (Fig. 40), 66, 71 Potamogeton rufescens, 69 (Fig. 44) Potamogeton trichoides, 62, 66, 71 Potamogeton varians, 195 Potamogeton zosterifolius, 61 (Fig. 38) Potamogetonaceae, fresh- water, 58-72; marine, 123-129, 131-135, 237, 331; other references, 205, 248, 314-316, Primulaceae, 180, 312, 318 Proserpinaca palustris, heterophylly, 159-161 Protection of embryo in aquatics, 242, 243 Pseudo-callitriche, 134, 237, 306, 307 Pseudo-lamina, 339, 340, 341 Quercus, 207 Rafts of wood of Herminiera, 192 Ranales, 146, 238, 308, 319, 320, 321, 346 Range, wide, of aquatics, 295 Ranunculaceae, 309, 313, 314, 320 Ranunculus, affinities, 318; air spaces, 1 76 (Fig. 113), 185; altitude, 290; am- phibious, aquatic and terrestrial types, 200, 309, 320; dimensions of dissected leaves, 140; geotropic curvature of peduncle, 145 (Fig. 93), 239; germina- tion, 280; heterophylly, 144 (Fig. 92), 145, 146, 155; in biological classifica- tion, 6; land and water forms, 195, 196 (Fig. 126), 198, 203 (Figs. 134, 135); luxuriance, 214; non-hetero- phyllous form, 145 (Fig. 93); root- system, 204, 264; sinking of seeds, 297; stem anatomy, 175, 176 (Fig 113); sub-aquatic flowering, 233, 234; submerged leaves, 29, 140, 142; sub- mergence of inflorescence, 228; sup- port of inflorescence, 228; toleration of salt, 134; winter-buds, 219 Ranunculus aquatilis, 144, 145, 155, 196 (Fig. 126), 204, 234, 280, 297 Ranunculus Baudotii, 134 Ranunculus carinatus, 228 Ranunculus circinatus, 145 Ranunculus confusus, 228 Ranunculus divaricatus, 234 Ranunculus Flammula, 145, 198, 203 (Figs. 134, 135), 309, 320 Ranunculus fluitans, 145, 214, 228, 233 Ranunculus hederaceus, 145 (Fig. 93) Ranunculus heterophyllus, 145 Ranunculus Lingua, 146, 219 Ranunculus Purschii, 144 (Fig. 92) Ranunculus repens, 200 Ranunculus sceleratus, 146, 284, 320 Ranunculus trichophyllus, 140, 176 (Fig, 113), 228, 290 Ranunculus sect. Batrachium, 6, 29, 142, 144-145, 175, 185, 195, 228, 239, 264, 309, 318, 320 Rheotropism, 282 Rhizomatous plants of Jura Lakes, 323 Rhizome, and polystely, 182; rhizome of Castalia, 24—26 (Fig. n), 217; Gunner a, 182; Hippuris, 173 (Fig. 112); Hottonia, 197 (Fig. 127); Lim- nanthemum, 39-41 (Figs. 22, 23), 217; Nymphaea, 24, 25 (Fig. 10), 27 (Fig. 12), 217; Potamogeton, 58-60 (Fig. 37) Rhizopods, as food of Utricularia, 94 Rhyncolacis macrocarpa, 120, 121 Ribbon leaves, n (Fig. 3), 12, 13 (Fig. 4), 14 (Fig. 5), 19, 20, 22, 23, 140, 141, etc. Riccia, 225 Rice, aliens accompanying, 303 Ricinus, 200 River-basins, isolation of, 296 Rodriguez, 129 Root-caps, of Brasenia, 205 ; of Lemna- ceae, 74 Root-hairs, absence of, in Lemna tri- sulca, 208, in water roots of Elodea, 208; in Hydrocharis, length of, 42, 43, 208; protoplasmic rotation, 43 Rootless Duckweed. See Wolffia Roots of water plants, 204-209 ; aeren- chyma, secondary, 188, etc.; air tissue, 185, 186 (Fig. 121), 187; anatomy, 65 (Fig. 41), 208, 209 (Figs. 138-140); assimilation, 207; differen- tiation, 207; equilibrium, 207; im- portance in life of aquatics, 264-266; reduction of, 208, 244; spiral or tendril, 127, 205 (Fig. 136), 206 (Fig. 137) INDEX 433 Roraima, 109 Resales, 310. 319 Roslyn Pits, Ely, 147, 215, 241 Rotala, alien accompanying rice, 303; cleistogamy, 234 Rotala indica, 303 Rotifers, 94, 142 Rubiaceae, 151, 313 Rubus jruticosus, 200 Ruppia, in brackish water, 134; macro- podous embryo, 319 (Fig. 166); pollination, 70; reduction of primary root, 244 Ruppia brachypus, 319 (Fig. 166) Sacred Lotus. See Nelumbo Nelumbo Sagittaria, band or ribbon leaves, 9, n (Fig. 3), 12, 13 (Fig. 4), 140, 141 (Fig. 90); diaphragms, 19 (Fig. 8), 167; effect of freezing on fruit, 243 ; floating of fruit, 297; flowers, 10 (Fig. i); fruits, 10 (Fig. 2); heterophylly, 9, ii (Fig. 3), 12, 13 (Fig. 4), 14 (Fig. 5), 1 6 (Fig. 6), 22, 23 ; in biological classifi- cation, 5; leaf anatomy, 344, 345 (Fig 171); mericarps and seeds, 17, 18; phyllodic interpretation of leaf, 161, 162, 339, 34.0, 344, .345 (Fig- squamulae intravaginales, 13, 15; stolons, 15, 16 (Fig. 6), 18 (Fig. 7); tubers, 11 (Fig. 3), 13 (Fig. 4), 15, 1 6 (Fig. 6), 17, 18 (Fig. 7), 217, 223, 224 Sagittaria montevidensis, 344, 345 (Fig. 171) Sagittaria natans, 12, 156 Sagittaria sagittifolia, 5, 9, 10 (Figs. 1, 2), 11 (Fig. 3), 12, 13 (Fig. 4), 14 (Fig. 5), 15, 16 (Fig. 6), 17, 18 (Fig. 7), 19 (Fig. 8), 23, 32, 34, 61, 141 (Fig. 90), 156, 157, 217, 223, 224, 297, 344, 345 (Fig- i?i) Sagittaria sagittifolia f. v alii snerii folia, II, 12 Sagittaria teres, 7, 22 Sagittayia variabilis, 17 St John's River, Florida, 213 Salix, development of aerenchyma, 188 ; effect of submergence, 200 " Salix viminalis, 188 Salt, toleration of, by certain hydro- phytes, 133, 134 Salvinia, 311, 337 Santalaceae, 312 Santarem, 291 Sarracenia, 93 Sarraceniales, 310, 319 Saxifragaceae, 310 Scania, 303 Scirpus, affinities, 317; heterophylly, 154; in Jura lakes, 287, 288 Scirpus fluitans, 317 Scirpus lacustns, 154, 288, 317 Scotland, 290 Screw Pine, 317 Scrophulariaceae, 151, 313, 318 Scutellaria, 188 Sedges in Lake Ellis 286 Seed dispersal, 17, 18, 35, 71, 72 (Fig. 46), 297-303 Seedlings, 243-249; oiAlisma, 151, 153 (Fig. 101); Castalia, 28 (Fig. 13), 29; Ceratophyllum, 85, 86 (Fig. 55); Dicraea, 114, 115 (Fig. 78); Elating, 245 (Fig. 158); Hippuris, 245 ; Lawia, H7;Lemna, 80, 81 (Fig. 52); Limnan- themum, 248; Nymphaea, 35 (Fig. 18), 36; Rhyncolacis, 121; Trapa, 245- 247 (Fig. 160); Utricularia, 100 (Figs. 67, 68); Victoria, 32, 33 (Fig. 16), 34; Zannichellia, 245, 246 (Fig. 159) Seeds, 239-249; of Aldrovandia, no; Alisma, 242, 297; Callitriche, 297; Castalia, 243; Hippuris, 242, 297; Hottonia, 297; Hydrocharis, 241; Lemnaceae, 80, 297; Limnanthemum, 240 (Fig. 156), 241, 243, 297; Litto- rella, 242 ; Lobelia, 297 ; Mayaca, 243 ; Myriophyllum, 242; Nymphaea, 243; Potamogeton, 297; Ranunculus, 297; Sagittaria, 17, 18, 297; Stratiotes, 241 ; Utricularia, 99, 100 Sensitive Plant, Floating. See Neptunia oleracea Sesbania, 191 "Shade leaf" characters of submerged leaf, 45 (Fig. 27), 164, 165, 171, 279 Siberia, 302 "Sifting" in evolution, 162, 203 Sium latifolium, heterophylly, 147, 149 (Fig. 97), 150 (Fig. 98); in biological classification, 5 Sleep movements, 281 Snails, as food of Utricularia, 93 Snails, water, absence in peaty water, 287; as pollinators of Sagittaria, 9; unable to eat turions of Utricularia, 101 Sneezewort. See Achillea ptarmica Solanum Dulcamara, 198 Soldier, Water. See Stratiotes aloides Sparganium, fruits eaten by wild ducks, 301; resemblance to Pandanaceae, 317 Spargamum natans, $ij Spathicarpa, 315 Spathiflorae, 314 Spearwort, Greater. See Ranunculus Lingua Spearwort, Lesser. See Ranunculus Flammula Specific characters, constancy of ana- tomical, 131, 331; lack of utility of, I31, 330, 33i Specific vital energy, 212 Speedwell, Water. See Veronica Ana- gallis 434 INDEX Spirodela polyrrhiza, 7, 74 (Fig. 47), 75, 76, 78, 80, 215 Spurwing, 300 " Squamulaeintravaginales," 15, 126,271 "Starch-leaved" plants, 164 Starwort, Water. See Callitriche Stems, aquatic, condensation of vascular tissue, 174-180; morphology and anatomy, 172-185, 187-194; poly- stely, 180—182; reduction of xylem, 172, 173; sympodial growth, 172, 173 Stomates, on submerged leaves, 165, 166, 167 (Fig. 107); water stomates, 30, 267, 268 (Fig. 163), 269 Stratiotes, calcophil, 287; chalky in- crustation on leaves, 51; dehiscence, 241; dioecism, 54; flowers, 54 (Fig. 33); fruit, 239; geographical distribu- tion of sexes, 54; habit, 53 (Fig. 32); in biological classification, 7; in- versely orientated leaf bundles, 52; leaves, 49~53, 57. I4I. J57> J69; mechanism of rising and sinking, 50, 51 ; Pleiocene and Pleistocene records, 54;roots, 50, 185, 186 (Fig. 121), 187, 207, 244; stem, 49 (Fig. 31), 172; stomates, 51, 52; winter-buds, 53 (Fig. 32), 54 Stratiotes aloides, 7, 49 (Fig. 31), 50-53 (Fig. 32), 54 (Fig. 33), 185-187, 215, 219, 241 Submerged leaves. See Leaves, sub- merged Substratum, influence of, 286—287 Subularia, affinities, 309; in biological classification, 7 ; submerged flowering 233 Subularia aquatica, 7, 309 "Sugar-leaved" plants, 164 Sundew, in Surface-heating of ponds, 274 "Swan's Potatoes," 17 Sweden, 46, 303 Sweet William, 155 Switzerland, 302, 303 Sympetalae, 312, 313, 320 Tanqui, 181 Temperature, 273-275 Tendril roots, of Gentianaceae, 205; Haloragaceae, 205 ; Hydrocharita- ceae, 205 (Fig. 136); Philodendron, 206; Potamogetonaceae, 205, 206 (Fig- 137) Terrestrial forms. See Land forms Thalassia, leaf anatomy, 131, 169; marine Angiosperm, 57, 123 Thallus of Podostemads, 7, 114, 115 (Figs. 76, 77, 78), 116 (Fig. 79), 117 Thames, 210, 273 Theophrastus, on ecology of aquatics, 285; on Trapa, 207 Thyme, Water. See Elodea canadensis Tibet, 291 Tillaea. See Bulliarda Tillandsia, 108, 109 Tornelia, 142 Transcaucasia, 303 Transpiration current, 260-272 Trapa natans, affinities, 311; assimila- tory roots, 207, 255; buoyancy due to lacunae, 192; changes in distribution, 302, 303; cotyledons, 245; fixation of seedling, 245; fossil records, 303; hypocotyl, negatively geotropic, 245; reduction of primary root, 244 ; seed- ling, 247 (Fig. 160); transverse helio- tropism, 281; use in magic and medicine, 302 4 ; heterophylly, Trap ell a sinensis, 234 Trianea, 45 Tri folium, sleep habits of, 161 Trifolium resupinatum, 199 (Fig. 131) Tristicha, dorsiventral root, 113; primi- tive Podostemad, 306 Tristicha ramosissima, 113 Tristichaceae (see also Podostemaceae), 7, 112, 113, 120, 306 Turions, 217-225; and unfavourable conditions, 222-224 ; m relation to in- florescences, 224, 225 ; oiAldrovandia, no, 219; of Caldesia, 22, 225 (Figs. 148, 149) ; oiHydrocharis, 47 (Fig. 29), 48, 49 (Fig. 30), 219; of Lemnaceae, 75-77, 219; of Myriophyllum, 219, 220, 221 (Fig. 144), 222 (Fig. 145), 223 (Fig. 146); of Potamogeton, 66, 67 (Fig. 42), 68 (Fig. 43), 69 (Fig. 44); of Utricularia, 101, 102 (Fig. 69), 103, 219, 220 (Fig. 143) Ulva, 27, 146 Umbelliferae, 5, 147, 312 Unconscious memory, 333, 347 Upware, 216 Utricularia, 91-109; absence of roots, 91, 204, 244; affinities, 313; "air shoots," 96-98 (Fig. 65); anatomy, 107, 108 (Fig. 74), 22 7; apical develop- ment, 106 (Fig. 72), 107 (Fig. 73); benzoic acid in utricles, 96; capsule, 239; carnivorous habit, 93-96, in, 270; comparison with Aldrovandia, no; dispersal, 299; "earth shoots," 96 (Figs. 63, 64), 97, 270, 336, 337; effect of strong light, 279 ; epiphytism, 108, 109; floats on inflorescence axis, 99, 229 (Fig. 150); freezing, 220; hairs inside utricle, 92 (Fig. 60), 93 (Fig. 61), 95 ; hairs on valve of utricle, 92 (Fig. 60), 94, 95; land forms, 91; morphology, 73, 103-107; position in biological classification, 8 ; regene- ration, 104 (Fig. 70), 105 (Fig. 71), INDEX 435 1 06; relation to Pinguicula, 181; "rhizoids," 96, 98, 99 (Fig. 66); seed- lings, 100 (Figs. 67, 68); seeds, 99, 100; turions, 101-103, 219, 220; utricles, 91, 92 (Figs. 59, 60), 93 (Fig. 61), 94, 95 (Fig. 62), 96 (Figs. 63, 64), 97, 98 (Fig. 65); vegetative luxuriance, 215 ; water absorption, 2 70 Utricularia Bremii, 93 (Fig. 61), 95 (Fig. 62), 97, 299 Utricularia exoleta, 100 (Fig. 68) Utricularia flexuosa, 92 (Fig. 60), 93 Utricularia Hookeri, 93 Utricularia Humboldtii, 109 Utricularia inflata, 99, 229 (Fig. 150) Utricularia inflexa, 99 Utricularia intermedia, 91, 94, 97, 101, 219, 220 (Fig. 143) Utricularia minor, 91, 96 (Figs. 63, 64), 97, 100, 102 (Fig. 69) 108 (Fig. 74), 168, 299 Utricttlaria neglecta, 92 (Fig. 53), 93, 97, 99 (Fig. 66), 104 Utricularia nelumbi folia, 108, 109 Utricularia ochroleuca, 97 Utricularia quinqueradiata, 99 Utricularia stellaris, 99 Utricularia vulgar is, 91, 93, 94, 97, 98 (Fig. 65), 100 (Fig. 67), 101, 102, 104 (Fig. 70), 105 (Fig. 71), 106 (Fig. 72), 107 (Fig. 73), 215, 220, 227 Valerian, Grew on heterophylly in, 155 Vallisnerta, contraction of peduncle, 235. 239; experimental cultures, 265; leaves, 57, 140, 169; ovules, 314; pollination, 57, 235, 236; root anatomy, 208, 209 (Fig. 139); tolera- tion of salt, 134 Vallisneria spiralis, 134, 140, 209 (Fig. 139), 235, 236 Vegetative reproduction, 210-226, etc. Venezuela, 113, 122, 191, 291 Veronica, anatomy of submerged and air shoots, 201, 259 Veronica Anagallis, 201, 259 Vicia, effect of water, 200; nutation of roots, 207 Vicia sativa, 200 Victoria regia, affinities, 38; evocation of heat from flower, 34; in shallow water, 31; leaf succession, 32-34; peltate leaf, 30; rate of growth, 214; ripening of fruit under water, 300; seedlings, 32, 33 (Fig. 16), 34; size, 80; Spurwing nesting on leaf, 300 Villarsia. See Limnanthemum Villarsia ovata, 166 Violet, Water. See Hottonia palustris Vorticellidae, 237 Vosges, 225, 287 ""Wapatoo," 17 Water Aloe. See Stratiotes aloides Water Buttercup. See Ranunculus sect. Batrachium, R. aqualilis, etc. Water Chestnut. See Trapa natans Water Crowfoot. See Ranunculus sect. Batrachium, Ranunculus aquatilis, etc. Water forms, of Achillea, 199; Caltha, 198, 199 (Fig. 129); Cirsium (Cnicus), 198, 199 (Fig. 130), 200; Cuscuta, 199 (Fig. 131); Hydrocotyle, 200, 201 (Fig. 132); Menyanthes, 199; Ranun- culus, 198, 200, 203 (Figs. 134, 135); Trifolium, 199 (Fig. 131), etc. Water-fowl and dispersal of hydro- phytes, 35, 298-302 Water Hyacinth. See Eichhornia spe- ciosa Water Lobelia. See Lobelia Dortmanna Water Loosestrife. See Lythrum Sali- caria Water Milfoil. See Myriophyllum Water Parsnip. See Sium lati folium Water Plantain. See Alisma Plantago Water Plantain, Lesser. See Echino- dorus ranunculoides Water pores, 267-269, etc. Water-shield. See Cabomba Water-snails. See Snails, water Water Soldier. See Stratiotes aloides Water Speedwell. See Veronica Ana- gallis Water Starwort. See Cattitriche Water stomates. See Stomates, water Water Thyme. See Elodea canadensis Water Violet. See Hottonia palustris Waterlily, Giant. See Victoria regia Waterlily, White. See Castalia alba Waterlily, Yellow. See Nymphaea lutea Waterweed, American, or Canadian. See Elodea canadensis Weddelina squamulosa, 113 Whitchurch Weir, 210 White Moss Loch, Perthshire, 288 (Fig. 165), 289 White Waterlily. See Castalia alba Wicken Fen, 20 Willow Herb, 311 Wolffia, conveyance by birds, 300; in biological classification, 7; member of Lemnaceae, 74; size, structure, wintering habits, 80 Wolffia brasiliensis, 80, 300 Wolffia Michelii, 80 Xerophyte, aquatic, 310 Xylem and water conduction, 173, 174 Yellow Waterlily. See Nymphaea lutea Yew-tree, 316 Yorkshire, 303 "Youth forms" of Conifers, 155 Zannichettia, anatomy, 62, 63, 173; 436 INDEX flowers, 70 (Fig. 45), 71, 215, 315, 316; of leaves, 269; association with Algae, fruit and seedling, 246 (Fig. 159), 123; distribution, 302; food of Brent 248; hydrophilous pollination, 70, 71, Geese, 302; fruit, 248 (Fig. 161); 134, 346; in biological classification, habit, 124, 140; in biological classi- 6; rheotropism, 282; root-hairs from fication, 6; leaf anatomy, 128 (Figs "collet," 245; stability of seedling, 85, 86), 131, 164; macropodous 245; twining roots, 205, 206 (Fig. 137) embryo, 248 (Fig. 161); pollen, Zannichettia palustris, 134, 173, 205, thread-like, 125; pollination, 127, 206 (Fig. 137), 215, 282 129, 237 Zannichettia polycarpa, 70 (Fig. 45), 71, Zostera marina, 123, 127, 128 (Figs. 85, 246 (Fig. 159) 86), 129, 248 Zonation, 287-289 Zostera nana, 125, 302 Zostera, affinities, 315; apical openings "Zostera ocean-ica." 125 CAMBRIDGE: PRINTED BY j. 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