THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES tf 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 I-MACMILLANANDCO., 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). WATER PLANTS A STUDY OF AQUATIC ANGIOSPERMS BY AGNES ARBER, D.Sc., F.L.S. FELLOW OF NEWNHAM COLLEGE, CAMBRIDGE, AND KEDDEY FLETCHER-WARR STUDENT OF THE UNIVERSITY OF LONDON 45781 WITH A FRONTISPIECE AND ONE HUNDRED AND SEVENTY-ONE TEXT-FIGURES CAMBRIDGE AT THE UNIVERSITY PRESS 1920 451 W TO THE MEMORY OF E. A. N. A. 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 obconka 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 mu^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 CHAP. 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 . 195 204 210 227 239 PART III THE PHYSIOLOGICAL CONDITIONS OF PLANT LIFE IN WATER XX. GASEOUS EXCHANGE IN WATER PLANTS XXI. ABSORPTION OF WATER AND TRANSPIRATION CURRENT IN HYDROPHYTES .... XXII. THE INFLUENCE OF CERTAIN PHYSICAL FACTORS IN THE LIFE OF WATER PLANTS XXIII. THE ECOLOGY OF WATER PLANTS . . PART IV THE STUDY OF WATER PLANTS FROM THE PHYLOGENETIC AND EVOLUTIONARY STANDPOINTS XXIV. THE DISPERSAL AND GEOGRAPHICAL DISTRIBUTION OF WATER PLANTS ..... XXV. THE AFFINITIES OF WATER PLANTS AND THEIR SYSTEMATIC DISTRIBUTION AMONG THE ANGIO- SPERMS ....... (i) The Affinities of Certain Aquatic Angio- sperms ..... (ii) Theoretical Considerations . XXVI. THE THEORY OF THE AQUATIC ORIGIN OF MONO- COTYLEDONS 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 253 260 273 285 295 308 308 317 3" 327 BIBLIOGRAPHY . INDEX TO BIBLIOGRAPHY INDEX . 336 349 422 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.] . . . » . . . . . 1 8 8. Sagrfttrria sagittifolia, L. Diaphragm of petiole. [Blanc, M. le (i912)] '9 9. Echinodorusranunculoides^LJEngelm. 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. (19*7)] 39 22. Limnanthemum nymphoides, Hofhngg. 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. Hydrocharis 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 aloides, 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 pulcher, 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. [Gluck, 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. Lemna gibba, L. [Hegelmaier, F. (1868)] 76 49. Lemna trisulca, 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. Ceratophyllum demersum, L. Flowers. [A. A.] .... 85 55. Ceratophyllum demersum, L. Seedling. [Guppy, H. B. (1894!)] . 86 56. Ceratophyllum demersum, L. Stem-stele. [Schenck, H. (1886)] . 87 57. Ceratophyllum demersum, L. Rhizoid. [Gluck, H. (1906)] . . 89 58. Ceratophyllum demersum, L. Leaves of water shoot and rhizoid. [Gluck, H. (1906)] 89 59. Utricularia neglecta, Lehm. Leaf with bladders. [Gluck, H. (1906)] 92 60. Utricularia fiexuosa, Vahl. Section through bladder. [Goebel K (1891-1893)] 02 61. Utricularia Bremii, Heer. Glands from bladder. [Meierhofer, H. 93 LIST OF ILLUSTRATIONS xiii FIG. PAGE 62. Utricularia Sremii, Heer. Part of leaf with bladder. [Meierhofer, H. 95 63. Utricularia minor, L., with earth-shoot. [Gluck, H. (1906)] . . 96 64. Utricularia minor, L. Leaves of water- and earth-shoots. [Gluck, H. (1906)] ........... 96 65. Utricularia vulgaris, L., with air-shoot. [Goebel, K. (1891-1893)] 98 66. Utricularia neglecta, Lehm. Rhizoids. [Gluck, 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. [Gluck, H. (I906)] ........ • 102 70. Utricularia vulgaris, L. Leaf with adventitious shoots. [Goebel, K. (J904)] ..... ...... 104 71. Utricularia vulgaris, L. Inflorescence axis with adventitious shoots. [Luetzelburg, P. von (1910)] . . ..... 105 72. Utricularia vulgaris, L. Apical development of shoot. [Pringsheim, ^(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)] • • IJI 76. Hydrobryum olivaceum, (Gardn.) Tul. [Warming, E. (i8832)] . 115 77. Dicraea elongata, (Gardn.) Tul. [Warming, E. (i8832)] . .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£arberi,Wi]]is. Cleistogamic flower. [Willis, J.C. (1902)] 121 83. Cymodocea aequorea, Kon. [Bornet, E. (1864)] .... 124 84. Cymodocea aequorea, Kon. [Sauvageau, C. (iSgi1)] . . .125 85. Zostera marina, L. Anatomy of leaf. [Sauvageau, C. (iSgi1)] . 128 86. Zostera marina, L. Median bundle of leaf. [Sauvageau, C. (iSgi1)] 128 87. Halofbila ovalis, (R. Br.) Hook. fil. [Balfour, I. B. (1879)] . .130 88. Halodule uninervis, Boiss. [Sauvageau, C. (1891 *)] . . .132 89. Posidonia Caulini, Kon. Anatomy of leaf. [Sauvageau, C. (iSgi1)] 132 90. Sagittaria sagittifolia, L. Young plant with ribbon leaves. [A. A.] 141 91. Aponogeton fenestralis, (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. Callitriche verna, L. Heterophylly. [A. A.] .... 147 95. Hippuris vulgaris, L. Water leaves and air leaves. [Gluck, H. (1911)]* 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 ampbibium, 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. (!9H)] 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. (iSgi1)] . . 167 109. Myriophyllum spicatum, L. Leaf anatomy. [Schenck, H. (1886)] . 168 no. Myriophyllum verticillatum, L. Trichomes. [Perrot, 6. (1900)] . 170 in. Callitriche verna, L. Leaf anatomy of land and water forms. [Schenck, H. (1886)] 170 112. Hippuris vulgaris, L. Rhizome. [Innisch, 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. Myriophyllum spicatum, L. Stem anatomy. [Vochting, H. (1872)] 179 117. Myriophyllum spicatum, L. Details of stem anatomy. [Vochting, H. (1872)] 179 118. Potamogeton natans, L. Diaphragm of stem. [Blanc, M. le (1912)] . 184 119. Hippuris vulgaris, L. Development of stem diaphragms. [A. A.] . 184 120. Hippuris vulgaris, L. Origin of cortical lacunae in stem. [Barratt, K. (1916)] 185 121. Stratiotes aloides, L. Origin of cortical lacunae in root. [Arber, A. (i9H)] 186 122. Jussiaea peruviana, L. Aerenchyma. [Schenck, H. (1889)] . . 190 123. Neptunia oleracea, Lour. Floating shoot. [Rosanoff, S. (1871)] . 191 124. Nesaeaverticillata,B..E.&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. (191 1)] 199 131. Water forms of Cuscuta alba, J. & C. Presl, Eckinodorus ranuncu- loides (L.), Engelm. and Trifolium resupinatum, L. [Gluck, H. (I911)] 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. [Gliick, H. (1911)] . 203 136. Hydnlla verticillata, Presl. Tendril roots. [Kirchner, O. von, Loew, ETand Schroter, C. (1908, etc.)] 205 137. Zannichellia palustris, L. and Potamogeton densus, L. Tendril roots. [Hochreutiner, G. (1896)] 206 138. Callitricbe stagnalis, Scop. Root stele. [Schenck, H. (1886)] . 209 139. Vallisneria spiralis, L. Root anatomy. [Schenck, H. (1886)] . 209 140. Naias major, All. and N. minor, All. Root anatomy. [Sauvageau, C. (I8891)] 209 141. Cardamine pratensis, L. Leaves bearing adventitious plantlets. [A.A.] 217 142. 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. Myriopbyllum verticillatum, L. Germinating turion. [A. A.] . . 222 146. Myriophyllum verticillatum, L. Land form with turions. [Gliick, H. (i9°6)] 223 147. Ecbinodorus ranunculoides, (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, R. B. (19171)] 234 xvi LIST OF ILLUSTRATIONS FIG. PAGB 154. Callitriche verna, L. Flowering shoot. [A. A.] .... 237 155. Pontederia rotundifolta, L. Flowering shoot. [Hauman-Merck, L. (I9J31)] • 24° 156. Limnanthemum nympboides, Hoffmgg. and Link. Fruit and seed. [A.A.] . 240 157. Limnantbemum nympboides, 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. Trapanatans,L. Seed and germination. [Goebel, K. (1891-1893)] 247 161. Zostera marina, L. Fruit. [Raunkiaer, C. (1896)] .... 248 162. Transpiration experiment. [Sauvageau, C. (iSgi1)] . . . 262 163. Callitricbe 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)] . . 288 166. Ruppia brachypus, J. Gay. Fruit. [Raunkiaer, C. (1896)] . . 319 167. Potamogeton lucens, L. Range of leaf form. [Raunkiaer, C. (1896)] 339 168. Potamogeton natans, L. Range of leaf form. [Raunkiaer, C. (1896)] 339 169. " Lamina " of Pontederia cordata, L. and Eicbhornia 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 "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^ 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 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 Stum tati/olium, L. (Water Parsnip). C. Plants which produce three types of leaf, (a) submerged, (b) 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 alba, 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., Myriophyllum (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). Zoster a (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 aloides, 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). Spirodela polyrrhiza, Schleid., 1 (Duck- Lemna minor, L. and L. gibba, L.J weeds). (ii) Rootless. Wolffia (Rootless Duckweed). 8 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 trisulca^ L. (Ivy-leaved Duckweed), (ii) Rootless. (a) Inflorescence raised out of the water. Aldrovandia. Utricularia (Bladder wo it). (£) Flowers submerged; hydrophilous pollination. Ceratophyllum (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 i62o5 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); Gluck'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). s MacLeod, J. (1893). 4 Bauhin, G. (1596). 5 Ibid. (1620). 10 ALISMACEAE [CH. FIG. i. Sagittaria sagittifolia, L. Top of inflorescence, August 17, 1917. 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 water*. 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/£/V.(i886). 5 Kirschleger, F. (1856). 6 Wachter, W. (1897!). ii] 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. (f nat. 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 sagittifolia, 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 variant/is , 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^rowth3. 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 16, 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. (1881). 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 (<). (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 fist 4. 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^arent 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 (sf^) 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. (1842). 3 Anon. (1895). 4 Qsbeck, P. (1771). 5 Paillieux, A. and Bois, D. (1888). « Buchenau, F. (1882). i8 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 (s/t) from plant of previous year bearing tuber (t) with scale leaves (sc) ; the plant of the current year has also produced a stolon (stj 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. (f nat size.) [A.A.I 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). n] 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 landltfe, while the latter is typically a water plant. A. Plantago, (L.) Mich, generally lives in FIG. 8. Sagittaria sagittifoUa, L. Diaphragm (£>) shallow water, where air leaves form the chief as- similatory organs. These are preceded, however, by band-shaped primary leaves (Fig. 101 B, 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). 2 Gluck, H. (1905). 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 arcualum, 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 orT-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 FIG. 9. Echinodorus ranunculoides, (L.) Engelm. A, plant from a dried up fen, August 5, 191 1 ; 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 of Caldesiaparnassifolia, (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. Gltick, 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. Gliick'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 lib. 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 1 'Vb. Similar to Stage I Fa, 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 parnassifolia, (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 THE Nymphaeaceae, like the Alismaceae dealt with in 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 = Nupharluteum, Sibth. et Sm. CH. m] WATERLILY RHIZOMES 25 be seen as rudiments and at rz as scars, while numerous groups of three mature roots are also shown (e.g. r^). Fig. 10 B repre- sents such a group in further detail. The root system is very elaborate, since the adventitious roots bear branches (Fig. loB] 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; r3, 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.] 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 (st. in Fig. 1 1 5, C, D). A larger number of roots is associated with each leaf than in the case of the Yellow Waterlily. These i = Nymphaea a!ka, L. 26 NYMPHAEACEAE [CH. roots may be seen in Fig. 1 1 A^ and their rudiments (r.) in Fig. 1 1 B and D. In Nymphaea lutea stipules are absent but the FIG. ii. Castalia 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 Irmisch, T. (1853). m] THE YELLOW WATERLILY 27 Sometimes, if a young rhizome ofNymphaea lutea 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 Uha (Fig. 12). In a wood-cut in the famous Herbarum vivae 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 — 1 Royer, 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). * 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- FIG. 14. Cabomba 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). 3o NYMPH AEACEAE [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 Nymphaea^. 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). * Schrenk, J. (1888). » 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). sjahn,E.(i897). Ill] FLOATING LEAVES 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. 1 5 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 plants which are rooted at the bottom of the water and bear floating leaves. Potamogeton natans2 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). 2 Jahn, E. (1897). 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 diagram. (Reduced.) May 30, 1911. [A. A.] 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 ^ 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- lowing account of the series is derived^rpm his work : The first leaf is acicular and without a blade (/j in Fig. 1 6). The second leaf is elon- gate lanceolate, sometimes with two hastate lobes, and resembles the adult leaves of Earclaya (/2 in Fig. 16). 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. 16. Victoriaregia.'Lmdl. Seedling, showing tViprp is a litt-lp nnrket- or acicular first leaf 'i. and hastate second and r third leaves /, and /,. (According to Gwynne- pouch on the adaxial side, Vaughan, D. T. (1897), the second leaf is more which.ppeu.to be formed 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). 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- foliay 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? xymphaealuteai the apices of the carpels, apparently at the L- Fruit showing Per- expense of the starch which they contain3. 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). 4 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 lute a (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 cbjjs 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 , , , J , ' J FIG. 1 8. Nymphaea lutea.'L. ^.seedling nad become water-logged and April 23, 1915 (x 2). B, seedling May 28, B had sunk to the bottom of the larof rootnairs;c,c=cotyledons;/>/.=plu- bell-jaH. It has been shown3 mule; llt /„ 13, first, second and third that these detached loculi are ^esofPlumule^=Seed;0=oPerculum. clothed with thin-walled cells which secrete much mucilage outwards. The cells have at first iGuppy, H. B. (1893). 2 On dehiscence of Casta/ia alba see p. 302. 3 Raciborski, 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 ^,p. 35 ; the seed-coat opens by means of an operculum (0) to emit the radicle. During the summer these seedlings developed a number of submerged leaves with lanceolate blades (Fig. 18 B\ 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 11 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 ancfttistinct stele," •in which about twenty bundles form a ring. However the most typi- /r 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 leaf . (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 xin. 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. 45781 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). /"\ii»- "R«-«<-«^U ~\\T 4. 1*1* FIG. 20. Brasenia Schreberi, J. F. Gmel. Part Our British Waterlilies of ^^ section of youjng leaf to show belong to the Central tribe thesecretoryhairs.w.A., surrounded by alayer of the family— Nymphaeoi- of dear mudlage' m' [G° 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). 5 Wettstein, R. von (1888). 6 Gwynne-Vaughan, D. T. (1897). Ill] 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 (Fillarsia\ a member of the Gentianaceae, which is represented in Britain by the beautiful L. 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 Jr\. 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.parnassifolia'2', 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). * Solereder, H. (1914). 3 Mer, £. * Gliick, H. (1906). * Qoebel, 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; flt /2, /3, successive foliage leaves; stlf st2, stipules belonging to /! and/2; s and s, outer scale leaves; r± and r2, 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 (j/J enclose the next foliage leaf (/2). The young stolon of the next genera- tion (tf#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. st. <* 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; dv = 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. Hydrochans Morsus-ranae, L. T.S. upper epidermis passing through a T.S. leaf of young plant growing entirely stomate. (xaiS.) [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 5). Attention has been drawn by i 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- Px , camL 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 Cf. 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 1 Goebel, 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 16 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 subterranean1. 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. FIG. 30. Hydrocharis Morsus-ranae, L. Young plant developed from a turion, showing the stage reached on May 16, 1911. (Nat. size.) [A. A.] FIG. 31. Stratiotes aloides, L. Semi- diagrammatic sketch of stem, as it appears in August, bisected longitudi- nally (v.c. — vascular region of stem; c=stem cortex; l.t. = leaf- trace; /=leaf ; st. = young stolon ; s = squamula intrava- ginalis ; ^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). 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 abides 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). 8 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 * 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 Nolle, 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). 4 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 (squamulae 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 Glttck, 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 A B C FIG. 33. Stratiotes aloides, L. A, unopened female flower emerging from two bracts (b). 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 2 Geldart, A.M. (1906). 3 Reid, €.(1893). 4 Nolte, E. F. (1825); see also Caspary, R. (1875). iv] THE CANADIAN WATERWEED 55 verttdflata, 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, nas 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 livfc' independently, while special winter- FIG. 34. Elodea shoots may also be produced (Fig. 34). The %£%£*%£. small leaves, which are arranged in whorls [Raunkiaer, c. of 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. ioensis6 (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. (i 8 5 2) and (i 857), Caspary,R. (i 8582) and Siddall, J. D. (1885). See pp. 210-213 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 E. A/1.5.. deL. 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. (18582) 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 Vallisneria'2', while in Hydromystria the pollination is sometimes effected by wind and sometimes by water3. In Elodea densa 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 Hydrocharis, 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. Stratiotes. 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). * Hauman-Merck, L. [58] 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 POND WEEDS 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- dial2. 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. 3 7, 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, (i 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'") 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, II', III', 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 IrmJsch, T. (18583) 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 ah 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 « c -iii j i -r-i i FIG. 38. Potamogeton zosteri- lammae or variable breadth. Examples foiius, Schum. Upper part of this group are P. lucens, P. perfoliatus of ,leaf; *»*• «*i. snz- <». vas- & " J cular bundles; s, bast bun- and P. crispus. In these and related dies; rs, bast bundle along species the blade is exceedingly thin, margi£jrX( ^^ .[faun' 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 (j 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. 62 POTAMOGETONACEAE [CH. graceful way. A similar undulation is characteristic of Apono- geton ufoaceus. 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. tt, Tlf tlt traces of next higher leaf; ts, r2> ta, 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. (i 3 Schenck, H. (1886) and Raunkiaer, C. (1903). v] 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 (/! , 7\ 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. 39-8) differs from that of P. Quicker in the fact that the traces beiwiging 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 Ay 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 B) 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 -D) 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 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 (x2go). [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). 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. 4 1 E] has the same type of structure, but the cell-walls remain thin. In P. pectinatus (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. pectinatus, 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. (i SSg2) describes the roots of P. 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 crispus2 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 fos^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. Potamogeton rufescens, Schrad. lated to P. natans—thzy T-s/ ®x™fr a .tu?on; / and, B> ™*? ' scale leaves equivalent to stipules; I— IV, OCCUr in this situation Only. foliage leaves, whose stipules are marked r> , / ?• . T i-4 and put in in solid black. Squamulae Potamogeton perfoltatus, L. in^avagilnales are omitted. (Enlarged.) forms winter-buds which are [After Giuck, H. (1906), Wasser- und , • , r i -\ • Sumpfgewachse, Bd. u, p. 160, Fig. 23.] not deciduous but unfold tn 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 floats, and the 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 m.c. 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- tinatusy 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 Zannichellia^olycarpa theflowersareunisexual(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 (st Fig. 45 E\ 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. CQje 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.perfo/iatus, and the cuticularised epidermal layer (o.e.\ 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. 8 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 1 8 9 1 , 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 otter 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. Hegelmaier3, 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 Goebel3 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 e^'ceSchl5f d ' 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; / . /., lateral shoot. [Hegel- examination or Puna 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 plant6. 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. (io.i94). 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^fcJae 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). 3 Guppy, H. B.(i894«). 76 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- 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). FIG. 48. Lemna gibba, L., with fruit,/. [Hegelmaier, F. (1868).] 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 tha^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 surface 2. 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. trisulca, L., the Ivy-leaved Duckweed, a submerged plant, floating beneath the surface level1. The fronds of L. trisulca 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). 4Schenck, 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.).] IG. 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).] 80 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 I -5 mm., while W. brasiliensis, 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 polyrrhiza^ (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 (st-i and J/2) and a female flower simply formed of a gynae- ceum (c.) with one or two ovules. Lemna minor3, 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). 3 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. 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 Sfrafiotes, 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 1 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 Hornwort, 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 (/> in Fig. 54 B). 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, ^chleiden, M.J. (1837). 2 Delpino, F. and Ascherson, P. (1871). 3 Ludwig, F. (1881). 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 r-st. "P- C B FIG. 54. Ceratophyllum demersum, L. A, node bearing two male flowers ( H- (I9°2)'] it was maintained that the bladders were to be interpreted as floats, which buoyed up the plant in the water. This idea has been discounted, however, since many terrestrial Utricularias produce large numbers of bladders; moreover it has been shown that the Utricularias do not sink when all the bladders are removed3. A third hypothesis now holds the field — namely, that the bladders act as traps for small animals which serve as food for the plant; this theory may now be considered to be fully proved. Before the middle of the last century, Treviranus 4 had recorded the finding of a beetle and some small snails in the bladder of a terrestrial Utricuiaria (U. Hookeri) and had suggested the comparison between these organs and the pitchers of Sarra- cenia. Nepenthes, and other carnivorous plants. But it was not until 1875 tnat tne fact that our native Utricularias preyed on small animals was definitely proved. In this year Cohn5 1 Darwin, C. (1875). 2 Crouan (Freres) (1858). 3 Darwin, C. (1875), Biisgen, M. (1888), Goebel, K. (18892) and (1891-1893). 4 Treviranus, L. C. (1848!). •5 Cohn, F. (1875). See also Darwin, C. (1875). 94 UTRICULARIA [CH. showed that in herbarium specimens of Utricularia vulgaris the bladders often contained skeletal tissues of Crustacea and insect larvae. He then tried the experiment of putting a living shoot of this plant, which had empty utricles, into water rich in Cypris\ next morning nearly all the bladders contained Crustacea, swimming about in a restless manner but unable to escape. Rotifers, Infusoria, Rhizopods and other animals were also present; certain bladders containing as many as six living Crus- tacea, as well as other animals, were described by the observer as " a little menagerie of the microscopic water fauna." The number of animals secured may sometimes be very great. It has been recorded, for instance, that a plant of the Common Bladderwort, introduced into water rich in Daphnidae, in one case was found after i \ hours to have caught as many as twelve of these little Crustacea in a single bladder1. Another plant, which was about 1 5 cms. long, and bore fifteen fully developed leaves, each with about six bladders, is reckoned to have en- trapped at one time as many as 270 individuals of Chydorus spkaericus1. It is a curious fact that different species of Utricu- laria, even when growing associated in the same water, may, owing to some slight difference of habit, catch quite different animals. In one case Goebel2 observed U. intermedia and U. vulgaris growing together, but while U. intermedia had caught chiefly Cypris, U. vulgaris had caught only Copepods. This is to be explained by the fact that U. intermedia^ being anchored at the bottom of the water, was only able to secure the Cypris, which is a creeping form, while the Copepods, because they were free-swimming, were entrapped by the bladders of the unattached U. vulgaris. The animals are said to be attracted by edible mucilage secreted by the hairs which grow on the blad- ders of the Utricularias (Fig. 62), and especially on the valve at the aperture2. The observations which we have enumerated and many others which might be cited, leave no room for doubt that the ifiusgen, M. (1888). 2 Goebel, K. (1891-1893). vin] CARNIVOROUS HABIT 95 Utricularias do, as a matter of fact, catch animals in their utricles, but the questions still remain whether the absorption of organic material actually takes place, and, if so, whether the carnivorous habit is of definite benefit to the plant. The inner epidermis of the bladders is cuti- cularised except as regards the four-armed hairs (Fig. 61, p. 93) which are thin-walled. These hairs, in the case of a bladder enclosing decaying animals, have been seefr*to include oil-drops, which may be presumed to be derived from the animal tissues, since the hairs in a bladder which had received no food, showed no such drops1. Experimental work FlG 62 utricuka Bremii, Heer. has also demonstrated that treat- . . ment with ammonium nitrate, etc., produces changes in the hairs which suggest that absorp- tion has occurred2. These observations would not be sufficient in themselves to prove that the entrapped animals serve as a source of food for the plant, but a demonstration of this point was supplied by certain comparative cultures of Utri- cularias growing in water with or without animal life. From the upshot of these experiments it appeared that the plants deprived of animal food only showed about one-half of the growth of those that were allowed to catch their prey in the normal way3. A further problem which presents some difficulty is that of the causes which bring about the death and absorption of the entrapped animals. No highly poisonous substance can be present in the bladders, since the imprisoned animals 1 Goebel, K. (1891-1893). 2 Darwin, C. (1875). 3 Busgen, M. (1888). See also Darwin, C. (1888), footnote to P- 365- Part of leaf with bladder. (Enlarged.) [Meierhofer, H. (1902).] 96 UTRICULARIA [CH. may remain alive in them for some days1. There is no doubt that the bladders are capable of digesting small animals, algae, etc., and, although no enzyme has yet been recognised, the presence of benzoic acid has been demonstrated2. Owing to the small size of the bladders, it must obviously be difficult to obtain an adequate quantity of the secretions for investiga- tion. FIG. 63. Uiricularia minor, L. Part of a shallow- water plant, E= earth-shoot. Two branches marked S at the base of the inflor- escence axis have been cut off. i and 2. = bracts on the inflorescence axis. (Reduced.) [Modified from Gluck, H. (1906), Wasser- und Sumpf- gewachse, Bd. n, PI. II, Fig. 18.] FIG. 64. Utricularia minor, L. a, green leaf of normal submerged shoot ; 6,colour- less leaf of an earth-shoot. In the latter the leaf seg- ments are reduced to rudi- ments indicated by S. (En- larged.) [After Gluck, H. (1906), Wasser- und Sumpf- gewachse, Bd. 11, Figs. 20 and b, p. 42.] Besides the normal leafy branches, which serve for assimila- tion and also bear bladders, no less than three modified types of vegetative shoot are borne by certain of the European Utricularias — the 'earth-shoot,' the breathing shoot or 'air- shoot,' and the so-called 'rhizoid3.' 1 Cohn, F. (1875). 2 Luetzelburg, P. von (1910). 3 Goebel, K. (189 1-1893) and Gluck, H. (1906). vin] < EARTH-SHOOTS ' AND < AIR-SHOOTS ' 97 In Utricularia minor > Bremii, intermedia* and ochroleuca, cer- tain shoots are formed which bear bladders on leaves of a reduced type (E in Fig. 63, and b in Fig. 64). These branches, which are known as 'earth-shoots,' penetrate the mud at the bottom of the water and apparently serve for purposes of an- chorage, and for the absorption of raw food materials. They have retained their power of entrapping small animals, but have substituted the functions characteristic of roots for the assimi- latory activities of the water-shoots. The bladders make such efficient hold-fasts that, unless the soil be very soft, it is difficult to pull the earth-shoots out of the substratum without snapping the leaves and leaving the bladders behind. Every transition can be observed between earth- and water- shoots. The British species of Utricularia which produce 'earth- shoots ' never show the second form of modification, the ' air- shoot ' (L in Fig. 65, p. 98), which occurs only in U. vulgaris and in the closely allied U. neglecta. These curious organs were ob- served by Pringsheim2, who did not, however, understand their nature, but called them 'Ranken' (tendrils). It is toGoebel3 that we owe a very plausible suggestion as to their biological value, and to Gluck4 a definite view as to their morphological status. They are, apparently, reduced inflorescences, and their function is said to be to serve as breathing organs and to connect the submerged vegetative body of the plant with the atmo- spheric air. In the case of Utricularia vulgaris , the air-shoots are fine, whitish, thread-like bodies, some centimetres long. They bear very small undivided leaves, closely appressed to the shoot and with stomates on their outer surfaces. The lower internodes are much elongated. The tips reach the water surface and pro- trude from it into the air, where the stomates can perform their usual function. The 'air-shoots' are said to occur especially 1 Benjamin, L. (1848) described U. intermedia as 'rooted,' so it is evident that he had observed the ' earth-shoots,' though mistaking their morphological nature. 2 Pringsheim, N. (1869). 3 Goebel, K. (1891-1893). 4 Gluck, H. (1906). 98 UTRICULARIA [CH. when the plants are growing in a thick tangle — that is to say under circumstances in which the oxygen starvation, to which submerged plants are liable, must be particularly acute. For the third type of modified shoot, the misleading term 'rhizoid' has been used; this name would have been more fitly FIG. 65. Utricularia vulgaris, L. Part of shoot with bladder-bearing leaves, and an air-shoot (L). (Enlarged.) [Goebel, K. (1891-1893).] applied to the 'earth-shoots,' since in function they approxi- mate to roots, and to the 'rhizoid shoots' of Ceralophyllum'*-. The 'rhizoids' are developed at the base of the inflorescence in certain species of Utricularia (R in Fig. 66). They bear no 1 See pp. 88 and 89. w w vni] « RHIZOIDS ' OF BLADDERWORTS 99 bladders, but their leaves are highly glandular and often bent in a claw-like fashion1. They are firmer than the ordinary shoots and do not collapse when lifted from the water. Their function is obscure, but it seems possible that they play some part in holding the inflorescence erect. The Utricularias evidently have a strong tendency towards the production of specialised shoots below the aerial part of the flowering axis. Certain extra- Europearrwnembers of the genus (U. stellaris, U. inflexa and U. inflata^ Fig. 150, p. 229) have a wreath of air-containing organs surrounding the base of the inflorescence, and un- doubtedly serving to keep it erect in the water2. A vivid de- scription is given by Spruce3, in his account of his travels on the Amazons, of a similar arrangement in U. quinqueradiata. This is a small species with the usual submersed, finely divided leaves bearing numerous bladders, but the flower-stalk, which is about two inches high, has, midway, a large involucre of five horizontal rays resembling the spokes of a wheel. This floats on the surface and keeps the stalk always erect, and the solitary flower well out of the water, " the whole recalling a floating night- lamp, especially as the large yellow flower may be considered to represent the flame." Reproduction by seed appears to be less important among 1Goebel, K. (18892). 2 Benjamin, L. (1848), Treviranus, L. C. (1848!) and Wight, R. (1849). 3 Spruce, R. (1908). 7—2 FIG. 66. Utricularia neglecta, Lehm. Base of inflorescence axis, /, with two 'rhizoids/ R. Three water-shoots, W, cut away for simplicity. (Slightly re- duced.) [After Glflck, H. (1906), Wasser- und Sumpfgewachse, Bd. n, PL IV, Fig- 34 «•] ioo UTRICULARIA [CH. the Utricularias than the method of asexual propagation shortly to be described. In the case of Utricularia minor, for instance, ripe seeds are seldom obtained. When they occur, they are found to be well suited to floating on water, as the surface of the seed-coat is pitted and capable of retaining air bubbles for a considerable time1. Eventually the testa becomes thoroughly wetted and the seed sinks. The seedling is unique in structure (Fig. 67). In U. vulgaris, which may serve as an example, germination begins in spring at the bottom of the FIG. 67. Utricularia vulgaris, L. Geminating seed; s, seed coat; /, primary leaves, (x about 19.) [Adapted from Kamienski, F. (1877)-] FIG. 68. Utricularia exoleta, R.Br. A and B, stages in germination; c= ? cotyledons. In A the seed-coat is removed. TGoebel, K. (1891).] water. The following organs are produced2 — a number (6-12) of simple primary leaves (/ in Fig. 67), a bladder, a conical stem apex, from which the main axis develops laterally, and an adventitious shoot (? an air-shoot). No root appears in the seedling, and there is not even any rudiment of this organ in the embryo3. In Utricularia exoteta*, a small and simple aquatic form found in Asia and tropical Australia, only two primary leaves (? cotyledons) are formed, but this is perhaps to be inter- preted as a case of reduction (Fig. 68). 1 Meister, F. (1900). 2 Warming, E. (1874) and Kamienski, F. (1877). 3 Merz, M. (1897). 4 Goebel, K. (1891). VHI] TURIONS OF BLADDERWORTS 101 The Bladderworts are able to reproduce themselves success- fully for long periods without having recourse to flowering and fruiting. Utricularia intermedia, for instance, was observed in a certain district in Germany to propagate itself for years by vegetative means, when the ditches in which it lived were cleared too frequently to give it an opportunity of flowering1. The organs of vegetative reproduction — the turions or winter-buds 2 — are spherical or egg-shaped bodies developed at the ends of the shoots. The case of Utricularia vulgaris may be taken as typical. In this species turion formation takes place, in normal circumstances, between the beginning of August and the middle of November. The apical region of the shoot produces a numbeP^f reduced leaves separated by highly abbreviated internodes. The concave leaves cover one another in imbricate fashion and are closely packed into a firm ball, clothed with a protective layer of mucilage. When the plant is grown in an aquarium, water-snails are its chief enemies, but the winter-buds, with their coat of hairs and slime, seem immune from the depre- dations of these creatures3. The parent plant sinks to the bottom in the autumn, owing to its tissues becoming water-logged, and carries the turions with it. These, in spite of their firm texture, are lighter than water, and, but for their attachment to the decaying axis, would rise to the surface like pieces of cork. As it is, they remain all through the winter stationary at the bottom, but with their apices directed upwards. In the spring, the turion is at last able to rise to the surface — the parent axis having been reduced by months of rotting to little more than a string-like vascular cylinder, which often adheres persistently to the base of the winter-bud. The axis of the turion elongates with remarkable rapidity, attaining three to six times its ori- ginal length. The composition of the bud then becomes mani- fest; a number of bud-scales occur at the base, followed by several transition leaves and then normal foliage leaves which iSchultz, F. (1873). 2 Benjamin, L. (1848), Glttck, H. (1906), etc. 3 Meister, F. (1900). UTRICULARIA [CH. 102 receive additions by the apical growth of the germinating turion. The bud-scales resemble reduced foliage leaves, but are specially suited to be protective organs. They are firmer than the other leaves and do not collapse on removal from the water. They are also less subdivided, and bear a more conspicuous development of hairs on their terminal segments — the hairs of the successive leaves amounting, indeed, to a protective felt — so that altogether they form an effective envelope for the bud. In Utricularia minor, though the hairs are absent, a similar result is obtained by the leathery texture of the bud-scale and by its form, which is less divided than that of U. vulgaris. The contrast between the foliage leaf and bud-scale of U. minor is shown in Fig. 69 a and b. In U. intermedia the turion generally becomes free before the winter, and swims among the shore plants instead of spending the dead season at the bottom of the water. The fact that the turion is protected by an especially thick coat of hairs, probably permits it to lead this more exposed existence1. Figs. 143 A and 143 5, p. 220, show the bud-scale and normal leaf of this species. Though, under normal conditions, the turions are only formed in the autumn, and carry the plant over the winter season, their formation can be induced at any period of the year by condi- tions of poor nutrition. In certain experiments made a few years ago2, some turions of Utricularia minor, germinated under starvation conditions on sand, after seventeen days had pro- duced plants 14 cms. long. These were transferred to a culture solution, and after five days, when they had had time to become FIG. 69. Utricularia minor, L. a, normal leaf of the shallow water form, with a bladder; 6, leaf belonging to a turion. (Enlarged.) [After Gliick, H. (1906), Wasser-und Sumpfge- wachse, Bd. n, Figs. 14 a and 6, p. "7-1 Schenck, H. (1885). Luetzelburg, P. von (1910). vin] MORPHOLOGY OF BLADDERWORTS 103 vigorous, they were returned to the sand. By the end of twenty-seven days they had formed turions. These were cut off, and the same alternation of sand culture and nutritive solutions was repeated three times. Each time the effect of the starvation culture was to induce the formation of turions, so that the plant went through the entire vegetative cycle, culminating in ' winter ' buds, no less than four times between May and the middle of December! The last turions produced were only the size of a pin's head. In the preceding pages we have, for convenience, used the terms 'shoot' and 'leaf for descriptive purposes, but it now remains fe^onsider how far current morphological conceptions can be applied to so anomalous a genus as Utricularia. There has probably been more controversy about the morphological nature of the different organs of these plants, than about such problems in the case of any other Angiosperm. It is not pro- posed here to enter into the details of the discussion1 which seems to have been singularly fruitless. In the upshot, the main point, which emerges from a study of the literature, is that in this genus the distinction habitually drawn by botanists be- tween stem and leaf, breaks down completely. The bladder is probably best interpreted as a modification of part of the " leaf2," but even if this be conceded it does not carry us far, since the nature of the " leaf " itself still stands in dispute. By some authors, the entire vegetative body, apart from the in- florescence axis, has been regarded as a root system, while others view it either as wholly axial or as consisting of stem and leaves. A view which has received considerable promi- nence, is that the entire plant is a much divided leaf3, but if this be so, it must, as Goebel has pointed out, be admitted that this " leaf " possesses many characters which we are accustomed to 1 For an historical survey of the literature, see Goebel, K. (1891) and Gliick, H. (1906). 2 Meierhofer, H. (1902). Another interpretation is illustrated in Fig. 72 By p. 1 06. 3 Kamieriski, F. (1877). io4 UTRICULARIA [CH. attribute to stems alone, viz. long continued apical growth1, as well as power of bearing leaves and axillary branches and of developing in more than one plane2. The fact that adventitious shoots are produced on the leaves of other Lentibulariaceae is, however, favourable to this view3. The unique plasticity of the Utricularias is indicated by the many observations on regenera- tion phenomena in the genus, which show that almost any part A" FIG. 70. Utricularia vulgaris, L. Detached leaf with four adventitious shoots, A, Alt Az, A3. (Enlarged.) [Goebel, K. (1904).] of these plants can produce new shoots at will. For instance, in U. neglecta^ detached leaves, or leaves connected with a dying axis, can produce adventitious shoots which arise endogenously at the points of forking of the leaves, or, more rarely, from the stalks of the bladders4. Fig. 70 represents a case in which four 1 Hovelacque, M. (1888). 2 Goebel, K. (1891). 3 Goebel, K. (1904). * G1(ick> H< (l()06). vm] REGENERATION IN BLADDERWORTS 105 shoots (A) A 1 , A<± , y/3) arose from a leaf of U. vulgaris. Again, the inflorescences of various species, if cut off and immersed in a culture solution, have been seen to give rise to lateral shoots from the axils of their scale leaves. These branches may occur in extraordinary abundance: in Utricularia vulgaris as many as FIG. 71. Utricularia vulgaris, L. Inflorescence with numerous lateral shoots arising in axils of scale leaves on inflorescence axis, after 47 days culture under water on peat, and, later, with the addition of a culture solution. (Enlarged.) [Luetzelburg, P. von (1910).] nineteen lateral shoots have been observed to develop in con- nexion with one scale1; Fig. 71 shows a large number of branches growing from a submerged inflorescence of this species. As illustrations of the numerous abnormalities on 1 Luetzelburg, P. von (1910). io6 UTRICULARIA [CH. record, it may be noted that an inflorescence-bract sometimes develops into a water-leaf or even an entire water-shoot, while a bladder rudiment may develop into a water-shoot1. In the development of the seedling, the primary leaves may be re- placed by stolons2. The apical development of the Bladderworts gives little help in interpreting their morphology. In Utricularia vulgaris (Fig. 72), for example, the apex of the shoot is coiled up in a singular FIG. 72. Utricularia vulgaris, L. A, spirally coiled end of a shoot, of which a is the apex ; si-s5 , young shoots ; /', youngest leaves ; I, older leaves (between / and /' some leaves have been removed) ; h, hairs (mucilage glands) ; i, young inflorescence growing from the base of s5. B, developing bladder; a, curved apex of shoot; SL first shoot, and /, single leaf or two leaves fused; a, sx and / fuse to form bladder ; s2 is second shoot which may give rise to a branch or a secondary bladder. [Adapted from Pringsheim, N. (1869).] way which recalls a young fern frond. The " leaves " (/) arise in two lateral rows, and there is a third row of rudiments (^— J5) on the concave face, which give rise to air-shoots. The develop- ing bladders on a leaf are indicated in Fig. 73, while Fig. 72 B illustrates that view of the composite origin of the single bladder which regards it as derived from both axial and foliar elements3. In general, the only safe conclusion to be drawn from a study 1 Gluck, H. (1906). 2 Goebel, K. (1891). 3 Pringsheim, N. (1869). -m.g. FIG. 73. Utricularia vul- garis, L. Developing leaf vm] ANATOMY OF BLADDERWORTS 107 of the available evidence regarding the nature of the organs in the Bladderworts, seems to be that — in the present state of our ignorance — the attempt to fit so elusive a genus into the Procrustean bed of rigid morphology, is doomed to failure. It is probably best, as a purely provisional hypothesis, to accept the view that the vegetative body of the Utricularias par- takes of both stem nature and leaf nature. How such a condition can have arisen, historically, from an ancestor possessing well-defmed stem and leaf organs, remains one of the unsolved mysteries of phylo- geny. The anatomy1 of the water Utricu- larias, though Showing SOme CUrioUS showing two young blad- c . dersOj and b,; m.g., muci- features, is less anomalous than their lage gland. (Enlarged.) morphology. In the stem of U. vu/garis, [Meierhofer> H- ('9°2)-3 the tracheids, of which one or more are present, are placed sub-centrally, and surrounded by little groups of phloem. Some degree of dorsiventrality is given to the structure by the thin-walled character of the small lower sector of the vascular cylinder in which the tracheids lie, while the con- junctive tissue of the rest of the stele, towards the upper side of the axis, is fibrous. The tracheal elements are of the nature of "imperfect vessels," being formed from a file of superposed cells, with imperforate, oblique, separation walls. The incompleteness of the conducting elements is probably to be associated with the relative unimportance of the transpira- tion stream in a rootless submerged plant. The vascular cylinder is surrounded by an endodermis, and the cortex is lacunar. The structure of the inflorescence-axis differs very markedly from that of the submerged stem; the tracheids form a discontinuous ring enclosing a large central pith containing phloem islands. 1 Tieghem, P. van (1868) and (1869!), Russow,E. (1875), Schenck,H. (1886) and Hovelacque, M. (i! io8 UTRICULARIA [CH. The submerged and aerial parts of the axis differ, in fact, so conspicuously in their internal structure that van Tieghem1 suggested that, if they were submitted separately to an anato- mist, he would probably attribute them to distinct and un- related plants! The leaf of Utricularia minor is typically that of a submerged plant (Fig. 74) 2. The bundle is extremely small, consisting generally of a single annular tracheid surrounded by thin- walled, elongated elements. The air spaces in the mesophyll reach to the epidermis, which contains the greater part of the chlorophyll, and is the most conspicuous region of the leaf. FIG. 74. Utricularia minor, L. T.S. lower part of leaf. (xi75.) [Schenck, H. (1886).] It seems thoroughly in keeping with the uncannily abnormal morphology and the exceptional carnivorous habits of the Utricularias, that they should sometimes locate themselves in odd situations. The oft-quoted case of those Bladderworts which live in association with certain South American Bromeliads, is an instance in point. The leaf rosettes of some Tillandsias form vase-like cavities, which collect and retain water. Utricularia nelumbifolia has been described by a traveller in Brazil3 as only to be found growing in the water which collects in the bottom of the leaves of a large Tillandsia occurring on an arid, rocky part of the Organ Mountains, at about 5000 feet above the sea. 1 Tieghem, P. van (1868). 2 Schenck, H. (1886). 3 Gardner, G. (1846). vm] EPIPHYTIC BLADDERWORTS 109 Such a habitat would be impossible for the Bladderwort without the help of the Bromeliad's store of water, while the rich fauna of this water gives it every chance of catching suitable prey 1. In the observer's own words, the Utricularia "propagates itself by runners, which it throws out from the base of the flower stem; this runner is always found directing itself towards the nearest Tittandsia, when it inserts its point into the water, and gives origin to a new plant, which in its turn sends out another shoot; in this manner I have seen not less than six plants united." In British Guiana a similar case has been described2. A huge aloe-like Bromeliaceous plant, Brocchinia cordylinoides, Baker, grows inthe Kaieteur savannah. It may be fourteen feet high, and, in older specimens at least, the crown of leaves is supported on a tall bare stem. Floating in the water retained in the axils of the leaves, is found a beautiful Utricularia (U. Humboldtii^ Schombk.) "with flower stems 3 or 4 feet long, supporting its many splendidly large violet flowers." This form of epiphytism is not obligatory, since in Roraima, although both the Bromeliad and the Utricularia occur, the Utricularia may live a terrestrial life on marshy ground, instead of being associated with the Bromeliad. Many of the unusual characteristics of the Utricularias are shared by another flowering plant, extremely remote from them in its affinities — Aldrovandia vesiculosa, L., a member of the Droseraceae. This plant has long had a peculiar fascination for botanists, and a detailed memoir upon it by an Italian writer appeared before the middle of the eighteenth century3. Like the Bladderworts, Aldrovandia is rootless and free-floating, and, but for its flowers, lives entirely submerged. It has a slender axis bearing whorls of leaves; the older internodes and leaf whorls die away successively, as new parts are formed at the apex. 1 Goebel, K. (1891-1893). 2 Im Thurn, E. F. and Oliver, D. (1887). 3 Monti, G. (1747). For an analysis of this paper see Auge de Lassu (1861). no ALDROVANDIA [CH. Aldrovandia, like the Bladderworts, is able to form turions; these are the size of a pea and consist of a highly abbreviated axis, which may bear as many as thirty-two leaf whorls. The turions normally sink to the bottom of the water in the autumn, owing to the weight of starch which they contain1, and rise again in the spring; but it seems that they sometimes fail to reach the surface in the succeeding season, and that the develop- ing plant may even in June be found at the bottom, held there by the remains of the winter-bud2. When the turions are kept in an aquarium indoors, it is said that they sometimes fail to sink, but remain floating throughout the winter3. In warmer climates these winter-buds are not formed; in Bengal, for in- stance, the plant is described as vegetating continuously through- out the year4. Reproduction by seed also takes place. The flowers are raised above the water, but the young fruits bend down, and the ripening of the seeds takes place beneath the surface5. The structure of the embryo recalls the other Drosera- ceae, the only difference being that the primary root remains rudimentary. The leaves of Aldrovandia are highly peculiar in structure, and serve, like the bladders of Utricularia^ for catching small animals6. The broad petiole terminates in a roughly circular bilobed lamina, and also bears, in its apical region, a number of stiff projections, which at first glance suggest leaflets 7, but are probably only petiolar emergences8 (Fig. 75). Long sensi- tive hairs are produced from the upper surface of the lamina in the neighbourhood of the midrib ; the touching of these by any passing animal results in the closure of the lobes 9, thus im- 1 Caspary, R. (1859 and 1862). 2 Maisonneuve, D. de (1859). 3 Schoenefeld, W. de (1860). 4 Roxburgh, W. (1832). 5 Caspary, R. (1859 and 1862). 6 The proof that Aldrovandia is carnivorous is due to Cohn, F. (i 875 j, though Delpino, F. (1871) had previously shown that the suffocation of small animals occurs in the leaves. i Cohn, F. (1850). 8 Caspary, R. (1859 and 1862). 9 Mori, A. (i 876) noted that the central region of the leaf was irritable. VIH] CARNIVOROUS HABIT 1 1 1 prisoning the prey. The sensitiveness of the leaves is greatest at rather high temperatures1. The Linnean name, " vesiculosa" is an unfortunate one, since it suggests that the leaves form actual bladders, whereas the lobes merely fold together like those of Dionaea. Besides the irritable hairs, glands2 are also present, which apparently secrete a digestive fluid and absorb organic matter3. There is good reason to suppose that both Aldrovandia and the water Utricularias are descended from terrestrial ancestors FIG. 75. Aldrovandia vesiculosa, L. i, whorl of leaves (about £ nat. size) ; 2 and 3, individual leaves (x -z\ circa}. Leaf is shown in natural position in 2, and with the lobes open in 3. [Adapted from Caspary, R. (1859).] which were already carnivorous. Aldrovandia is the only aquatic member of the Droseraceae, a family which contains well-known insectivorous types such as the Sundew, while the aquatic Utricularias are associated both with terrestrial car- nivorous members of the same genus, and with the insect- catching Pinguiculas, which are not hydrophytes. The habit of consuming animal food has thus not arisen de novo in connexion with an aquatic existence, though this mode of life undoubtedly affords unique opportunities to a carnivorous plant4. 1 Stein, B. (1874). 2 Fenner, C. A. (1904). 3 Darwin, C. (1875). 4 On Aldrovandia, in addition to the papers cited in this chapter, see Caspary, R. (1858*) and Hausleutner (1850!) and (1851). 112 ] CHAPTER IX THE LIFE-HISTORY OF THE TRISTICHACEAE AND PODOSTEMACEAE1 A^L the families of aquatics hitherto considered are represented in our own country; some of them, e.g. the Potamogetonaceae, show a marked preference for temperate regions, while others, e.g. the Lemnaceae, seem equally at home in both the hotter and colder parts of the world. The Tristi- chaceae and Podostemaceae, however, whose life-history we propose to touch upon in the present chapter, are, with rare exceptions, confined to the tropics. That they are essentially plants of hot regions, is indicated by the statement of Dr Willis 2 that the forms living in the low-country of Ceylon and S. India inhabit water which maintains a very constant temperature of 80° F. (27° C.). The two families together form an anomalous group, characterised, as regards their morphology, by remark- able variety, but agreeing, as regards their ecology, in one singular feature — a preference for inhabiting water which flows rapidly or even torrentially over a rocky substratum. This peculiarity, sometimes rendered more noticeable by reason of the striking colour of the plants, has been observed from the earliest time at which Podostemads became known to botanists. The first recognition of a member of this group as the type of a 1 General accounts of these plants will be found in Gardner, G. (i 847), Tulasne, L. R. (1852) and Warming, E. (1881, 1882, 1888 and 1891). They have only recently been divided into these two families (Willis, J. C. 19 1 51), and many authors still refer to them all as Podo- stemaceae. 2 Willis, J. C. (1902). This interesting memoir has been largely drawn upon in the present chapter; it contains a bibliography of previous work. See also Willis, J. C. (I9H1), (iqiS1) and (1915*) and Matthiesen, F. (1908). CH. ix] HABITAT OF THE PODOSTEMADS 113 distinct family, occurred when Aublet1, nearly a century and a half ago, discovered Mourera in rapidly running water in French Guiana. In the case of a certain Venezuelan river, Goebel2 describes the bed, in places where the water flows quickly, as quite green with a Podostemad, Marathrum utile, growing on the stones, and he points out that it flourishes more freely the stronger the current ; when the stream is slow it is replaced by Mosses and Algae. Another writer3 observed Mourera fluvia- tilis in the cataracts of a tributary of the Amazon, growing in such abundance that the rocks, amongst which the waters rushed, were veiled by it, and the colour was so vivid that the river seemed — to use his own expression — "to flow over a carpet of roses. >Si*rhis red hue of the vegetative organs, due to antho- cyanin in the surface cells4, has been noted in many cases. Miss Lister5, for instance, in her account of the occurrence of a species of Tristicha in rapidly flowing water below the first cataract of the Nile — one of the rare records of the appearance of a member of these families outside the tropics — mentions that, when the plant was wet and fresh, the colour was crimson. The majority of the peculiarities of the Podostemaceae and Tristichaceae are closely related to the nature of their habitat. Life in rushing water — on rocks which are often water-worn to smoothness and into which no roots can penetrate — is obviously impossible except to plants which have a special capacity for clinging to the substratum. In the Tristichaceae4 (e.g. Tristicha ramosissima and Weddellina squamulosd) a creeping, thread-like organ is formed, which, though morphologically a root, is dorsiventral in structure, and gives rise to leafy shoots endogenously in acropetal succession. But this thread-like root is not apparently competent to anchor the leafy shoots with the necessary firmness, and additional organs called ' haptera ' are formed. They are produced exogenously from the creeping root, and by their positive geotropism and power of flattening them- 1 Aublet, F. (1775). 2 Goebel, K. (1891-1893). 3 Weddell, H. A. (1872). 4 Willis, J. C. (1902). 5 Lister, G. (1903). n4 PODOSTEMACEAE [CH. selves against the substratum, form firm attachment organs. They also secrete a kind of cement which renders their adhesion to the rock very close and permanent. These haptera are found in many Podostemaceae. In Mourera fluviatilis^ for instance, they are sometimes almost tendril-like1, while in certain cases they serve as storage organs for reserve carbo- hydrates2. In many of the Podostemaceae the creeping root discards its root characteristics even more completely than in the Tristi- chaceae, and becomes converted into a thallus, which either follows out every irregularity in the substratum, or, remaining more or less free, develops into all sorts of curious shapes3. It still produces secondary shoots bearing leaves, but as the root thallus becomes more important, the secondary shoots become less so, until, in such genera as Hydrobryum (Fig. 76), Farmeria, Dicraea (Fig. 77 and Fig. 79, p. 1 1 6), and Griffithiella they are much reduced, and assimilation is mainly performed by the thallus. A seedling of Dicraea stylosa^ with the young thallus (//£.) developed as a lateral outgrowth from the hypocotyl (hyp.\ and bearing secondary shoots (s.s.) is shown in Fig. 78 ; the mature plant is represented in Fig. 79, p. 1 16. The thallus of the Podostemads is sometimes amazingly polymorphic; its capacity for developing in exceptional forms depends, apparently, on the fact that it is not restricted by a rigid skeletal system, and that nearly all the cells possess the capacity for renewed meristematic activity. Griffithiella Hooker- ianay for instance, has a thallus which may develop into various shapes recalling different Algae that grow in moving water; one of its forms resembles the basal cup of Himanthalia lorea. Farmeria metzgerioides^ again, recalls Delesseria Leprieurii, while Podostemon subulatus simulates such an Alga as Eostrychia Moritziana, which also grows in rapids. Willis, who draws attention to these cases of simulation, alludes to the great difficulty of interpreting such resemblances between plants far 1 Went, F. A. F. C. (1910). 2 Matthiesen, F. (1908). 3 See Willis, J. C. (1902) for further details. IX] HYDROBRYUM AND DICRAEA FIG. 76. Hydrobryum olivaceum, (Gardn.) Tul. Thallus bearing endogenous flower- ing shoots. (Enlarged.) [Warming, E. (1883*).] cot. ..th. hyp FIG. 78. Dicraea stylosa, Wight, f. fucoides, Willis. Seedling with hypocotyl (hyp.), cotyledons (cot.), thallus (th.), and secondary shoots (s.s.). [Adapted from Willis, J. C. (1902).] FIG. 77. Dicraea elongata, (Gardn.) Tul. Plant with three vertical roots bearing flowers. These float in the water: they spring from a horizontal creeping root. (Nat, size.) [Warming, E. (i8832).] 8—2 1 1 6 PODOSTEMACEAE [CH. distant in relationship from one another, and adds, "it is impossible at present to do more than point out these very suggestive analogies of form which accompany analogy of the FIG. 79. Dicraea stylosa, Wight. Plant somewhat reduced, showing the shoots (g, g) arising from the band-like root thallus. [Warming, E. (i8832).] conditions of life, and which seem to indicate that an experi- mental and comparative morphological study of the forms of the Algae and Podostemaceae should be attended with inter- ix] THALLUS OF PODOSTEMADS 1 1 7 esting results1." Even the Tristichaceae, which do not possess these polymorphic thalli, show "remarkable similarities in morphological features, and in the arrangement and anatomy of the leaves, to many mosses or liverworts, especially to those of wet situations1." The specific and varietal names given to various members of these families — such as bryoides, jucoides, selaginoides and lichenoides — speak eloquently of their striking resemblance to the lower plants, which the botanists who named them have felt impelled to emphasize1. The genus Lawia differs from those hitherto mentioned in having a thallus which is not of root nature, but which origi- nates by the fusion of flattened, dorsiventral shoots, while Castelnavia also has a shoot thallus. In Lawia foliosa^ the small thallus adheres so closely to the stones that it cannot be separated from them. There are no haptera, but the thallus is attached by hairs. The small simple leaves are without sto- mates or vascular bundles. They have a midrib of elongated cells, but their structure is altogether more simple than that of the leaves of many Liverworts. In Lawia zeylanica1 the hypo- cotyl, produced on the germination of the seeds, bends down to the rock and becomes attached to it by unicellular rhizoids from the superficial cells. The hypocotyl then expands and forms a relatively large surface of attachment. The internal structure of the Podostemads is similar to that of many other submerged plants in reduction of xylem, absence of stomates, and the presence of chlorophyll in the epidermis. On the other hand, a character in which these plants diverge from other hydrophytes is the presence of large quantities of silica in the cells3. It seems on the whole most probable that this silica is merely a useless by-product of the plant's meta- bolism4. It has been suggested that it serves as a protection against the attacks of animals5, but there seems little evidence 1 Willis,J. C. (1902). 2 Goebel, K. (iSSg3) and (1891-1893). In Goebel's earlier account this plant is called Terniola (longipesf). 3 Goebel, K. (1891-1893). * Matthiesen, F. (1908). 5 Wachter, W. (1897*). 1 1 8 PODOSTEM ACEAE [CH . for this view. That these plants, with their large stores of reserve starch, are, as a matter of fact, liable to be preyed upon, is indicated by Im Thurn's1 observation that, in British Guiana, when the rivers are low, and the rocks which underlie the rapids are partially uncovered, a certain fish (Pacu myletes] collects at the falls to feed on the leaves of the Podostemads, which clothe the rocks, and at this time of year come into flower. This fact is so well known that, at this season, large numbers of Indians camp on the sides of the falls, in order to seize the opportunity of shooting the fish. The most important anatomical peculiarity of the Podostemads is the extreme reduction of the inter- cellular spaces2; in this respect the members of these families contrast most markedly with other water plants (Fig. 80). This feature is probably to be associated with the thorough aeration of the tor- rential water which they frequent 3. Certain species, however, possess FlG 8o Dicraea styhsa> Wight> f. delicate outgrowths from the sur- fucoides, \vniis. x.s. thaiiustoshow face of the leaves which have been absence of intercellular spaces. ep.= epidermis ; p.c. = parenchymatous interpreted as "gill-tufts" (Fig- cortex^-6-=vascularbundle-(xl5o o N r> -ui L circa^ [Willis- J- c- (1902).] 81). Possibly these structures to some extent compensate for the lack of an internal aerating system. The water in which the Podostemaceae live is liable to variations in level, and their habit of blooming when the sinking UmThurn, E. F. (1883). 3 See pp. 256, 257. 2 Warming, E. (1881). ix] " GILL-TUFTS " OF OENONE 119 of the water exposes them to entire or partial desiccation, has been repeatedly noted by travellers. Barrington Brown]1,' in describing his explorations up the Cuyuni River in British FIG. 81. Oenone mvltibranchiata, Matthiesen. Part of flowering plant showing the numerous "gill-tufts" on the upper surfaces of the leaves. [Matthiesen, F. (1908).] Guiana, mentions the occurrence of Podostemaceae on the rocks under water where the current runs strongest, and adds, 1 Brown, C. Barrington (1876). 120 PODOSTEMACEAE [CH. "These plants bear very pretty flowers at this season of the year [September] as soon as they are left uncovered by the subsiding of the waters after the rainy season, but still kept moist by the wash of the water's edge. One small-leaved species has a little white star-shaped flower, on a short delicate stem, which has a slight perfume and proves an attraction to numerous species of wild bees." Im Thurn1, again, in his account of the same regions, mentions Mourera fluviatilis and Lads alata as growing "on the half-submerged rocks in most of the falls. As the water decreases in the dry season, the tall spikes of bright pink flowers of the former plant rise from their large leaves, the edges of which are cut and curled into the like- ness of moss, which lie flat on the rocks ; and at the same time and place innumerable tiny pink stars rise an inch or two over the equally moss-like leaves of the Lads." The vegetative parts of the Podostemads die very quickly when out of their element, and the flowering and seed-setting, both of which take place with the utmost rapidity when the plants are exposed to the air, represent, as it were, their swan- song. In Lawia zeylanica, Willis2 has observed that the enor- mous amount of starch stored up in the flowering shoots accounts for the great rapidity with which anthesis and seeding take place. In the case vfRhyncolads macrocarpa, Goebel3 points out that each inflorescence-bud is enclosed in a cavity formed by the connate union of two leaves. These cavities are full of water, so that the life of the flower-stalks is passed in an environ- ment resembling that of ordinary aquatics inhabiting still water; it is thus not surprising that these stalks differ from the other vegetative organs in developing an aerating system, such as is characteristic of water plants in general. Both entomophily and anemophily occur among the Podo- stemads. According to Willis, we can trace a series from certain American Tristichaceae with conspicuous, entomophilous 1 Im Thurn, E. F. (1883). 2 Willis, J. C. (1902). 3 Goebel, K. (1891-1893). IX] FLOWERS AND SEEDS g-Hf- spa.. -,per. 121 flowers, to members of the Podostemaceae in which anemo- phily or autogamy is associated with gradually increasing dorsi- ventrality. To this subject we shall return in Chapter xxvu. Cleistogamous flowers are also sometimes produced (Fig. 82). The peduncles of the Podostemaceae contain little water-conducting tissue, and, possibly in correlation with this, the seed- development proves to be of a decidedly xerophilous type1 — an illustration of the conservatism of the reproductive organs of aquatics and their tendency to retain terres- trial characters. By disappearance of nucellar tissue, a cavity is formed beneath the embryo- sac which, at the time of fertilisation, is filled with fluid. This cavity is bounded by the strongly cuticularised inner wall of the inner integument and the suberised cells of the chalaza. It is open only on the side towards the developing embryo, and is described as "an ideal water reservoir." Mucilage is often present in the neighbouring cells of the inner integument and this may perhaps form an additional protection against loss of water. The seeds of the Podostemads are often small and numerous. Those of Rhyncolacis macrocarpa are about as large as the largest known pollen-grains (e.g. those ofMirabilis). The seeds of this species often germinate when caught in some cranny of the parent, so that the old plant may support a number of seedlings. The embryo is strictly rootless, but haptera grow out from the hypocotyl2. The morphology of the Tristichaceae and Podostemaceae positively bristles with problems for the botanist, but great caution has to be exercised in dealing with them, since it must 1 Magnus, W. and Werner, E. (1913). 2Goebel, K. (1891-1893). FIG. 82. Podostemon Barberi, Willis. Clei- stogamic flower in spathe (spa.), the front of which is re- moved to show the gynaeceum (g). the single stamen (st.) and one of the two thread- like organs represent- ing the perianth or staminodes (per.). [Simplified from Willis, J. C. (1902).] 122 PODOSTEMACEAE [CH. ix not be overlooked that the data are still highly incomplete, for, as a recent writer has pointed out, we probably know only a small proportion of the existing species belonging to these families1. It was recorded a decade ago, for instance, that the examination of a few kilometres of a river in Venezuela — hitherto unexplored in this respect — yielded no less than four species of Podostemaceae new to science2. The extremely local distribution of many forms, their anomalous morphology and progressive dorsiventrality, and the great variety of types of structure which they present, offer every incentive to specula- tion. Dr Willis has put forward certain far-reaching theoretical views, based on his study of the group, and to these and related questions we shall return in Chapter xxvn, when we are touching upon the problem of Natural Selection. 1 Went, F. A. F. C. (1910). 2 Matthiesen, F. (1908). CHAPTER X THE LIFE-HISTORY OF THE MARINE ANGIOSPERMS THE small group of Phanerogams inhabiting the sea consists of about thirty species1 belonging to two fami- lies of that Cohort of Monocotyledons known as Helobieae. The Hydrocharitaceae are represented by Halophila, Enhalus and Thalassia, and the Potamogetonaceae by Zostera, Phyllo- spadix, ^osidonia, Cymodocea and Halodule (Diplanthera). The thorough way in which the marine Helobieae have identified themselves with their environment, is shown by the fact that Cymodocea antarctica was actually included by Agardh2 in his Species Algarum under the name of "AmphiboKs zoster ae- folia"; injustice to this author it should, however, be mentioned that he lays stress upon the uncertainty of its position "in catena entium." Zostera marina, the Grass- wrack, often grows among Seaweeds as if it were one of themselves ; in lagoons of the Mediterranean coast it has been observed in association with Enteromorpha, Codium tomentosum, Padina pavonia, Dictyota dichotoma, and other Algae3, while in Danish waters it grows in the midst of varied assemblages of brown, red and green Seaweeds4. Zostera is even able to descend to considerable depths in the sea; in the Baltic its occurrence at 1 1 metres from the surface has been recorded3. A species of Phyllospadix (a genus allied to Zostera} is noted for its power of withstanding the violence of the waves; it grows on the Californian coast "in the heaviest surf and on the most exposed ocean shores5." Ascherson, whose work has done much to elucidate this difficult group, pointed out about fifty years ago6 that the 1 Sauvageau, C. (iSgi1). 2 Agardh, C. A. (1821). 3 Flahault, C. in Kirchner, O von, Loew, E. and Schroter, C. (1908, etc.). 4 Ostenfeld, C. H. (1908). 5 Dudley, W. R. (1894). 6 Ascherson, P. (1867). I24 MARINE ANGIOSPERMS [CH. Phanerogams inhabiting the sea were, at that time, less well known than most of the higher groups of Algae. These marine Angiosperms often grow in deep water, and botanists have been obliged to depend chiefly on the study of casual fragments washed up by the waves, and thus have been apt to miss the organs of fructification altogether. The marine Helobieae all show a strong affinity, both as regards vegetative habit and reproductive methods. They all have alternating leaves in two ranks arising from creeping stems. Supple, ribbon-like leaves, sessile, sheathing and capable of following all the undulations of the water, are most charac- teristic, occurring in Enhalus, Posidonia, Phyllospadix, Z.QS- tera, etc. Several Halophilas, on the other hand, have broad petiolate leaves with Potamo- geton-like nervation, while Cymodocea isoetifolia is dis- tinguished by awl-shaped succulent leaves1. Submerged pollination and Conferva-like pollen are characteristic of all the marine Angiosperms. The thread-like pollen was figured as early as 1792 by the Italian FIG. 83. Cymodocea aequorea, Kon. Plant in the middle of the third year of vegeta- tion ; /= fruit from which plant has grown. (Nat. size.) [Bornet, E. (1864).] 1 Ascherson, P. (1867) and Sauvageau, C. and x] CYMODOCEA 125 botanist, Cavolini1, who described it in the case of Posidonia as "lanae instar gossipinae." The cases which he records are those of "Zostera oceanica"(= Posidonia Caulini, Kon.), "Phucagros- tis major " (= Cymodocea aequorea^ Kon.), and " Phucagrostis minor " (= Zostera nana, Roth). As a typical life-history of one of the marine Potamogeton- aceae, that of Cymodocea aequorea, Kon. may be briefly outlined. This plant was made the subject of a classic memoir by Bornet2, FIG. 84. Cymodocea aequorea, Kon. T.S. leaf near base of limb showing median bundle; t, t, sieve tubes. (x22o.) [Sauvageau, C. from which the following account is derived. Cymodocea aequo- rea (Fig. 83) is an herbaceous plant with a creeping stem, which forms submarine meadows after the manner of Zostera marina. It occurs in a number of localities in the Mediterranean, growing on muddy sand, in shallow creeks which are not greatly ex- posed to the shock of the waves. It is a perennial plant, which is in full vegetation from May to October; during the other months it is difficult to detect, for only a few short narrow leaves 1 Cavolini, F. (i 792*) and (i 7922). 2 Bornet, E. (i 864). 126 MARINE ANGIOSPERMS [CH. remain, with their green colour masked by a layer of various animal and vegetable growths. Probably the plant does not attain its full development until the fifth or sixth year, and an individual may live for another six years or more after reaching maturity. The rhizomes are fixed in the soil by long, whitish, fibrous roots, which put out a great quantity of tortuous laterals. The roots form a network, which holds in its meshes the gravel and mud, and thus contributes towards maintaining the stability of the bed of the creek in which the plant grows. The leaves, which are linear and membranous, attain the length of 20 to 30 cms. Fig. 84, p. 125, shows the appearance of a trans- verse section of the leaf near the base of the limb. At the junction of the sheath with the blade there is a ligule which Bornet compares with that of the Grasses. At the extreme base of each young leaf, ten ' squamulae intravaginales ' occur, and the same structures are associated with the stamens and carpels. The male and female flowers of Cymodocea aequorea, which are borne on separate plants, and are buried 2 or 3 cms. deep in the soil of the sea-shore, mature about the end of May or the beginning of June. Only the stamens and styles emerge into the water. The flowers are solitary, and are borne without any perianth in the axils of ordinary foliage leaves. The male flower consists of a pedicel bearing two stamens, completely fused as to their filaments. The double nature of the stamen is revealed in the single large anther of a vivid red hue, which has eight pollen sacs, and is supplied by two vascular strands. The female flowers are only manifested externally by white, filamentous styles, which emerge in groups of four from the sheaths of certain leaves. Two of these styles correspond to each of the two carpels which constitute the gynaeceum. The ovary is unilocular with one ovule. Until the disappearance of the pollen-mother- cell, the pollen grains are roundish, but at this stage they elon- gate, without increasing in diameter, until they attain the dimensions of about 2 mm. by yi^- mm., thus becoming thread- like. The fruits, which are ripe by August, are flat and oval being roughly i cm. long by 0-5 cm. wide. The endocarp is x] CYMODOCE A AND ZOSTERA 1 2 7 filled by the embryo with its enlarged hypocotyl, enclosed in a brown membrane. As the fruits develop, mature, and become detached, while still buried in the soil, there is no chance of their becoming disseminated, unless tempests or other accidental causes stir up the sea bottom ; this explains the rarity of their occurrence among shore debris. Bornet several times found branches bearing two or three generations of fruits. An Australian species of the same genus, Cymodoceaantarctica, Endl., exhibits an interesting variant on C. aequorea in the matter of the fruit1. The plant is annual, or at most biennial, and the germination is viviparous. When the seedling attains a length of 3 to 4 inches, it breaks away from the parent, but carries wftfc it a cup-like body (? the remains of the ovary wall) which has been described as bearing " two unsymmetrical pairs of basket-like spines." The " cup," on account of its relative density, " retains the floating waif in an upright position, and soon proves its ultimate use by acting as a grappling apparatus, catching in the tangles of small algae etc." The young plants develop spirally twisted roots, which presumably also serve for anchorage2. In Zostera marina, L., the Grass-wrack of our shores, the fertile and sterile plants are readily distinguishable from one another, since in the fertile plant the stem is slender, erect, and much branched, while that of the sterile individual is thick, creeping, more luxuriantly leafy, and anchored to the soil by adventitious roots developed in bundles beneath each leaf base3. Figs. 85 and 86, p. 128, illustrate the leaf anatomy. The inflo- rescence, unlike that of Cymodocea, consists of a number of male and female flowers, reduced to stamens and carpels and enclosed in a spathe. A French observer4 has given a vivid description of a successful attempt to observe the actual pollination. Having found a good locality for the purpose, in the month of June, 1872, in his own words, "j'allai m'installer avec mon micro- 1 Tapper, J. G. O. (1882) and Osborn, T. G. B. (1914). 2 See p. 205. 3 Gronland, J. (1851). 4 Clavaud, A. (1878). 128 MARINE ANGIOSPERMS FIG. 85. Zoster a marina, L. T.S. leaf at base of limb between the median nerve and a lateral nerve, phloem indicated by shading. [Sauvageau, C. (1891').] FIG. 86. Zostera marina, L. T.S. median bundle of leaf; t, sieve tubes, (x 220.) [Sauvageau, C. x] ZOSTERA AND HALOPHILA 129 scope dans la maison cTun ami, a quelques centaines de metres de la plante, et je re'solus de n'en point partir que je n'eusse de*- couvert, si c'etait possible, le mode de reproduction de Zos- tera — II m'importait de ne pas rester plus longtemps dans une incertitude qui commencait a me peser." On a favourable day, hot and absolutely still, he went out in a boat and examined some flowering plants. The three conditions in which the in- florescences were found proved that cross pollination is ensured by protogyny. Some were still enclosed in the spathes, with the anthers intact ; others showed stigmatic branches, ready for pollination or recently pollinated, emerging from the spathe, while the stamens were still enclosed and not completely ripe; in others>*Rgain the stigmatic lobes had all fallen, while the anthers were exposed, and either all empty, or the lower ones empty and the upper ones in the act of dehiscence. The anthers were seen to open, and eject the thread-like pollen which formed a floating cloud. In pollen-grains, which had just been expelled from the anther, an outgrowth was observed at a little distance from one end. When pollinated stigmas were examined, it was noticed that these outgrowths, which were, in fact, young pollen- tubes, were forcing their way into the stylar tissue, between the cells whose walls were becoming mucilaginous and separating from one another. The pollination of Zostera is scarcely possible except in still water, as any movement would carry the pollen completely away from the scene of operations. The best-known genus among the marine Hydrocharitaceae is Halophila, three species having been investigated in detail by Bayley Balfour1 and Holm2. Bayley Balfour himself col- lected his material of H. ovalis, (R. Br.) Hook. fil. (H. ovata, Gaudich.) and of H. stipulacea, (Forsk.) Asch. on the reefs surrounding the island of Rodriguez — east of Mauritius. H. ova/is (Fig. 87) grows on spots just uncovered at full ebb tide, while H. stipulacea prefers localities where it is always submerged and subjected to a constant current. The rhizomes are creeping, and produce numerous long filiform rootlets 1 Balfour, I. B. (1879). 2 Holm, T. (1885). 1 3o MARINE ANGIOSPERMS [CH. bearing a thick matting of root hairs ; this tangle of roots fixes the plant in the sand. The flowers are typically hydrophilous. The filiform styles, which may be 26 mm. long, are receptive throughout their entire length, and, though the individual pollen- grains are not thread-like, the same result is secured by their being united into strings1. The seed-coats form an admirable protection for the embryo. The outermost cell-layer is conspi- cuously thickened on all the walls except that forming the sur- face of the testa. The next three cell-layers are cuticularised. FIG. 87. Halophila ovalis, (R.Br.) Hook. fil. Portion of mature plant showing two female flowers in spathes with three thread-like stigmas (st.) . (Enlarged.) [Balfour, I. B. (1879).] Since the testa of Zostera is similar in structure, it seems not unlikely that in both cases the histological features bear some relation to the mode of life. Bayley Balfour concludes, from the general result of his researches, that Halophila forms a link between the Hydrocharitaceae and Potamogetonaceae. The leaf anatomy of the marine Helobieae has been studied in great detail, partly because these plants are nearly always 1 The thread-like character of the pollen of Halophila was observed by Gaudichaud, C. (1826) who also noticed the same feature in Cymodocea antarctica. x] LEAF ANATOMY 1 3 1 collected in a sterile and often fragmentary condition, and it has thus become a matter of importance to systematists to be able to identify them even when no organs of fructification are present. It might have been expected that the examination of the leaves of these plants, which show great similarity in external form and all live completely submerged in a fairly uniform environment, would reveal a monotony of internal structure. But this expectation is far from being realised. Duchartre1 showed in 1872 that the genera Cymodocea (Fig. 84, p. 125) and Zostera (Figs. 85 and 86, p. 128) could be distinguished from one another, even in the absence of the flowers and fruit, on anatomical grounds alone. This conclusion was carried much further bySauvageau2, who proved, as a result of detailed and critical studies of the anatomy of the marine Phanerogams, that (except among the Halophilas) the anatomy of the leaf gives sufficient data for their exact generic and even specific deter- mination. The variation occurring in the leaf structure is illus- trated in Figs. 84, p. 125, 85 and 86, p. 128, 88 and 89, p. 132. Sauvageau pointed out, for instance, that the development of the lignified fibres differs markedly in the three genera, Enhalus, Thalassia and Halophila, and that it is thus impossible to regard this mechanical system merely as an adaptive response to the milieu. The differences that are displayed by the different species afford, indeed, another example of the fixity and lack of utility so often observed in specific differences ; for it is not conceivable that each of the detailed distinctions between the closely related types of anatomy met with in the leaves of these marine Angiosperms, is to be interpreted as having some definite 'survival value,' though it may be broadly true that some structural variations are more suited to life in a boisterous sea and others to existence in calmer waters. But though we cannot explain the different types of skeletal system of the leaves on adaptive grounds, there are other leaf- characters which seem definitely related to submerged life. In 1 Duchartre, P. (1872). 2 Sauvageau, C. (1890*), (iSgo2), (iSgo3) and (iSgi1). 9—2 I32 MARINE ANGIOSPERMS [CH. FIG. 88. Halodule uninervis, Boiss. T.S. leaf at base of limb; a, a, secretory cells. (X220.) [Sauvageau, C. (iSgi1).] FIG. 89. Posidonia Caulini, Kon. T.S. limb of leaf. A, i cm. from apex; B, at base of limb, (x 145.) [Sauvageau, C. (iSgi1).] x] THE ORIGIN OF THE GROUP 133 the marine Potamogetonaceae, the epidermis is characteristi- cally free from stomates and very rich in chlorophyll. Liquid exchange between the plant and the surrounding medium is facilitated by the occurrence of openings at the leaf apices, with which the median nerve is in direct communication. These openings come into existence quite early in the history of the leaf, and are due to the disappearance of the epidermis. In the genus Posidonia, again, the fibres of the sheath survive and form a protective covering for the younger leaves. Another small peculiarity, which may be adaptive or may more probably be an indication of community of origin — since it is common to certain genera in the two families under consideration, but is not found elsewhere — is the occurrence of " Flossenzahne " or " dents nageoires " on the leaf margins1. These teeth are formed by a peculiar elongation and wall-thickening of the marginal cells. The fact that a considerable number of Phanerogams live and flourish in the sea, and that yet, on examination, these marine types all prove to be restricted to representatives of two related families, stimulates conjecture as to the origin of this biological group. Both the families to which the flowering plants of the sea belong are typically aquatic, and are widely represented in fresh waters; no marine Angiosperm has a close affinity with any terrestrial plant. These facts suggest that the flowering plants now living in the sea are not the immediate descendants of land plants, but have been derived from ances- tors which had already accommodated themselves to life in inland waters. It would seem that, in order to be capable of em- barking upon life in the sea, a flowering plant requires four special faculties. These are, firstly, toleration towards a saline medium; secondly, the power of vegetating while wholly submerged; thirdly, the knack of developing a sufficiency of anchoring roots to withstand the wash of waves and tide; and, fourthly, the capacity for hydrophilous pollination, since any aerial method must be doomed to failure, except in halcyon weather in a non- 1 Ascherson, P. and Graebner, P. (1907). i34 MARINE ANGIOSPERMS [CH. tidal sea. Both the families to which the marine Angiosperms belong, fulfil these four conditions in the persons of some, at least, of their fresh-water representatives. The existence of such species as Potamogetonpectinatus and Zannichelliapalustris^}\\ch inhabit both fresh and brackish waters, and also of Ruppia and Althenia (Potamogetonaceae), which typically occur in a brack- ish medium — as well as of Fallisneria spiralis1 (Hydrocharita- ceae) and Callitriche autumnalis2 (Callitrichaceae), which are able to tolerate some salt — indicates how the transition from fresh to saline water may have been bridged. The vegetative organs, again, are entirely submerged in such genera as Elodea and Vallisneria among the Hydrocharitaceae, and Zannichellia,, Naias and many Pondweeds among the Potamogetonaceae. Both families also contain a number of species with well- developed root systems. Finally, floating pollen is carried by water to the stigmas in some species of Elodea and Ruppta, while actual submerged pollination is found in Naias and Zannichellia. These families are thus in every respect prepared, as it were, for the evolution of marine members. The reason why other families have not produced any forms adapted to life in the sea, seems to be that, though certain of their species may fulfil some of the conditions which we have enumerated, they fail in others — the one which is most rarely exhibited being a tendency to sub-aquatic pollination. Myriophyllum spicatum and Ranunculus Baudotii*, for example, have been observed to live under conditions of slight salinity, but they are handicapped for entry on marine life by the fact that they cannot be cross- pollinated, unless the flowers are raised into the air. Cerato- phyllum and Pseudo-callitriche, on the other hand, owing to their hydrophilous pollination, suggest themselves as possible candidates for marine life, but Ceratophyllum lacks roots en- tirely, and Pseudo-callitriche has no rhizome — obstacles that may well prove insuperable. Conceivably in future ages, if the 1 Chatin, A. (18552). 2 Lebelj 3 Ostenfeld, C. H. (1908). x] THE FUTURE OF THE GROUP 1 35 evolution of fresh-water plants proceeds on its present lines, a greater number may reach the specialised stage of hydrophilous pollination, and some of these may colonise the sea, thus demo- cratising the narrow and exclusive circle of the Marine Angio- sperms1. 1 In addition to the papers cited in this chapter the following references may be mentioned: Ascherson, P. (1870) and (1875); Chrysler, M. A. (1907); Cunning- ton, H. M. (1912); Delpino, F. (1870); Delpino, F. and Ascherson, P. (1871); Engler, A. (1879); Hofmeister, W. (1852); Magnus, P. (18702) and (1872); Martens, G. von (1824); Sauvageau, C. (i8893)and(i89i3); Solereder, H. (1913); Walsingham, Lord,and Payne-Gallwey,R.(i886); Warming^. (1871). PART II THE VEGETATIVE AND REPRODUCTIVE ORGANS OF WATER PLANTS, CONSIDERED GENERALLY "If then the Anatomy of Vegetables be so useful a Mean, we ought not to streighten it ; but to force this, as well as the rest, to its utmost Extent. And therefore, first of all, To go through all the Parts, with equal care ; examining the Root, Trunk, Branch, Leaf, Flower, Fruit, and Seed. . . . Together with the Knife it will be necessary to joyn the Microscope; and to examine all the Parts, and every Way, in the use of That. As also, that both Immediate, and Micro- scopical Inspections, be Compared: since it is certain, That some things, may be demonstrated by Reason and the Eye conjunct, without a Glass, which cannot be discovered by it." Nehemiah Grew, The Anatomy of Plants, 1682. [ 139 ] CHAPTER XI LEAF TYPES AND HETEROPHYLLY IN AQUATICS (i) TYPES OF LEAF IN WATER PLANTS THE types of leaf characteristic of aquatics — excluding those that rise wholly into the air and are thus comparable with the leaves of terrestrial plants — fall into two groups: firstly, those which float on the water surface, and thus preserve con- tact on tiie*Ventral side with the atmosphere and on the dorsal side with the water, and secondly, those which have more com- pletely adopted the water life, since they keep up no direct contact with the atmosphere, but live entirely submerged. The general question of the relation of floating leaves to their en- vironment has been discussed, in connexion with the Water- lilies, on pp. 30-32. There is considerable monotony in the out- line and structure of a large proportion of such leaves, associated no doubt with the very definite and uniform physical condi- tions to which they are subject. Submerged leaves, on the other hand, are characterised by much greater variety. With a number of exceptions, they fall mainly into two groups — those that present a very thin, entire lamina, generally ribbon-shaped but sometimes broad, and those in which the leaf blade is finely subdivided, either by fenestration or dissection. In both these types of leaf, the ratio of surface to volume is higher than is the case in a normal, terrestrial lamina, and many botanists regard their peculiarities as definite adaptations for obtaining from the water an adequate supply of gases in solution. It is generally assumed that the dissected type of leaf is the more efficient form for the purpose. A Russian writer1 has recently proved, however, that this assumption is scarcely borne out by a critical examination of the facts. By measurements and calculations 1 Uspenskij, E. E. (1913). 140 TYPES OF LEAF IN WATER PLANTS [CH. he shows that a cylindrical leaf, in order to have as high a ratio of surface to volume as, for example, the broad, flat leaf of Pota- mogeton -perfoliatus, must be only 120^ in diameter, whereas the diameter of the segments of Myriophyllum spicatum leaves varies from 22o/t to 38o/z, and of Ceratophyllum demersum, from 6oo/x to 75o/x, while even the ultimate divisions of the leaves of Ra- nunculus trichophyllus reach 1 90/^1. He admits that, apart from the actual ratio of surface to volume, the dissected leaf may possibly have an advantage over the corresponding flat leaf, in tapping a greater volume of the medium1 ; he thinks, however, that though this factor would be of importance in absolutely still water, its significance is much reduced if, as is nearly always the case, movement has to be taken into account. It may be added that the dissected leaf possibly interferes less with its neighbours' light than the undivided type of submerged leaf. From this enquiry and from a general study of submerged leaves, it may perhaps be concluded that both the dissected and flat types of leaf are organs of tolerably equal efficiency for subaqueous gaseous exchange, though the dissected leaf has the advantage of offering less resistance to currents. Which type of leaf a sub- merged plant shall produce is probably ultimately decided by the general leaf morphology of its terrestrial ancestors, rather than by environmental causes, much as coast scenery is often determined by the forms of the pre-existing land surfaces, rather than by the direct action of the ocean itself. _'. Among the undissected types of submerged leaf, the ribbon leaf is conspicuous (Fig. 90) ; it is probably better adapted to resist tearing than, for instance, the large, U/va-like submerged laminae of the Waterlilies. Ribbon leaves are found among many of the marine Angiosperms, such as Zostera, which are subjected to the wash of waves and tide. Leaves of this type sometimes grow to a notable length ; those of Sagittaria sagitti- folia, as we have shown in Chapter n, may be more than two yards long, while those o>i V allisneria spiralis are said to be often a yard or more in length, though hardly a quarter of an inch wide. iSchenck, H. (1885). xi] SUBMERGED LEAVES 141 Other types of submerged, radical leaf are the small, almost cylindrical leaves of Lobelia Dortmanna and Littorella lacustris (Fig. 142, p. 218), and the linear serrate leaves of Stratiotes aloides (Fig. 32, p. 53), which are too firm and stiff to be called ribbon leaves. In Lobelia and Littorella^ the shortness of the leaves obviates the necessity of pliability to the motion of the water, while in Stratiotes the need for flexibility is diminished by the partially free-swimming habit of the plant and its pen- chant for quiet waters. FIG. 90. Sagittaria sagittifolia, L. Young plant produced from a tuber (T) and bearing ribbon leaves only; tuber, with axis and scale leaves, and roots, indicated in solid black. Drift at bank of Cam, May 31, 1911. (Nat. size.) [A. A.] When the leaves, instead of being radical, are borne on a pliable, elongated stem, the function of flexibility seems to be taken over by the axis and the leaves are generally small and simple, as in the case of Elodea canadensis. In Hippuris vulgaris, however, the whorled, submerged leaves may reach a consider- able length. The finely divided type of submerged leaf takes two different forms, according to whether the species to which it belongs is Dicotyledonous or Monocotyledonous. There are numerous examples of dissected, submerged leaves among the Dicoty- i42 TYPES OF LEAF IN WATER PLANTS [CH. ledons, the most familiar case being that of various Batrachian Ranunculi. Among Monocotyledons the submerged leaves are nearly always entire; the character- istic venation of this group does not lend itself readily to the formation of a dissected leaf. As Henslow1 has pointed out, dissection among Dicotyledons is represented, in the very few equivalent cases among Monocotyledons, by fenestration, which produces a similar result. He adds the ingenious, but probably untenable, suggestion that the fene- stration of the aerial leaves of Tor- nelia, Monstera, etc., is a character handed down to them from aquatic ancestors. Among the Aponogetons we meet with a slight and irregular perforation of the leaves in A. Bernerianus, (Decne.) Hook, fil.2, while in A. (Quvirandrd) fenes trails the mature leaves are completely reticulate (Fig. 91). According to Mlle Sergueeff3, who has made a detailed study of the subject, the young leaves are imperforate, the perforations arising at a later stage by destruction of the tissues. When the perforations are formed, a fauna ,rn r _ „ ' .r FIG. 9 1. Perforated leaf of Apono- and flora or b lagellates, Rotifers, geton fenestrdiis, Hook. f.=o«v«- Bacteria and Algae accumulate in ran.d™ Sf™traiis Poir. Lace- o . plant. [Sergueeff, M. (1907).] 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). 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, Art!)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.and Engler, A. (1906) 3 Hansgirg, A. (1903). 4 See Section (3) of this Chapter. 5 Arber, A. (igig3) has been largely incorporated in Sections (2) and (3) of this Chapter. i44 HETEROPHYLLY [CH. Lyte's Herball(i$7%) contains a vivid description of hetero- phylly in the Water Buttercup — a free translation of that given in Dodoens' Histoire des Plantes of 1557. 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- A ri/w, L.1, it is impossible to say at an early stage whether a leaf B FIG. 92. Ranunculus Purschii, Rich. A, water leaf (| nat. size) and B, leaf of the land form (reduced). [Goebel, K. (1891-1893)-] rudiment will produce the floating or submerged form. 1 Askenasy, E. (1870). See also Rossmann, J. (1854). Up 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 whicfr**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, €.(1887), West, G. (1910), Gliick,H. (1911); references will be found in West, G. (i 9 1 o) 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.] H6 HETEROPHYLLY [CH. been recorded in R. sce/eratus1, 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 Ufoa-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 vutgaris 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 (<:). 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 Gliick, 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 i48 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 (b) transition leaves and (a) water leaves. An axillary shoot (ax) bore water leaves. (J 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 d\n Fig. 97). Some small plants were found bearing the submerged type of leaf alone. 1 50 HETEROPH YLLY [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- pin nate air leaves were intermediate. Oenanthe Phellandrium, Lamk. vnr.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. (i 844). FIG. 98. Sium lalifolium, 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 Beckii5 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. 1 01, p. 1 53). After between two and three months, it had developed into the typical water plant shown in Fig. 102, p. 153. 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). » 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 A . 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, 1911, and submerged in shallow water until August 9, 1911 (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. 1 54, illustrates this experiment. i54 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 1Goebel, 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 Batrachos-permum. The exclusion 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 Costantin3, 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. InAlismaPlantago (Fig. 101 ^and 5, 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. (1897!). xi] WATER LEAVES AND POOR NUTRITION 157 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 Boscii (Hydrocharitaceae) that, in this case also, the heterophylly is not a direct adaptation to land or water life, but that^die 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 sagittijolia 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 Potamogetons5, 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 Montes^ntos, 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; blt £>., and 63 are the surviving land leaves; the internodes i1 and i2 show some elongation as a result of the changed conditions; 64 and fc5 are floating leaves; bG-b10 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 internodes 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 leaf of Nymphaea lutea was about eleven times the dry weight of a submerged leaf of the same area. Another Dicotyledon, Proserpinaca pa/ustris, which was in- 1 Wachter, W. (18972). 2 Arcangeli, G. (1890). 3 Brand, F. (1894). 160 HETEROPHYLLY [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 palustris2" 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 sho'ots, 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). 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. 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 vutgaris, 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 palisl?Se 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, £. (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, '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 Crispins, 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 Callitriche*. In some cases, e.g. Zostera, 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. (18551), 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 Bellis 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 sin5Tnerged plants receive. f/ The statement, frequently made, that stomates are absent from submerged leaves, and from the lower surface of floating FIG. 106. Elodea canadensis, Michx. T.S. leaf; t, 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 iStohr, 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). 4 Goebel, K. (1891-1893). 5 Mer, £. (iSSo1) and (I8821). 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 epidermisj 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, Fillarsia ovata2 and Pontederia cordata2^ 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. Callitriche verna and Hippuris vulgaris. (2) The aperture may be permanently closed, as in the case of Potamogeton natans (Fig. 107 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. (I8551). 4 Goebel, K. (1891-1893). 5 Porsch, O. (1903) and (1905). 6 Porsch uses the specific name "Oenanthe aquatilts, L."; he is pro- bably referring to Oe. Phellandrium, Lamk. var. fluviatilis, Colem. L. T.S. submerged stomate from leaf stalk of floating leaf. This stomate is entirely roofed in with cuticle. [Porsch, O. (1905).] 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- stris (Fig. 107 A}. In addition to ordinary stomates, which, in sub- \.c . ,. FIG. 107. A. Calla palustris, L. T.S. stomate merged lite, are incapable in submerged leaf stalk; the thickening bands Of exercising their normal fit closely together. B. Potamogeton natans, function, submerged leaves also very commonly bear water stomates, which are probably of importance in keeping up the 'transpiration' stream by exudation1. A longitudinal section parsing 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 by porous diaphragms, and the same feature is markedly developed in the elongated petioles of such leaves as Sagit- taria (Fig. 8, p. 19) These diaphragms form points d'appui for the secondary &eau- c- (l89x1)-] 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). c.S. FIG. 1 08. Potamogeton densus, L. L.S. apex of leaf passing through the median nerve and showing the apical opening . ( Upper surface of leaf to right hand.) (x 220.) [Sauva- i68 SUBMERGED LEAVES [CH. FIG. 109. Myriophyllum spicatum, L. T.S. through a segment of the leaf of the water form . The epidermis contains chloroplasts and the mesophyll is laden with large starch grains, only indicated ,7.) TSchenck, H. spongy parenchyma. In many cases the assimilatory activity seems, in great measure, confined to the epidermis, the meso- phyll serving rather for storage purposes. Myriophyllum1 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 tissues. Littorella, Utricularia minor (Fig. 74, p. 108) and Ceratophyllum2i\\ show the same approximately radial type of in a few cells- 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 nijolium, 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 verticil- latum and M. spicatum show a peculiarity which has repeatedly 1 Schenck, 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, Callitriche, Elodea and Alisma may be mentioned. In Fig. 1 1 1, 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 alreaTty been made. Callitriche autumna/is2, 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, £. (1900). 2 Hegelmaier, F. (i 864). 3 Solereder, H. (i 9 1 3). 170 SUBMERGED LEAVES [CH. FIG. no. Trichomes ol Myriophyllum 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, £. (190°)-] , 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 epictermis 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 Schenclc, 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. Potamogetotiy Fig. 37, p. 60 and Myriophyllum, Fig. 144, p. 22 1). 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. 1 12). 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 LIGNIFI CATION 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 the internodes, and may survive only at the nodes, e.g. Potamogeton /ucens1, Zannichellia palustris1^ Althenia filiformis2, etc., while in the case of Elodea 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 tftea that such a current is absent in these plants, seems often to have been accepted on totally inadequate grounds. In FIG. 112. Hippuris 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 Caspary, R. (18582). Prillieux, E. (1864). i74 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 lignified 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 Callitrichf\ the Hydrilleae, Aldrovandia^ Naias, Hippuris2-^ 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. 1 13, p. 176), in which the bundles remain p«jcfectly 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 Peplis Portula 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 5, 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. 114-6) 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). aSchenck, H. (1886). 176 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 tnchophyllus, 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.] FIG. 114. Callitriche stagnalis, Scop. Central cylinder of stem. A, land form, (x 475.) 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). XIH] 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*cT>nsidered 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 5) 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. 1 1 5 5), 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 178 AQUATIC STEMS [CH. 'pith' are actually xylem, 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. lA' X)-n O O L+L' L FIG. 115. Hippuris vulgaris, L. Series of transverse sections of stele of a stem near apex to show relation of cauline and leaf trace xylem ; the dotted line in each case represents the periphery of the stele. ( x 280 circa.) A , appearance of first cauline 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 cauline 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 cauline elements can still be counted. D, a lower level at which traces (L') from a second node have entered. Fusion of traces from the two nodes or of cauline elements with either is indicated by (L + L1), (C + L), etc. [A. A.] Myriophyllum (Figs. 1 16 and 117) closely resembles Hippuris in vascular anatomy and has the same cauline tracheal elements in the pith, but the xylem is more reduced1. 1 Vochting, H. (1872). xm] 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 [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). i8o 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 Gunnera* (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 1875 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; it thus adopted the alternative of branching, and reproducing its structural peculiarities in each branch. (Russow, E. 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. Tfctf 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 Barbe*4 — 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 Barb6, C. (1887). 1 82 AQUATIC STEMS [CH. xin 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 also 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 irnjjspecial 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 natans, in which 1 Haberlandt, G. (1914). 2 Schenck, H. (1886). 8 Duval-Jouve, J. (1872), Blanc, M. le (19 12) and Snow, L. M. (1914). 1 84 AERATING SYSTEM [CH. the cortical lamellae are connected by a diaphragm (D) with small intercellular spaces (m) at the angles of the cells. Fig. 119, m. 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.] A) B, 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. 120. 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/is For tula* 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^ (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. Lacr 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). xiv] 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 verna1, the air spaces in the root cortex may be increased by the replace- ment of small schizogenous air spaces by large cavities of a partialty*4ysigenous 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 JSchenck, 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 ofEpibtium hirsutum, Lycopus europaeusy 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 weeks"1. The list of our native waterside plants, in which aerenchyma occurs under suitable conditions, includes Lysimachia, Lotus, 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 Jussiaea2; 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 ^e»ter the mud (m.r.\ and in certain erect roots which seem to serve entirely for aeration (a.r.). Fig. 122 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 Bonpland4, for instance, more than a hundred years ago, recorded that in "'Mi- mosa lacustris" (Neptunia oleracea^ Lour.), the Floating Sensitive Plant (Fig. 12 3, p. 191), 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). z 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. 1 9o AERATING SYSTEM [CH. have made it clear that the spongy mass (/ 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 Sesbania1, 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.'Lour. 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). 1 92 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 \ feet long and about 4 inches in diameter, is described as weighing less than i J 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 mirabi/is, Kotschy. 2Kotschy, T. (1858), Hallier, E. (1859), Jaensch, T. (1884!) and (i8842), Klebahn, H. (1891). See also Hope, C. W. (1902). 3 Goebel, K. (i 89 i-i 893). 4 Schenck, H. (1889). 5 Costantin, J. (i 884). ' 6 Martins, C. (i 866). xiv] NESAEA 193 as floating organs. For this particular case, this view can scarcely be maintained, since Goebel1 has shown that Jussiaea re-pens 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). i94 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. 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 whicrfftormally 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 QiHydrocharis^^ 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. variant, 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. Myriophyllumy Callitriche* and the Batrachian Ranunculi (Fig. 126, p. 196) agree in producing land forms which are close-growing and tufted. When Myriophyllum spicatum^, for 1 Schenck, H. (1885). 2 Mer, £. (iSSi1). 3 Bachmann, H. (1896), and Mer, £. (1882*). See also p. 32, Ch. in. 4 See Chapter n and Gliick, H. (1905). 5 Fryer, A. (1887). 6 Uspenskij, E. E. (1913). 7 Fryer, A., Bennett, A., and Evans, A. H. (1898-1915). 8Lebel, E. (1863). 13—2 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. iA, seedling which germinated in water, and which is shown in iB at a somewhat older stage. zA, seedling which germinated on land, and which is shown in zB 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 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. 152. It is notable that in this case 1 Prankerd, T. 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 135, p.2O3), Ca/thapa/ustris(}<\g. 129), Cnicuspratensis*(Fig. 1 30 1 Massart, J. (1910). 2 Schenck, H. (1885). H. (191 1 j. 4 Gliick uses the name Cirsium anglicum, D.C. FIG. 128. Littorella lacustris, L. 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.] xv] WATER FORMS OF LAND PLANTS 199 A and 5) and Menyanthes trijoliata, but also by typically terres- trial plants such as Achillea ptarmicay Trifolium resupinatum (Fig. 131 B) and Cuscuta alba (Fig. 131 A}. Gliick1 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 Gliick, 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 Gliick, H. (1911), Wasser- und Sumpfgewachse, Bd. in, Figs, i a and b, 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 arvensis 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 repens2 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, Ficia 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 fruticosuSy 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 Sa/ix, also, little anatomical change was induced by submergence5. 1 Compton, R. H. (1916). 2 The existence of these forms was noted by Cluck, H. (191 1). On Hydrocotyle see West, G. (1910). 3 Costantin, J. (1884). 4 Lewakoffski, N. (18732). 5 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 pratensis 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 #). In the case of C D FIG. 133. Cardamine pratensts, 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 D). 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. A » B FIG. 134. Ranunculus Flammula, L. A, form with floating leaves. B.land form. (Reduced.) [ After Gliick, H. (1911), Wasser-undSumpfgewachse, 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.] J 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 (peltata)1, 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)*, 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 4. 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,J. (1888). 2 Graebner, P., in Kirchner, O. von, Loew, E., and Schroter, C. (1908, etc.). 8 Irmisch, T. (1861). 4 Tepper, J. G. O. (1882). 5 Hochreutiner, G. (1896). io6 ROOTS OF WATER PLANTS [CH. pression, " elles grimpent en has." 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 x has described a differentiation between absorb- ing and anchoring roots in the case of Hetera nthera 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 communis2, 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 natans3, 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). 208 ROOTS OF WATER PLANTS [CH. character of the growths which spring from the stalk; for these are neither leaves nor stalk l . ' ' 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 Elodea 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. 138), 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. Fallis- 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). * Sauvageau, C. xvi] ROOT ANATOMY 209 r FIG. 138. Callitriche stagnalis, 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, s, 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 Q. [Sauvageau, C. 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 Potamo- geton. But the classic example in England of the extremely rapid growth and multiplication of a water plant, is the behaviour of Elodea canadensis2' , 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 ^chenck, H. (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 1 8491 — that 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 1847, introduced it into a tribu- tary of the Cam in 1848. By 1852 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 commums, 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 a^ 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 speriosa, 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). 2 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 fluitans, 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). xvn] LUXURIANCE AND PERENNIATION 215 branches of Utricularia vulgaris 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-ranae, Stratiotes aloides, Spirodela polyrrhiza, Lemna trisulca, Myriophyllum sp., Oenan- the Phellandrium vzr.flu (1907) Vol. 38, 1907, pp. 29-80, 4 text-figs. Parkin, J . j (This paper is partly devoted to a reconstruction of the primitive [pp. 308, 315] tvPe of Angiospennic flower. Among aquatics, the Nym- phaeaceae, Alismaceaeand Butomaceae are regarded as showing certain primitive features of flower structure.) Arcangeli, G. (1890) Sulle foglie delle piante acquatiche e specialmente [pp. 27, 159] sopra quelle della Nymphaea e del Nuphar. Nuovo Giornale Botanico Italiano, Vol. xxn. 1890, pp. 441- 446. (A study of heterophylly in these genera.) Areschoug, F. W. C. Om Trapa natans L. och dess i Skane annu lefvande (18731) 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).] Areschoug, F. W. C. On Trapa natans L., especially the form now living (18732) in the southernmost part of Sweden. Journ. Bot. [pp. 302, 303] Vol. xi. N.S. Vol. n. 1873, pp. 239—246, i pi. [Thispaper is a translation, revisedby the author, of Areschoug, F.W.C. (1907) Graebner, P. ) [pp. 133. 291, 315] Ascherson, P. (1875) Die geographische Verbreitung der Seegraser, in [pp. 135, 302] 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.) Ascherson, P. (1883) Bemerkungen uber 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.) Ascherson, P.j Hydrocharitaceae, in Die Natiirlichen Pflanzen- and / (1889) familien, n. i (Engler, A. and Prantl, K.). Leipzig, Gtirke, M. | 1889, pp. 238-258, n text-figs. (A systematic treatment of the family.) Ascherson, P. See Delpino, F. and Ascherson, P. (1871). Askenasy, E. (1870) Ueber den Einfluss des Wachsthumsmediums auf die [pp. 144, 228 and Gestalt der Pflanzen. Bot. Zeit. Jahrg. 28, 1870, pp. Fig. 126, p. 196] 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.) Aublet, F. (1775) Histoire des plantes de la Guiane Fran£oise, T. i. [p. 113] 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.) Auge de Lassu (1861) Analyse du memoire de Gaetan Monti sur I'Aldro- [p. 109] vandia, suivie de quelques observations sur 1'irrita- bilite" desfollicules 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.) Bachmann, H. (1896) Submerse Blatter von Nymphaea alba. Landformen [pp. 32, 195] 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 localities in the dry summer of 1895.] 352 BIBLIOGRAPHY Bailey, C. (1884) Notes on the Structure, the Occurrence in Lancashire, [pp. 237, 275, 303] and the Source of Origin, of Naias graminea Delile, var. Delilei Magnus. Journ. Bot. Vol. xxn. 1884, PP- 305-333, 47 text-figs., 4 pis. [This account of an Egyptian species, which has been introduced into Lancashire, in some points supplements Magnus, P. (iSyo1). Magnus, P. (1883), Ascherson, P. (1874) andWeiss, F. E. and Murray, H. (1909) deal with the same plant.] Bailey, C. (1887) Forms and Allies of Ranunculus Flammula L. Journ. [p- J45] °f Bot- xxv- l887, pp. 135-138. (In this paper the existence of a form of Ranunculus Flammula with floating leaves is recorded.) Baillon, H. (1858) Recherches sur 1'organogenie du Callitriche et sur ses [p. 311] rapports naturels. Bull, de la Soc. hot. 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.) Balfour, I. B. (1879) On the Genus Halophila. Trans, and Proc. Bot. Soc. [p. 129 and Edinburgh, Vol. xin. 1879, pp. 290-343, 5 pis. Fig. 87, p. 130] [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.] Barbe, C. (1887) 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- 105-116, 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 1'anatomie 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) The Origin of the Endodermis in the Stem of [p. 185 and Fig. Hippuris. Ann. Bot. Vol. xxx. 1916, pp. 91-99, 120, p. 185] 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 submergees. 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 avoirl'importance cosmique qu'on lui attribue, ") Batten, L. (1918) [p. 188] BIBLIOGRAPHY 353 Observations on the Ecology of Epilobium hirsutum'. Journ. Ecology, Vol. 6, 1918, pp. 161-177, I5 text- Bauhin, G. (1596) [P- 9] Bauhin, G. (1620) [p. 9 and Fig. 3, p. n] 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- 55] Bennett, A. [A fully illustrated account of the "aerenchyma" of this species — a tissue whose existence had previously been recorded by Lewakoffski, N. (1873') and Schenck, H. (1889).] Phytopinax seu Enumeratio Plantarum...Basileae per Sebastianum Henricpetri 1596. (Bauhin describes the germinating tuber of Sagittaria as "Gramen bulbosum," p. ai.) 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 Botanici. . .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.) Hydritta 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- 23 BIBLIOGRAPHY Geologic History indicated by the Fossiliferous Deposits of the Wilcox Group (Eocene) at Meridian, Mississippi. U.S. Geol. Survey. Professional Paper 108 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 Nelttmbo.) 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 aerifdres 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 and leaves of certain aquatics are described, and they are figured in tne cases of Sagittaria sagittifolia, Pontederia cordata and Potamogeton natans.) Ueber die durchsichtigen Punkte in den Blattern. Flora, N. R. Jahrg. XLII. (G. R. Jahrg. LXVII.) 1884, pp. 49-57. 97-II2> 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 and Alisma 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 118, p. 184] Blenk, P. (1884) [P- 37] Bois, D. Bokorny, T. (1890) [p. 261] BIBLIOGRAPHY 355 Boresch, K. (1912) Die Gestalt der Blattstiele der Eichhornia crassipes [P- X54] (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.) Bornet, E. (1864) Recherches sur le Phucagrostis major Cavol. Ann. [p. 125 and Fig. 83, d. sci. nat. Ser. v. Bot. T. i. 1864, pp. 5-51, n pis. P- I24l (This finely illustrated memoir gives a singularly complete account of the structure and life-history of the plant now called Cymodocea aequorea, Kon.) Borodin, J. (1870) Ueber den Bau der Blattspitze einiger Wasser- [pp. 86, 169 and pflanzen. Bot. Zeit. Jahrg. 28, 1870, pp. 841-851, i pi. Fig. 163, p. 268] [A description of the stomates which occur in small numbers near the apices of the submerged leaves of Cattitriche 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).] Bottomley, W. B. Some Effects of Organic Growth-Promoting Sub- (1917) stances (Auximones) on the Growth of Lemna minor [p. 287] 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.) Boulger, G. S. (1900) Aquatic Plants. Journ. Roy. Hort. Soc. Vol. 25, [p. 321] 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.) Brand, F. (1894) Ueber die drei Blattarten unserer Nymphaeaceen. [pp. 27, 159] Bot. Centralbl. Bd. LVII. 1894, pp. 168-171. (A brief account of the submerged, floating and air leaves of Nymphaea lutea and Castalia alba.) Brongniart, A. (1827) Memoire sur la Generation et le DeVeloppement de [P- 3°9] 1'Embryon dans les v6ge"taux 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.) Brongniart, A. (1833) Note sur la structure du fruit des Lemna. Archives [p. 76] de Botanique, T. n. 1833, pp. 97-104. (An account of the structure of the seed and fruit in Lemna minor and L. gibba.) Brongniart, A. (1834) Nouvelles recherches sur la structure de I'fipiderme [p. 164] des Ve'ge'taux. Ann. d. sci. nat. SeY. 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 Ittcens and the existence of "une pellicule tout-a-fait incolore" (=cuticle) on the surface of the epidermal layer. In PI. Ill, Fig. 5, the characters of the epidermis are clearly demonstrated.] 23—2 BIBLIOGRAPHY Brown, C. Barrington Canoe and Camp Life in British Guiana, xi + 400 pp., (1876) 10 pis. and map. London, 1876. [p. 119] (On p. ii some Podostemaceae occurring in the Cuyuni River are described under the name of Lads spp.) Brown, R. (1814) General remarks on the Botany of Terra Australis. [p. 311] 89 pp. Reprinted in the Miscellaneous Botanical Works of Robert Brown, Vol. i. 1866. (The author includes Callitriche in the Halorageae; see p. 22.) Brown, W. H. (1911) The Plant Life of Ellis, Great, Little, and Long Lakes [p. 286] 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.) Brown, W. H. (1913) The Relation of the Substratum to the Growth of [pp. 253, 264, 265] 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 COZ supply.) Bruyant, C. (1914) Les Tourbieres du massif Mont-Dorien. Annales de [p. 291] 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.) Buchenau, F. (1857) Ueber die Bliithenentwickelung von Alisma und Butomus. Flora, N.R. Jahrg. xv. (G.R. Jahrg. XL.) l857, PP- 24I~254. * Pi- (A description of the development of the parts of the flower in Alisma Plantago and Butomus umbellatus, with a briefer mention of Sagittaria sagittifolia.) Buchenau, F. (1859) Zur Naturgeschichte der Littorella lacustris L. Flora, [pp. 217, 232] 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.) Buchenau, F. (1865) Morpliologische StudienandeutschenLentibularieen. 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.) Buchenau, F. (1866) Morphologische Bemerkungen iiber Lobelia Dort- [p. 245] 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.) Buchenau, F. (1882) Beitrage zur Kenntniss der Butomaceen, Alismaceen [p. 17] und Juncaginaceen. Bot. Jahrbiicher (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. (igo^1) Alismataceae, in Das Pflanzenreich, iv. 15 (heraus- [PP- 9, 314] gegeben von A. Engler), Leipzig, 1903, 66 pp., 19 text-figs. (The standard systematic account of this family.) Buchenau, F. (iQOS2) 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 nymphoidcs.) 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. 1 60, 161] Vol. xvin. 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.) Botanical Rambles in West Norfolk, with notes on and V (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] xxvu. 1899, pp. 37-66, 59 text-figs. (In this memoir special attention is paid to the gametophytes and fertilisation.) Cambessedes,J.(i829) Note sur les filatin^es, nouvelle famille de plantes. [p. 311] Mem. du museum d'histoire nat. T. xvin. 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] Carlo, R. (1881) Caspary, R. (1847) Caspary, R. (I8561) Caspary, R. (i8s62) [p. 214] Caspary, R. (1857) Caspary, R. Caspary, R. (i8582) [PP- 55. 56, 173. 210, 2ll] Caspary, R. (i8583) 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, 4i-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 Alsinastrum 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 Nymphe'acees fossiles. Ann. des sci. nat. Se>. 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. 39-51- (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. 377-513, 5 pis. (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, i pi. (A description of the female flower based on living material.) BIBLIOGRAPHY 359 Caspary, R. (1858*) [p. in] Caspary, R. (1859 and 1862) [pp. no, 239 and Fig. 75, p. in] Caspary, R. (1860) [P- 234] Caspary, R. (1861) [P- 276] Caspary, R. (1870*) Caspary, R. (i87o2) [p. 300] Caspary, R. (1875) [P- 54] Cavolini, F. 1 (Caulinus, P.)J ( 79 ' [P- I25] Cavolini, F. I, 7 ,* (Caulinus, P.) J(79 ' [P- 125] Sur I'Aldrovanda vesiculosa. Bull, de la Soc. bot. de France, T. v. 1858, pp. 716-726. [The observations in this paper are expanded and illustrated in Caspary, R. (1859 and 1862).] Aldrovanda vesiculosa Monti. Bot. Zeit. Jahrg. 17, *8 59* PP- 117-123, 125-132, 133-139, 141-150, 2 pis. Aldrovandia vesiculosa. Bot. Zeit. Jahrg. 20, 1862, 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.] Bulliarda aquatica D.C. Schriften d. konig. phys.-6k. 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.) Nuphar luteum L. var. rubropetalum. Schriften d. 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.) 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.) Welche Vogel verbreiten die Samen von Wasser- 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.) Die geographische Verbreitung der Geschlechter von 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.) Zosterae Oceanicae Linnei AN0H2I2- Contem- platus est Philippus Caulinus Neapolitan us. Annis 1787 et 1791, 20 pp., i pi. Neapoli, 1792. [An account of the flowering and vegetative organs of Posidonia Caulini = " Zostera oceanica." This paper and Cavolini, F. (1792*) are analysed in Delpino, F. and Ascherson,P. (1871).] Phucagrostidum Theophrasti AN0H2I2. Contem- platus est Philippus Caulinus Neapolitanus. Anno 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 = " Phucagr ostis minor.") 36° BIBLIOGRAPHY Chamberlain, C. J. See Coulter, J. M. and Chamberlain, C. J. (1904). Chatin, A. (1855 l) Note sur la presence de matiere verte dans 1'epiderme [pp. 164, 166] des feuilles de I'Hippuris vulgaris, du Peplis portula, des Jussiaea longifolia et /. lutea, de I'Isnardia 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.) Chatin, A. (i8552) Memoire sur le Vallisneria spiralis, L. 31 pp., 5 pis. [pp. 134, 235] Paris, 1855. (The morphology, anatomy and floral structure are dealt with in detail, and there is a habit drawing showing male and female plants.) Chatin, A. (1856) Chatin, A. Chatin, A. (1858*) Chrysler, M. A. (1907) [pp. 63, 65, 135 and Fig. 39, p. 62] Clarke, W. G. Clavaud, A. (1876) [P- 78] Clavaud, A. (1878) [p. 127] Anatomie 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. hot. 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.) The Structure and Relationships of the Potamo- getonaceae and allied Families. Bot. Gaz. Vol. XLIV. 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.) See Burrell, W. H. and Clarke, W. G. (1911). Sur une particularite du Lemna trisulca L. Actes de la Soc. Linn, de Bordeaux, T. xxxi. (Ser. iv. T. i.) 1876, pp. 309-311. (A note on the occurrence of raphides in this species and their possible biological significance.) Sur le veritable mode de fecondation du Zostera marina. Actes de la Soc. Linn, de Bordeaux, T. xxxii. (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 submerges sous 1'influence de la lumiere. Comptes rendus de 1'acad. des sciences, Paris, T. LVH. 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) , _ Recherches sur la vegetation. Comptes rendus de Gratiolet, P. P * > 1'academie 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] vni. (G. R. Jahrg. xxxin.) 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 hi an appendix.] Cohn, F. (1875) Ueber die Function der Blasen von Aldrovanda und [??• 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 CEnanthe. Annals [pp. 150, 204] and Mag. of Nat. Hist. Vol. xiu. 1844, pp. 188-191, i pi. (The author makes out what appears to be a good case for regarding Oenanthe fluviatHis 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. Se"r. 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.) BIBLIOGRAPHY Costantin, J. [pp. 165, 1 66] Costantin, J. (i8852) [P- 155] Costantin, J. (i8853) [P- 5i] Costantin, J. (1886) [pp. 12, 28, 30, 51, 145, 151, 155, 156] Coster, B. F. (1875) [P- 67] Coulter, J. M. and (1904) Chamberlain, C.J. [pp. 322, 325] Coulter, J.M. ] and [ (1914) Land,W.J.G.j [P- i5l Crocker, W. (1907) [P- 243] Observations critiques sur 1'epiderme des feuilles des vegetaux aquatiques. Bull, de la Soc. bot. de France, T. xxxii. (Ser. u. T. vn.) 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. xxxn. (Ser. u. T. vii.) 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.) 1885, 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.l fitudes 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 Delayed germination in seed oiAlisma Plantago. Bot. (1914) Gaz. Vol. 58, 1914, pp. 285-321, 8 text-figs. Crocker, W. and Davis, W. E. J (A detailed study of one case, Alisma Plantago, illustrating the [pp. 242, 243] 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. hot. de France, T. v. 1858, pp. 27-29. (These notes on U. minor are written without knowledge of the previous literature.) Cunnington, H. M. Anatomy of Enhalus acoroides (Linn, f.), Zoll. Trans. (1912) Linn. Soc. Lond. Ser. n. Bot. Vol. vn. Pt 16, 1912 [P- I35l (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.) Dangeard, j La Polystelie dans le genre Pinguicula. Bull, de la P. A. and /• (1887) Soc. bot. de France, T. 34, 1887, pp. 307-309. Barbe, C. ) (The authors show that the old axes of Pinguicula vulgaris [p. 181] 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.) Darwin, C. (1875) Insectivorous Plants, x + 462 pp. 30 text-figs. [PP- 93, 95, in] London, 1875. (Chapter xiv. deals with Aldrovandia and Chapters xvu. and xvin. with Utricularia.) Darwin, C. (1888) Insectivorous Plants. Second Edition revised by [p. 95] 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).] Darwin, C. (1890) Journal of Researches into the Natural History and [p. 181] Geologyof the. ..voyageof...H.M.S.' Beagle.' London, 1890. (See reference to Gunnera on p. 298.) Darwin, C. (1891) The Movements and Habits of Climbing Plants. [p. 206] 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.) Darwin, C. and F. The Power of Movement in Plants, x + 592 pp., (1880) 196 text-figs. London, 1880. [pp. 90, 161, 206] [Onp.2iitheobservationsmadebyRodieronthemovementsof Ceratophyllum are discussed. See Rodier, E. (I8771) and (l8772)-] Davie, R. C. (1913) [pp. 50, 287] Davis, W. E. Delpino, F. (1870) [p. 135] Delpino, F. (1871) [p. no] BIBLIOGRAPHY Stratiotes A loides, Linn., near Crieff . Trans, and Proc. Bot. Soc. Edinb. Vol. xxvi. 1913, pp. 180-183, i 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. xni. 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 A Idrovandia.) Delpino, F. and) , _ .Federico Delpino's Eintheilung der Pflanzen nach Ascherson, P. J 'dem Mechanismus der dichogamischen Befruchtung [pp. 84, 135, 236] 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, Ceratophyllum and Enhalus.) Desmoulins, C. (1849) Feuilles du Nymphaea et du Scirpus lacustris. Actes [p. 27] de la Soc. Linn6enne 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.) Devaux, H. (1889) Du mecanisme des echanges gazeux chez les plantes [pp. 253, 254, 256] aquatiques submergees. Ann. d. sci. nat. Ser. vn. 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.) Dodoens, R. (1578) A Nievve Herball, or Historic of Plantes:...nowe [p. 144] 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.) Dollo, L. (1912) Les Cephalopodes adaptes & la Vie Nectique Secon- [p. 39] 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 Arber, A. (1919*).] BIBLIOGRAPHY 365 Douglas, D. (1880) Notes on the Water Thyme (Anacharis alsinastrum, [P- 55] 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.) Duchartre, P. (1855) 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 Vattisneria do or do not become detached from their pedicels and float to the surface of the water.) Duchartre, P. (1858) Recherches experimentales sur la transpiration des [p. 261] 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.) Duchartre, P. (1872) Quelques observations sur les caracteres anatomiques [p. 131] des Zostera et Cymodocea, a propos d'une plante trouv6e 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. (1890') 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.) Dutailly, G. (1878) Sur la nature reelle de la " fronde " et du " cotyledon " [p. 73] 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 i la suite les uns des autres.") Dutailly, G. (1892) La fecondation chez les Ceratophyllum. Bull. mens. [p. 85] de la Soc. feinneenne 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 decouverte 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.) 366 BIBLIOGRAPHY Duval-Jouve, J. (1872) Diaphragmes vasculiferes des monocotyledones aqua- [pp. 167, 183] tiques. Academic des Sciences et Lettres de Mont- pellier. M6m. 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. 159-232, 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 [p. 314] 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. (i8722) 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 367 Beitrage zur Biologic 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- Esenbeck, E. (1914) [pp. 151, 157 and Figs. 104, p. 158, and 105, p. 159] Evans, A. H. Fauth, A. (1903) Beitrage zur Anatomic und Biologic der Friichte und [pp. 15, 18, 241, 242, Samen einiger einheimischer Wasser- und Sumpf- 246, 248, 271, 297] pflanzen. Beihefte zum Bot. Centralblatt, Bd. xiv. 1903, PP- 327-373. 3 Pis- (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.) Fenner, C. A. (1904) Beitrage zur Kenntnis der Anatomic, Entwicklungs- [p. in] geschichte und Biologic der Laubblatter und Driisen einiger Insektivoren. Flora, Bd. 93, 1904, pp. 335- 434, 1 6 pis. (One section of this paper is devoted to Aldrovandia.) Ferrero, F. See Gibelli, G. and Ferrero, F. (1891). Fischer, A. (1907) Wasserstofi- und Hydroxylionen als Keimungsreize. [p. 243] 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).] Fischer, G. (1907) Die bayerischen Potamogetonen und Zannichellien. Ber. d. Bayer. Bot. Gesellschaft, Miinchen, Bd. xi. 1907, pp. 20-162. (A detailed systematic monograph of the Bavarian Potamo- getonaceae, without illustrations.) Focke, W. O. (I8931) Erne Fettpflanze des siissen Wassers. Abhandl. '[p. 310] 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. XH. 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 17. 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] 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. Handbucher 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. Conn's Beitrage zur Biologic 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 (1873).] 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 (Fagersjd 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.) Fryer, A., Bennett, A. The Potamogetons (Pond Weeds) of the British Isles, and Evans, A. H. x + 94 pages, 60 pis., 2 text-figs. London, 1898-1915. (1898-1915) (A systematic monograph of the genus, as far as it is represen ted [pp. 58, 195, 303] in Britain, with fine coloured plates by R. Morgan.) Freyn, J. (1890) [p. 228] Fries, E. (1858) [p. 276] Fryer, A. (1887) [PP- 195, 330] 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.] Gardner, G. (1846) Travels in the Interior of Brazil, xvi + 562 pp., [p. 1 08] 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 nelumbifolia.) Gardner, G. (1847) Observations on the Structure and Affinities of the [pp. 112, 310] 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 antardica) is mentioned.] Geldart, A. M. (1906) Stratiotes Aloides L. Trans. Norfolk and Norwich [pp. 50, 54] 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.] GeneaudeLamarliere, Sur les membranes cutinisees des plantes aquatiques. L. (1906) Revue gen. de Bot. T. 18, 1906, pp. 289-295. [pp. 163, 260] (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, Potamogeton dertsus Glyceria spectabilis and Equisetum limosum.) Gibelli, G. ) Intorno allo sviluppo dell' ovolo e del seme della and (• (1891) Trapa natans L. Ricerchedianatomiaedimorfologia. Ferrero, F.) 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.) Gin, A. (1909) Recherches sur les Lythrac6es. 166 pages, 13 pis., [pp. 234, 295, 303] 28 text-figs. These Doct. Univ. Paris, 1909. (This memoir contains information about the structure, dis- tribution, etc. of the aquatic Lythraceae.) 370 BIBLIOGRAPHY Gliick, H. (1901) DieStipulargebildederMonokotyledonen. Verhandl. [p. 44] 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.) Gliick, H. (1902) 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.) Gliick, H. (1905) Biologische und morphologische Untersuchungen [pp. 9, 19, 195, 223, iiber Wasser- und Sumpfgewachse. I. Die Lebens- 280 and Figs. 147, geschichte der europaischen Alismaceen. xxiv + 312 p. 224, 148 and 149, pp., 7 pis., 25 text-figs. Jena, 1905. p. 225] [The species studied were Alisma Plantago, (L.) Michalet, A.graminifolium, Ehrh.,Elismanatans, Buchenau, Echinodorus ranunculoidcs, (L.) Engelm., E.ranunculoides var. repens,(Lam.), Caldesia parnassifolia, (Bassi) Part., 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.] Gliick, H. (1906) Biologische und morphologische Untersuchungen [Passim and Figs. 44, iiber Wasser- und Sumpfgewachse. II. Untersuchun- p. 69, 57, p. 89, 58, gen iiber die mitteleuropaischen Utricularia-A.rten, P- 89, 59, P- 92, 63, iiber die Turionenbildung bei Wasserpflanzen, sowie p. 96, 64, p. 96, 66, iiber Ceratophyllum. xvii + 256 pp., 28 text-figs., p. 99, 69, p. 102, 146, 6 pis. Jena, 1906. p. 223] (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.') Gliick, H. (1911) Biologische und morphologische Untersuchungen [pp. 145, 188, 198, iiber Wasser- und Sumpfgewachse. III. DieUferflora. 199, 200, and Figs. xxxiv + 644 pp., 8 pis., 105 text-figs. Jena, 1911. 95, p. 147, 128, p. [A detailed study of the manner of life of those plants which 198, 129, 130 and grow on the margin of fresh waters and have adopted an T^T' n rnn T?A anrl amphibious habit. As in his previous work, the author com- bines cultural experiments with observations in the field. He I35« P- 2O3] 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.] Gliick, H. (1913) 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. xxvir. 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 371 Goebel, K. (1879) Ueber Sprossbildung auf Isoetesblattern. Bot. Zeit. [P- 225] 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 IsoiUs lacustris and I.echinospora from the Vosges.) Goebel, K. (1880) Beitrage zur Morphologic und Physiologic des [p. 12] 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.) Goebel, K. (iSSg1) 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.) Goebel, K. (i8892) Der Aufbau von Utricularia. Flora, Neue Reihe, [PP. 93, 99] Jahrg. 47 (G. R. Jahrg. 72), 1889, pp. 291-297, i pl. (This paper forms a continuation of the author's previous work on Utricularia; U. affinis, U. longifolia, and U. bryophila are figured.) Goebel, K. (i8893) [p. 117] Goebel, K. (1891) [pp. 40, 100, 103,104, 106, and Fig. 68, p. IOO] 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.duJardinBot.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- Goebel, K.(i89i-i893) Pflanzenbiologische Schilderungen. Teil n. 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- p0int (pp. 217-373, pis. XXIV, XXV, etc.).] p. 29, 20, p. 38, 60, p. 92, 65, p. 98, 92, D JAA TO? n TiA « l*Z ;L> « 143, P- 22°, O0, 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 BIBLIOGRAPHY Goebel, K. (1896) Ueber Jugendformen von Pflanzen und deren [p. 156] kiinstliche Wiederhervorrufung. Sitzungsber. d. math.-phys. Classe d. k. b Akademie d. Wissensch. zu Miinchen, Bd. xxvi. 1897 (for 1896), 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.) Goebel, K. (1904) Morphologische und biologische Bemerkungen. 15. [p. 104 and Fig. 70, Regeneration bei Utricularia. Flora, Bd. 93, 1904, p. 104] pp. 98-126, 17 text-figs. (Includes an account of the formation of adventitious shoots from the leaves of the water Utricularias.) Goebel, K. (1908) Einleitung in die experimentelle Morphologic der [pp. 161, 281] 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- scrpinacoides and Limnophila heterophylla.) Goebel, K. (1913) Morphologische und biologische Bemerkungen. 22. [pp. 234, 344] 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.) Goppert, H. R. (1847) Ueber die Schlauche von Utricularia 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.) Goppert, H. R. (1848) Ueber den rothen Farbestoff in den Ceratophylleen. [p. 86] 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.) Graebner, P. (1901) Die Heide Norddeutschlands. (Engler, A. und [p. 290] 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.) Graebner, P. See Ascherson, P. and Graebner, P. (1907). Gratiolet, P. See Cloez, S. and Gratiolet, P. (1850). Gray, A. (1848) Remarks on the Structure and Affinities of the Order [p. 309] 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. 3or, 302] Guppy, H. B. (I8941) [pp. 85, 88, 273, 274, 275, 301 and Fig. 55, p. 86] Guppy, H. B. (i8942) [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- I32-i33, 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. xm. 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)-] 374 BIBLIOGRAPHY Guppy, H. B. (1897) On the Postponement of the Germination of the [pp. 243, 244, 280, Seeds of Aquatic Plants. Proc. Roy. Phys. Soc. 301] Edinburgh, Vol. xin. 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.) Guppy, H. B. (1906) Observations of a Naturalist in the Pacific between [pp. 88, 162, 241, 296, 1896 and 1899. Vol. n. Plant-dispersal, xxviii + 627 297, 301, 303, 304, pp., i pi. London, 1906. 305] (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.) Guppy, H. B. (1917) Plants, Seeds, and Currents in the West Indies and [pp. 303, 304, 333] 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.) Giirke, M. See Ascherson, P. and Giirke, M. (1889). Gwynne-Vaughan, On some Points in the Morphology and Anatomy of D. T. (1897) the Nymphaeaceae. Trans. Linn. Soc. Lond. Ser. II. [PP- 33> 37» 38, 182] 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.) Hall, J. G. (1902) An Embryological Study of Limnocharis emarginata. Bot. Gaz. Vol. xxxiii. 1902, pp. 214-219, i pi. (An account of the embryo-sac and embryo in this species.) Hallier, E. (1859) Aedemone mirabilis Kotschy. Ein neues Schwimm- [p. 192] 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.).] Hannig, E. (1912) Untersuchungen iiber die Verteilung des osmotischen [pp. 260, 266] 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 + 486pp., 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 g^otropisme hydrocarpique chez (19131) Pontederia rotundifolia L. Recueil de 1'Institut Bot. [p. 239 and Fig. 155, Le"o Errera, T. ix. 1913. pp. 28-32. i text-fig. P- 24°] (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 e"thologiques et syste"matiques sur deux (19132) especes argentines du genre Elodea. Recueil de [PP- 55. 57. 236] 1'Instit. Bot. L£o 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. vin. [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, (i8so2) Ueber eine neue Nymphaea aus Schlesien. Bot. Zeit. Jahrg. vin. 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) Monographic der Gattung Callitriche. 64 pp., 4 pis. [pp. 169, 175, 216, Stuttgart, 1864. 236, 311] (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.) BIBLIOGRAPHY Hegelmaier, F. (1868) [PP- 73, 74, 75, 77, 80, 314 and Figs. 48, p. 76, 50, p. 79, 52, p. 81] Hegelmaier, F. (1870) [P- 7<>] 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) 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 Fructinkationstheile 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.) Hiern, W. P. (1872) A Theory of the Floating Leaves in certain Plants, [p. 30] 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) Einige Beobachtungen aus dem Gebiete der Pflanzen- [p. 67] Anatomic. 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.) Hildebrand, F. (1870) 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. elata 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.) Hildebrand, F. (1885) t)ber Heteranthera zosterifolia. Engler's Bot. Jahr- [pp. 207, 228] 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).] Hiltner, L. (1886) UntersuchungeniiberdieGattungStt&M/ana. Engler's [P- 233] 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 5. aquatica are not true species, but owe their differences to their varying environments.) Hochreutiner, G. fitudes sur les Phane"rogames aquatiques du Rhdne (1896) et du Port de Geneve. Rev. g<§n. de Bot. T. vm. [pp. 174, 204, 205, 1896, pp. 90-110, 158-167, 188-200, 249-265, i pi., 245, 261, 281, 282 15 text-figs. and Fig. 137, p. 206] [The first part of these studies consists of a detailed account of the morphology, anatomy and development of Zannichcllia paluslris (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).] Hochreutiner, G. Notice sur la Repartition des Phanerogames dans le (1897) Rhdne et dans le Port de Geneve. Bull, de 1'Herbier Boissier, Ann6e v. No. i, 1897, pp. 1-14, i pi. (A study of the distribution and ecology of the water plants of this region.) Hoffmann, J.F. (1840) Beitrage zur naheren Kenntniss von Lemna arrhiza [p. 78] 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.) Hofmeister, W. (1852) Zur Entwickelungsgeschichte der Zostera. Bot. Zeit. [P- 1351 Jahr§- x- l852- PP- 121-^31. 137-149, i pl. [An account of the development of the pollen, ovule and embryo of Zostera, which supplements and corrects Gronland, J. (1851). Some account of Ruppia is also given. Forcriticism seeEngler, 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) Recherches anatomiques et morphologiques sur deux [p. 129] 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).] Hooker, J. D. (1847) The Botany of the Antarctic Voyage of H.M. Dis- [pp. 233, 311] 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 P1- [This paper is chiefly of systematic interest. Hydrothrix is a reduced and aberrant member of the family. See also Goebel, K. (1913)-] Hope, C. W. (1902) The 'Sadd' of the Upper Nile: its Botany compared [pp. 192, 214] with that of similar Obstructions in Bengal and American waters. Ann. Bot. Vol. xvi. 1902, pp. 495-5 1 6. (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 Lemnac6es. [pp. 74, 75, 76] Bull, de la Soc. Roy. de Bot. de Belgique, T. vin. 1869, pp. 15-88, i pi. [These observations, which deal largely with the hibernation of the I.emnaceae, 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 v6getatif des Bignoniacees, [pp. 104, 107] Rhinanthacees, Orobanchees et Utriculariees. 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 [ (1808) Paris, 1808. Bonpland, A. J [The aerenchyma of Mimosa lacttstris (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- 313, 32i, 324] P- 305, 4 text-figs. (An account of Bidens Beckii, Cotula myriophylloides, Pectis aquatica and Eriger&n heteromorphus.) ImThurn,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, \ The Botany of the Roraima Expedition of 1884: E. F. and [• (1887) being Notes on the Plants observed, by Everard F. Oliver, D. j im Thurn; with a list of the Species collected, and [p. 109] 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 Utricularia Humboldtii, which lives in the water collected in the leaf axils of a Bromeliad.) Irmisch, T. (1853) [pp. 26, 87] Irmisch, T. (1854) [Fig. 112, p. 173] Irmisch, T. Irmisch, T. (i8582) [pp. 52, 271] Kurze botanische Mittheilungen. 6. Nymphaea alba und Nuphar luteum. 7. Potamogetondensus. 8. Dauer derCeratophyllum-A-rien. Flora, N.R. Jahrg. xi. (G.R. Jahrg. xxxvi.) 1853, pp. 527-528, i pi. [In these notes attention is called to the stipula axillaris of Nymphaea (Castalia) 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.! Bemerkung iiber Hippuris vulgaris L. Bot. Zeit. Jahrg. 12, 1854, pp. 281-287, i pi. (A detailed account of the mode of branching of the sympodial stems.) Botanische Mittheilungen. i. Ueber Utricularia minor. Flora, Neue Reihe, Jahrg. xvi. (Ganz. Reihe, Jahrg. XLI.) 1858, pp. 33-37. i P1- (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. I77-J79- (This paper records the occurrence of "squamulae intra- vaginales" in a number of Helobieae.) 380 BIBLIOGRAPHY Irmisch, T. (i8s83) Ueber einige Arten aus der natiirlichen Pflanzen- [p. 59] 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 Potamozeton 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.) Irmisch, T. (1859*) Bemerkungen iiber einige Wassergewachse. Bot. [pp. 169, 245] Zeit. Jahrg. 17, 1859, pp. 353-356- (Notes on the morphology of Myriophyllum, Callitriche, Potamogeton trichoides, Hydrocharis and Stratiotes.) Irmisch, T. (i8592) 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).] Irmisch, T. (1861) Ueber Polygonum amphibium, Lysimachia vulgaris, [p. 205] 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.) Irmisch, T. (1865) Beitrag zur Naturgeschichte des Stratiotes Aloides. [p. 244] Flora, N.R. Jahrg. xxin. (G.R. Jahrg. XLVIII.) 1865, pp. 81-91, i pi. (The fruit, seed and seedling of Stratiotes divides, the seedling of Naias major and the axillary shoots of Hydrocharis Morsus- ranae, and Vallisneria spiralis are described and figured in this paper.) Ito, T. (1899) Floating-apparatus of the Leaves of Pistia Stratiotes, [p. 83] L. Ann. Bot. Vol. xin. 1899, p. 466. (Notes on the structure and mode of flotation of the leaves of this plant which was studied in its native habitat.) Jaensch, T. (I8841) Nachtrag zur Kenntniss von Herminiera Elaphroxy- [p. 192] Ion 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.) Jaensch, T. (1884*) Zur Anatomic einiger Leguminosenholzer. Ber. d. [p. 192] 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).] Jaggi, J. (1883) Die Wassernuss, Trapa natans L., und der Tribulus [p. 302] 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 381 Jahn, E. (1897) Uber Schwimmblatter. Funfstiick's Beitrage zur [pp. 30, 31] 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.) Jeffrey, E. C. (1899) The Morphology of the Central Cylinder in the [p. 180] 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).] Johnston, G. (1853) The Botany of the Eastern Borders, London, 1853. [p. 210] xii + 336 pp., 13 pis. (Pp. 191-192 give an early account of the spread of Elodea canadensis in this country.) Jonsson, B. (1883) Der richtende Einfluss stromenden Wassers auf [p. 282] 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.) Jonsson, B. Om befruktningen hos slagtet Najas samt hos (1883-1884) Callitriche autumnalis. Lunds Univ. Ars-skrift, Tom. [P- 236] 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.) Juel, 0. (1910) Cynomorium und Hippuris. Svensk. Bot. Tidskrift, [p. 312] 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.) Juel, O. (1911) Studien iiber die Entwicklungsgeschichte von Hip- [p. 312] puns vulgans. 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- (In this work Zostera is included among the Aroids, see p. 24.) Kalberlah A (1895) Das Bluhen der Wasserlinsen. Zeitschrift. f. Natur- [p. 76] 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.) 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 anatomj of Utricularia vulgaris.) Karsten, G. (1888) Ueber die Entwickelung der Schwimmblatter bei [pp. 146, 284] 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.] Keller, I. A. (1893) The Glandular Hairs of Brasenia peltata Pursch. [p. 272] Proc. Acad. Nat. Sci. Philadelphia, 1894 (for I893), pp. 188-193, I 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.) Kerner, A. and The Natural History of Plants. 2 vols., 1760 pp., Oliver, F. W. 1000 figs., 16 pis. (1894-1895) (This well-known book includes a good deal of information Q>t 301] 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. n. Abth. in. Stutt- Schroter,C. (1908, etc.) gart, 1908, 1909 and 1917. [pp. 50, 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'pies'^Q P 7Q Abth. in. pp. 57-8o, 23 text-figs.; Ceratophyllaceae, Bd. n. I36, p 205] Abth- '"• PP" 5I~73' l6 teXt'figS-) Kirchner, O. See Schroter, C. and Kirchner, 0. (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 Klebahn, H. (1891) Ueber Wurzelanlagen unter Lenticellen bei Her- [p. 192] 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. (1884*) 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.] Klebs, G. (1884) Beitrage zur Morphologic und Biologic der Keimung. [p. 245 and Fig. 158, Unters. bot. Inst. Tubingen, Bd. i. Heft 4, 1884 p. 245] pp. 536-635, 24 text-figs. (In this paper the seedlings of certain water plants come under consideration.) Klinge, J. (1881) Ueber Sagittaria sagittaefolia L. Sitzungsber. d. [pp. 15, 18] 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.) Klinsmann,F. (1860) Ein Beitrag zur Entwickelungsgeschichte von Stra- [p. 54] 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.) Knoch, E. (1899) Untersuchungen iiber die Morphologic, Biologic und [p. 34] 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.) Knupp, N. D. (1911) The Flowers of Myriophyllum spicatum L. Proc. [p. 232] Iowa Acad. Sci. (Des Moines), Vol. xvnr. 1911, pp. 61-73, 4 pis. (A study of the development and general structure of the flowers of this species.) Koch, K. (1852) Ueber Pistia in Allgemeinen und P.istia Turpini [pp. 82, 316] Blume insbesondere. Bot. Zeit. Jahrg. 10, 1852, PP- 577-585. i Pi- (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.) Koehne, E. (1884) 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 von A. Engler), 24 pp., 71 text-figs. 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. vin. 1858, No. 4, pp. 113—116, i pi. (The first scientific description of A edemone mirabilis, Kotschy = Herminiera Elaphroxylon, G.P.R.) Krause, K.and\, ,, Aponogetonaceae, in Das Pflanzenreich, iv. 13 pI9< ' (herausgegeben vo 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 heterophylly 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. M6m. 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 EpUobium hirsutum, Lycopus europaeus and two species of Lythrum, when grown in water.) Zur Frage uber 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 ) Kotschy, T. (1858) [p. 192] Engler, A. [pp. 62, 142, 143, 154, 239. 3°5> 3*4] Kurz, S. (1867) [pp. 73, 291] Lamarck, J. B. P. A. (1809) [P- J55] Land, W. J. G. Lebel, E. (1863) [pp. 134, 195, 216, 311] Lewakoffski, N. (I8731) [p. 188] Lewakoffski, N. (iSys2) [p. 200] BIBLIOGRAPHY 385 Lindberg, S. O. (1873) [p. 46] Lister, G. (1903) [P- "3l 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, IO2, IO5 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. fwlchella 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 phcnolphthalein 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 Suss- 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 Potarnogetons.) See Dodoens, R. (1578)- 25 386 BIBLIOGRAPHY McCallum, W. B. On the nature of the stimulus causing the change of (1902) form and structure in Proserpinaca palustris. Bot. [p. 160] 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).] MacCaughey, V. Gunner a petaloidea Gaud., a remarkable plant of (1917) the Hawaian Islands. American Journ. Bot. Vol. iv. [p. 182] 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.) MacDougal, D. T. The Determinative Action of Environic Factors upon (1914) Neobeckia acquatica Greene. Flora, N.F. Bd. vi. [p. 162] (G.R. Bd. 106), 1914, pp. 264-280, 14 text-figs. (A study of the heterophylly of this plant under a variety of conditions.) MacLeod, J. (1893 Over de bevruchting der bloemen in het kempisch and 1894) gedeelte van Vlaanderen. Bot. Jaarboek, Vol. v. [pp. 9, 230] 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.) Magnin, A. (1893) Recherches sur la vegetation des lacs du Jura. Rev. [pp. 274, 279, 287, gen. de Bot. T. v. 1893, pp. 241-257, 303-316, 290, 323] 8 text-figs. (An ecological survey of 62 out of the 66 lakes which occur in the Jura region.) Magnus, P. (iSyo1) 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.) Magnus, P. (iSyo2) Ueber die Anatomic der Meeresphanerogamen. [p. 135] 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).] Magnus, P. (1871) Einige Bemerkungen zu dem Aufsatze des Herrn J. [p. 169] 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) Untersuchungen iiber die Anatomic der Cymodoceen. [P- *35] 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.) Magnus, P. (1883) Ueber eine besondere geographische Varietat der [P- 332] Najas graminea Del. und deren Auftreten in England. Ber. d. deutsch. bot. Gesellsch. 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).] Magnus, P. (1894) 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. (I8701) 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. 1859, 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.) Marloth, R. (1883) Uber mechanische Schutzmittel der Samen gegen [p. 241] 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.) Marshall, W. (1852) Excessive and noxious Increase of Udora Canadensis [pp. 55, 210] (Anacharis Alsinastrum). Phytologist, Vol. iv. 1852, PP- 7°5~7I5- (An historical account of the introduction of this plant.) Marshall, W. (1857) Tne 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. [P- *35] ETnis 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 Magnus, W.) and [ Werner, E. ) (1913) [p. 121] Maisonneuve, D. de (i859) [p. no] 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. xin. pp. 169-189, 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 supplemen- Frgs. 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. xiu. 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, 101, 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 ThyUenfrage. Bot. Zeit. Jahrg. 44, 1886, pp. [p. 258] 745-753, i pl- [An account of a petiole of Nymphaea (Castalid) 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. xxvn. (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 aquatilis, R. Flammula, Littorella lacustris, etc. The author suggests a comparison between etiolated and submerged plants.) BIBLIOGRAPHY 389 Mer, £. (i88o2) Mer, £. (1881) Mer, £. (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 Standley, P. C. Des causes qui modifient la structure de certaines plantes aquatiques ve'ge'tant dans 1'eau. Bull, de la Soc. bot. de France, T. xxvu. (Se>. 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. in.) 1881, pp. 87-90. (Brief notes on the submerged and aerial forms of CallUriche Littorella, etc.) De la v6g6tation 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'here'dite 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. n 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- (1912) 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.) 39° Minden, M. von ( 1 899) [pp. 83, 266, 268, 269 and Fig. 53, p. 82] Mobius, M. (1895) [P- 281] Moeller, J. (1879) [p- 191] Monkemeyer, W. (1897) [p. 291] Montesantos,N.(i9i3) [pp. 50, 51, 52, 157, 239, 282] Monti, Gaetano (1747) [p. 109] Mori, A. (1876) [p. no] Moss, C. E. (1913) [p. 291] Miiller, F. (1877) [P- 3H] BIBLIOGRAPHY Beitrage zur anatomischen und physiologischen Kenntnis Wasser-secernierender Organe. Bibliotheca 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.) Ueber einige an Wasserpflanzen beobachtete Reizer- scheinungen. Biol. Centralbl. Bd. 15, 1895, pp. 1-14, 33-44, 8 text-figs. (Observations on the effect of light and darkness on water plants.) Aeschynomene aspera Willd. (Papilionaceen). Bot. Zeit. Jahrg. 37, 1879, pp. 720-724, i text-fig. (An account of the anatomy of the floating wood of this plant.) Die Sumpf- und Wasserpflanzen. Ihre Beschreibung, Kultur und Verwendung. iv + 189 pp., 126 text- 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.) Morphologische und biologische Untersuchungen iiber einige Hydrocharideen. Flora, N.F. Bd. v. (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.) De Aldrovandia novo herbae palustris genere. De Bononiensi Scientiarum et Artium Institute atque Academia Commentarii. Tomi secundi Pars tertia, 1747, PP- 404-4H. i pl- [This old and rare memoir on Aldrovandia is analysed in Aug6 de Lassu (1861).] Nota sull' irritabilita delle foglie dell' Aldrovandia vesiculosa. Nuovo Giornale Botanico Italiano, Vol. vin. 1876, p. 62. (The author shows that the irritability of the Aldrovandia leaf is confined to the central glandular region.) Vegetation of the Peak District, x + 235 pp., 36 figs., 2 maps. Cambridge, 1913. (Chapter vi. contains an account of the marsh and aquatic associations of the district.) Untersuchungen iiber die Struktur einiger Arten von Elatine. Flora, N.R. Jahrg. xxxv. (G.R. Jahrg. LX.) 1877, pp. 481-496, 519-526, i pl. (A description of the anatomy and flower structure of Elatine and a discussion of the affinities of the genus.) BIBLIOGRAPHY 391 Miiller, F. (1883) Einige Eigenthumlichkeiten der Eichhornia crassipes. [PP- 239, 282] Kosmos, Jahrg. vn. Heft iv. 1883, pp. 297-300. (Notes on the floral biology of this species.) Miinter, J. (1845) Beobachtungen iiber besondere Eigenthumlichkeiten [P- I5] 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.] Murray, H. See Weiss, F. E. and Murray, H. (1909). Nakano, H. (1911) The Vegetation of Lakes and Swamps in Japan. I. [p. 291] 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.) Nolte, E. F. (1825) Botanische Bemerkungen iiber Stratiotes und Sagit- [pp. 15, 50, 52, 54 taria. Kopenhagen, 44 pp., 2 pis., 1825. and Fig. 32, p. 53] [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).] Ohno, N. (1910) Ueber lebhafte Gasausscheidung aus den Blattern [p. 258] 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.) Oliver, D. See Im Thurn, E. F. and Oliver, D. (1887). Oliver, F. W. (1888) On the Structure, Development, and Affinities of [pp. 151, 234, 266] Trapella, Oliv., a new Genus of Pedalineae. Ann. Bot. Vol. n. 1888-1889, pp. 75-115, 5 pis., i text-fig. (A monograph of Trapella sinensis, Oliv., a Chinese water plant discovered by Dr Henry.) Oliver, F. W. (1889) 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. (1 888).] Oliver, F. W. (1894) See Kerner, A. and Oliver, F. W. (1894). Onslow, The Hon. See Wheldale, M. (1916). Mrs Huia Osbeck, P. (1771) A Voyage to China.... 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(1914) The transpiration of emersed water plants: its [p. 261] 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.) Overton, E. (1899) Notizen iiber die Wassergewachse des Oberengadins. [pp. 228, 280, 290] VierteljahrsschriftderNaturforschendenGesellschaft 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.) Paillieux, A.) Les plantes aquatiques alimentaires. Bull, de la Soc. and >- (1888) nat. d'acclimatation de France, Ser. iv. T. 5, Ann6e Bois, D. ) 35, 1888, pp. 782-793, 924-929, 1028-1035, 1 102- [pp. 17, 24] 1108. (An account of a number of aquatic plants which are used for food, including Aponogeton, Trapa, Eleocharis, Sagittaria and various Nymphaeaceae.) Pallis, M. (1916) The Structure and History of Plav: the Floating Fen [pp. 207, 21 1] 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.) Parkin, J. See Arber, E. A. N. and Parkin, J. (1907). Parmentier, P. (1897) Recherches anatomiques et taxinomiques sur les [p. 312] 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.) Payne-Gallwey, R. See Walsingham, Lord, and Payne-Gallwey, R. (1886). BIBLIOGRAPHY 393 Pearsall, W. H. The Aquatic and Marsh Vegetation of Esthwaite (1917-1918) Water. 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(1913) On the Buoyancy of the Seeds of some Britannic [p. 297] Plants. Sci. Proc. Royal Dublin Soc., N.S., Vol. xiv. 1913-1915. PP- 13-62. [This memoir is supplementary to the parts of Guppy, H. B. (1906) which relate to seed buoyancy.] Prankerd, T. L. (1911) On the Structure and Biology of the Genus Hottonia. [pp. 181, 197, 216, Annals of Bot. Vol. xxv. 1911, pp. 253-267, 2 pis. 228, 233 and Fig. and 7 text-figs. 127, p. 197] (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.) Preston, T. A. (1895) The Flora of the Cropstone Reservoir. Trans. Leic. [pp. 232, 291] 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.) Prillieux, E. (1864) Recherches sur la vegetation et la structure de [pp. 63, 173] I1 Althenia filiformis Petit. Ann. d. sci. nat. S6r. v. Bot. T. ii. 1864, pp. 169—190, 2 pis. (A general account of this Mediterranean member of the Zannichellieae.) Pringsheim, N. (1869) t)ber die Bildungsvorgange am Vegetationskegel von [pp. 97, io6andFig. Utricularia vulgaris. Monatsber. d. k. preuss. Akad. 72, p. 106] 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.) Pringsheim, N. (1888) Ueber die Entstehung der Kalkincrustationen an [p. 51] 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 CO% which has held the calcium carbonate in solution.) Queva, C. (1910) Observations anatomiques sur le " Trapa natans L." [p. 244] Association Fran£aise pour I'avancement des sciences. Compte rendu de la 380 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) Ueber die Inhaltskorper der Myriophyllumtrichome. Ber. d. deutsch. hot. Gesellsch. Bd. xi. 1893, pp. 348-351. (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 Ceralophyllum, Elatine, etc.) Raciborski , M. ( 1 8Q41) 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.) Raciborski, M.( 1 8942) Beitrage zur Kenntniss der Cabombeen und Nym- [pp. 29, 35, 272, 338] phaeaceen. Flora, Bd. 79 (Erganzungsband), 1894, pp. 92-108, i pi. [This paper is supplementary to Raciborski, M. (1894*) and includes a reply to the criticisms on the latter contained in Schumann, K. (1894).] Raciborski, M. (1895) DieSchutzvorrichtungenderBluthenknospen. Flora, Bd. 81 (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.) Raffeneau-Delile, A. Evidence du mode respiratoire des feuilles de (1841) Nelumbium. Ann. d. sci. nat. Ser. n. T. xvi. Bot. [pp. 38, 258] 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.) Raunkiaer, C. (1896) De Danske Blomsterplanters Naturhistorie. Bd. I. [Figs. 34, p. 55, 159, Enkimbladede i. Helobieae, 1896, 138 pp., 240 text- p. 246, 161, p. 248, figs. 166, p. 319, 167, (This fully illustrated account of the biology of the Helobieae P- 339. J68, p. 339] native to Denmark is in Danish.) Raunkiaer, C. (1903) Anatomical Potamogeton-Studies and Potamogeton [pp. 62, 65, 331 and fluitans. Botanisk Tidsskrift, Vol. 25, 1903, pp. 253- Fig. 38, p. 61] 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 Polagometon.) Ravn, F. K. (1894) Om Flydeevnen hos Fn/tene 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 rtsume.) Regnard, P. (1891) Recherches experimentales sur les conditions phy- [pp. 253, 255, 278] 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.) 396 BIBLIOGRAPHY Reid, C. (1892) On the Natural History of Isolated Ponds. Trans. [pp. 296, 298] 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.) Reid, C. (1893) On Paradoxocarpus carinatus, Nehring, an extinct [p. 54] 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 Stratiotes aloides L. (vide note by same author in Trans. Norfolk and Norwich Nat. Soc. Vol. vi. Pt 3, 1897, p. 328).] Reid, C. (1899) The Origin of the British Flora, vi + 191 pp. London, [P- 303] 1899- (This classical study, based on the flora of the Newer Tertiary beds, contains many references to water plants.) Reinsch, P. (1860) 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 " Bliithezeitblatter.") Rendle, A. B. (1899) A Systematic Revision of the Genus Najas. Trans. [P- 3J5] Linn. Soc. Lond. Ser. n. Vol. v. 1895-1901, Part xn. 1899, pp. 379-436, 4 pis- (This monograph includes a general introduction dealing with the morphology, structure and distribution of the genus.) Rendle, A. B. (1900) Supplementary Notes on the Genus Najas. Trans. Linn. Soc. Lond. Ser. n. Vol. v. 1895-1901, Part xin. 1900, pp. 437-444. [This paper supplements Rendle, A. B. (1899).] Rendle, A. B. (1901) Naiadaceae, in Das Pflanzenreich, iv. 12 (heraus- [P- 3°4] 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.) Rendle, A. B. (1904) The Classification of Flowering Plants. Vol. I. [p. 314] 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.) Richard, L. C. (1808) Demonstrations Botaniques ou Analyse du Fruit. [p. 311] 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, £. (iBjj1) Sur les mouvements spontanes et r^guliers d'une [p. 90] 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. (1877*).] Rodier, £. (i8772) The Movements of a Submerged Aquatic Plant, [p. 90] 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,6. (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.) Rohrbach, P. (1873) Beitrage zur Kenntniss einiger Hydrocharideen. Abhandl. d. naturforsch. Gesellschaft zu Halle, Bd. xn. 1873, pp. 53-114, 3 pis. (This memoir deals chiefly with the morphology and anatomy of Hydrocharis Morsus-ranae, Straiiotes aloides and Vallisntria spiralis. Special attention is paid to the shoot and inflorescence systems and to the development of the flower.) Roper, F. C. S. (1885) Note on Ranunculus Lingua, Linn. Journ. Linn. Soc. [p. 146] 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.) Rosanoff, S. (1871) Ueber den Bau der Schwimmorgane von Desmanthus [p. 189 and Fig. 123, natans Willd. Bot. Zeit. Jahrg. 29, 1871, pp. 829-838, p. 191] i pi. [A study of the aerenchyma of Desmanthus natans, Willd. (Neptunia oleracea, Lour.).] Rossmann, J. (1854) Beitrage zur Kenntniss der Wasserhahnenfiisse, [p. 144] 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.) Roux, M. le (1907) Recherches biologiques sur le lac d'Annecy. Annales [p. 291] de Biologic Lacustre, T. u. 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.) Roxburgh, W. (1832) Flora Indica. Vol. n. vi + 691 pp. Serampore, 1832. [p. Iio] (On p. 112 the author mentions that Aldrovandiaverticillatais " Found swimming on ponds of water over Bengal during the cold and hot season.") Royer, C. (1881-1883) Flore de la C6te-d'Or avec determinations par les [pp. 24, 27, 87, 216, parties souterraines. 2 vols., 693 pp. (2 vols. paged 234, 236] 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 BIBLIOGRAPHY Roze, E. (1887) Le mode de fecondation du Zannichellia palustris L. [p. 71] Journ. de Bot. T. i. 1887, pp. 296-299, i text-fig. (Observations on the submerged pollination of this species.) Roze, E. (1892) Sur le mode de fecondation du Najas major Roth et [p. 85] du Ceratophyllum demersum L. Bull, de la Soc. hot. 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.] Russow, E. (1875) Betrachtungen iiber das Leitbiindel- und Grundge- [pp. 107, 180] webe (Jubilaumschrift Dr Alexander von Bunge). 78 pp. Dorpat, 1875. (The anatomy of water plants is dealt with in this memoir in some detail.) Sanio, C. (1865) Einige Bemerkungen in Betreff meiner iiber Gefass- [pp. 65, 86, 175, 176, biind elbildung geausserten Ansichten. Bot. Zeit. 179] Jahrg. 23, 1865, pp. 165-172, 174-180, 184-187, 191-193, I97-200- [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).] Sargant, E. (1903) A Theory of the Origin of Monocotyledons, founded [p. 320] on the Structure of their Seedlings. Ann. Bot. Vol. 17, 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).] Sargant, E. (1908) The Reconstruction of a Race of Primitive Angio- [pp. 308, 320, 323] 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. [P- 135] Sauvageau, [pp. 124, 131, 164] Sauvageau, C. (1890*) [P- 131] Sauvageau, C. (1890*) [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. (1891*) Sauvageau, C. (i89i3) [PP- 135. 33i] Sauvageau, C. (1893) [p. 269 and Fig. 164, p. 270] Contribution a l'6tude du systeme m^canique 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 monocotyl&lones aquatiques. Ann. d. sci. nat. S6r. vn. Bot. T. xni. 1891, pp. 103-296, 64 text-figs. (Also published as Theses pr6sent6es a la facultS des sciences de Paris, S6r. 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 (1890*). 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 steins 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 Cymodoc6es 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 Butom6es. Ann. des sci. nat. S6r. 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.) 4oo Sauvageau, C. (1894) [PP. 59, 63, 71, 243 and Figs. 37, p. 60, 43, P- 68] SchaffnerJ.H. (1896) [P- 19] Schaffner,J.H.(i897) [P- 9] Schaffner, J.H. (1904) [PP- 309, 3M] 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. vin. 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. gemmi- 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- pfianzen. Ber. d. deutsch. bot. Gesellsch. Bd. II. 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. xvm. 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 aerencbyma 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 uber 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 uber 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 uber die Limosella-Arten. Bot. Zeit. von (1854) Jahrg. 12, 1854, pp. 900-918. (A critical account of the species and varieties.) Schleiden, M. J. (1837) Beitrage zur Kenntniss der Ceratophylleen. Linnaea, [pp. 63, 84, 86] Bd. ii, 1837, pp. 5 1 3-542. * 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 uber 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.( 1 838*) Berichtigungen und Nachtrage zur Kenntniss der Ceratophylleen. Linnaea, Bd. 12, 1838, pp. 344-34^, ipL [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.] 26 402 Schleiden,M.J.(i839) [P- 73] Schoenefeld, W. de (1860) [p. no] (1906) Schorler, B., Thallwitz, J. and Schiller, K. j [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.O.J [pp. 291, 322] Schroter, C. Schuchardt, T. (1853) Schultz, F. (1873) [p. 101] Schumann, K. (1892) [P- 7°] BIBLIOGRAPHY Prodromus Monographiae Lemnacearum oder Con- spectus Generum atque Specierum. Linnaea, Bd. xni. 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'Aldrovanda vcsiculosa en hiver et au printemps. Bull, de la soc. bot. de France, T. vn. 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 Biologie 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. II. 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 of Naias expressed by Magnus, P. BIBLIOGRAPHY 403 Schumann, K. (1894) Die Untersuchungen des Herrn Raciborski iiber die Nymphaeaceae und meine Beobachtungen iiber diese FamUie. Ber. d. deutsch. hot. Gesellsch. Bd. xii. 1894, PP- 173-178. [A criticism of Raciborski, M. (I8941).] Scott, D. H. (1891) Origin of Polystely in Dicotyledons. Annals of Bot. [p. 180] Vol. v. 1890-1891, pp. 514-5x7. (In this paper the hypothesis is put forward that the cases of polystely known to occur among Angiosperms maybe associated with an aquatic ancestry.) Scott, D. H.| On the Floating-Roots of Sesbania acuhata, Pers. and j- (1888) Ann. Bot. Vol. i. 1887-1888, pp. 307-314, i pi. Wager, H. j (An account of the aerenchyma developed on the roots of this [p. 191] Leguminous plant. The spongy tissue is produced by a cortical phellogen.) Scott, J. (1869) Note on the Isoetes capsularis, Roxb. Journ. Linn. [p. 235] 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.) Seehaus, C. (1860) Hydrilla verticillata (L. fil.) Casp. var. pomeranica [p. 286] (Rchb.) Casp. Verhandlung. d. bot. Vereins f. d. Provinz Brandenburg, Heft n. 1860, pp. 95-102. (Observations on the life-history of this species.) Seidel, C. F. (1869) Zur Entwickelungsgeschichte der Victoria regia [pp. 34, 309] 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.) Sergueeff, M. (1907) Contribution a la morphologic et la biologic des [pp. 142, 244, 281, Aponog6tonac6es. Universit6 de Geneve. These... 314 and Fig. 91, p. docteur es sciences, Institut de Botanique. Prof. 142] DrChodat,7mes£rie, vmmefasc. 1907, 132 pp., 5 pis., 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.] Shull, G. H. (1905) Stages in the Development of Sium cicutae folium. [p. 162] 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. 4o4 BIBLIOGRAPHY Siddall, J. D. (1885) The American Water Weed, Anacharis Alsinastrttm, [pp. 55, 210, 21 1] 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 canadtnsis, Michx. in this country.) Snell, K. (1908) Untersuchungen iiber die Nahrungsaufnahme der [pp. 208, 260, 265] 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.) Snell, K. (1912) Der Transpirationsstrom der Wasserpflanzen. Ber. [p. 266] 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).] Snow, L. M. (1914) Contributions to the knowledge of the diaphragms [p. 183] of water plants. /. Scirpus validus. Bot. Gaz. Vol. 58, 1914, pp. 495-517. J6 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.) Solereder, H. (1913) Systematisch-anatomische Untersuchung des Blattes [pp. 42, 46, 52, 135, der Hydrocharitaceen. Beihefte zum Bot. Centralbl. 165, 169, 340] 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.) Solereder, H. (1914) Zur Anatomic und Biologic der neuen Hydrocharis- [p. 42] 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. deCandolle'sMonographiae Graf zu (1883) Phanerogamarum, Vol. iv. 1883, pp. 501-535. [p. 317] (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 leave? of this type probably occur in other Nymphaeaceae, but that they have been overlooked.] BIBLIOGRAPHY 405 Spruce, R. (1908) Notes of a Botanist on the Amazon and Andes... [PP- 31, 99, 154. 190, during the years 1849-1864, edited by A. R. Wallace. 229, 290, 291, 311] 2 vols. London, 1908. (These volumes contain a number of notes on the aquatic plants observed by Spruce in S. America.) Stahl, E. (1900) Der Sinn der Mycorhizenbildung. Pringsheim's [p. 164] 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 transpirers and "sugar leaved" weak transpirers seems to have a bearing upon the nature of the epidermis of submerged leaves.) Standley, P. C. See Miller, G. S. and Standley, P. C. (1912). Stein, B. (1874) t)ber Reizbarkeit der Blatter von Aldrovanda vesicu- [p. in] losa. Zweiundfiinfzigster Jahres-Ber. d. Schlesischen Gesellsch. 1875 (1874), pp. 83-84. (The author records the sensitiveness of the Aldrovandia leaf to contact.) Stb'hr, A. (1879) Uber Vorkommen von Chlorophyll in der Epidermis [pp. 165, 171, 279] 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 in the case of terrestrial Monocotyledons.) Strasburger, E. (1884) Das Botanische Practicum. xxxvi + 664 pp., 182 [p. 37] 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. 182; Hippuris, pp. 185 and 249; Elodea, p. 187.] Strasburger, E. (1902) Ein Beitrag zur Kenntniss von Ceratophyllum sub- [pp. 85, 86, 272, 309] mersum und phylogenetische Erortemngen. 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.) Sykes, M. G. See Thoday, D. and Sykes, M. G. (1909)- Sylven, N. (1903) 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). Tackholm, G. (1914) Zur Kenntnis der Embryosackentwicklung von [p. 311] 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 Onagraceac 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, 191-5, pp. 294-361, 1 6 text-figs. (It is pointed out on p. 354 that in the true Onagraceae and in Trapa 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.) Tepper, 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) Notes on the Germination of the Winter Buds of [pp. 48, 280] 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.) Thallwitz, J. See Schorler, B., Thallwitz, J. and Schiller, K. (1906). Theophrastus (Hort) Enquiry into Plants, with an English translation by (1916) Sir Arthur Hort. 2 vols. London, 1916. [p. 208] (Book iv. Chapter 9 contains an exceedingly clear description of Trapa natans.) Thiebaud, M. (1908) Contribution a la Biologic du Lac de Saint-Blaise. [p. 291] Annales de Biologic Lacustre, T. in. Fasc. i, 1908, pp. 54-i4«>. 5 Pis. (This work contains a short section dealing with the plants of the Lake.) Thoday, D. ) Preliminary Observations on the Transpiration and >• (1909) Current in Submerged Water-plants. Ann. Bot. Sykes, M. G.) Vol. xxm. 1909, pp. 635—637. [pp. 262, 266] (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.) Thurn, E. F. Im See Im Thurn, E. F. BIBLIOGRAPHY 407 Tieghem,P.van(i866) [P- 256] Tieghem,P.van(i867) Tieghem,P.van(i868) [pp. 107, 108, 227] Tieghem, P. van (1869*) [p. 107] Tieghem, P. van (i8692) Tittmann, J. A. (1821) [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 submergees. 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 CO8 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 (1869*).] Note sur la respiration des plantes aquatiques. Comptes rendus de 1'acad. des sciences, Paris, T. 65, 1867, PP- 867-871. [A further communication dealing with the same results as Tieghem, P. van (1866).] Anatomic de 1'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.) Anatomie 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 luteum (Nymphaea luiea), Potamogeton natans, Trapa natans.] Recherches sur la structure et le deVeloppement du Nuphar luteum. Ann. des sci. nat. S6r. 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 organog^niques sur la Victoria regia, et anatomic compared du Nelumbiutn, du Nuphar et de la Victoria. Ann. d. sci. nat. Se>. 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 opercnlura 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 cri*p»s is alluded to on p. 76.) 408 BIBLIOGRAPHY Treviranus, L. C. Noch etwas iiber die Schlauche der Utricularien. (18481) Bot. Zeit. Jahrg. 6, 1848, pp. 444-448. [PP- 93- 99> I54l (Notes on the bladders of Utricularia which the author regards as of foliar nature.) Treviranus, L. C. Observationes circa germinationem in Nymphaea et (18482) 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.] Treviranus, L. C. De germinatione seminum Euryales. Bot. Zeit. (1853) Jahrg. xi. pp. 372-374. i853- [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.] Treviranus, L. C. Vermischte Bemerkungen. i. Hybernacula des Pota- (1857) mogeton crispus. 2. Hybernacula der Hydrocharis [p. 67] Morsus Ranae L. Bot. Zeit. Jahrg. 15, 1857, pp. 697-702, i pi. [As regards Potamogeton crispus Treviranus confirms the observations recorded by Clos, D. (1856). He also gives a short description of the winter buds of Hydrocharis.] Tulasne, L. R. (1852) Podostemacearum Monographia. Archives du Mu- [p. 112] 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.) Unger, F. (1849) 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.) Unger, F. (I8541) 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.) Unger, F. (i8S42) Beitrage zur Physiologic der Pflanzen. I. Bestim- [p. 256] mung der in den Intercellulargangen der Pflanzen enthaltenen Luftmenge. Sitzungsber. d. k. Akad. d. Wissenschaften, Math.-Naturwissens. Classe, Bd. xn. 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 Unger, F. (1862) [p. 260] Ursprung, A. (1912) [P- 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 r 17, p. 179] Volkens, G. (1883) [p. 267] Vries, H. de (1873) [p. 284] Wachter, W. (I8971) [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 PI. VI, figs. 5 and 6.) Die vitalistische Theorie und der Transversal- Geotropismus. Flora, N.R. Jahrg. xxxi. (G.R. Jahrg. LVI.) 1873, pp. 3°5-3I5- [A criticism of Frank, A. B. (1872).] Beitrage zur Kenntniss einiger Wasserpflanzen. I. and II. Flora, Bd. 83, 1897, pp. 367~397- 2I 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., HydrocUis nymphoides, Buchenau. The second part deals with the morphology and anatomy of WeddeUina squamulosa, Tul., one of the Podostemaceae.) 4io BIBLIOGRAPHY Wachter, W. (i8972) Beitrage zur Kenntniss einiger Wasserpflanzen. III. [P- X59] 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;] Wager, H. See Scott, D. H. and Wager, H. (1888). Wagner, R. (1895) Die Morphologic des Limnanthemum nymphaeoides [p. 39 and Fig. 23, (L.) Lk. Bot. Zeit. Jahrg. 53, Abt. i. 1895, pp. p. 41] 189-205, i pi.. 1895. (A general descriptive paper dealing with the development, branching, etc. of this species.) Walker, A. O. (1912) The Distribution of Elodea canadensis, Michaux, in [p. 212] 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.) Walsingham, Lord, Shooting (Moor and Marsh). Badminton Library. and Payne-Gallwey, xiii + 348 pp. London, 1886. R. (1886) (The authors mention, pp. 158 and 165, that Brent Geese feed [pp. jor 302] on Zostera, and that these birds are almost confined to those parts of the coast where Zostera occurs.) Walter, F. (1842) Bern erkungen iiber die Lebensweise einiger deutschen [pp. 15, 17] Pflanzen. Flora, Jahrg. xxv. Bd. u. 1842, pp. 737- 748, i pi. (A picturesque account of Walter's discovery of tuber- formation and heterophylly in Sagittaria sagittifolia.) Warming, E. (1871) Forgreningen hos Pontedenaceae og Zostera. Viden- [p. 135] 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.) Warming, E. (1874) Bidrag til Kundskaben om Lentibulariaceae. Viden- [p. 100] skab. Meddel. fra den naturhist. Forening i Kjoben- havn for Aaret 1874 (1874-5), PP- 33~58, 3 Pls- (This paper, which is in Danish, deals with Genlisea and Utricularia. The germination of Utricularia is described.) Warming, E. (1881, Familien Podostemaceae. Kongel. Dansk. Videnskab. 1882, 1888, 1891) Selskabs Skrifter. Sjette Raekke. i. Vol. n. 1881, [pp. 112, 118, 310] pp. 1-34, 6 pis. 2. Vol. u. 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. (x883») Botanische Notizen. Bot. Zeit. Jahrg. 41 1883 pp [p. 245] 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.) Warming, E. (i8832) Studien iiber die Familie der Podostemaceae. [Figs. 76 and 77, Engler's Bot. Jahrbiich. Bd. iv. 1883 pp 217-223 p. 115, 79, p. 116] 5 figs. (A German version of part of the author's work on this family.) Warming, E. (1909) (Ecology of Plants, xi + 422 pp. Oxford, 1909. [p. 291] (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.) Webber, H. J. (1897) The Water Hyacinth, and its relation to navigation [p. 213] 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 crassipcs, (Mart.) Britton, = Eichhornia speciosa, Kunth, = Eichhornia crassipes, (Mart.) Solms.] Weddell, H. A. (1849) Observations sur une espece nouvelle du genre [pp. 80, 300] 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 Podostemacees en general, et leur distribution [pp. 113, 295] g^ographique 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. (1899) Untersuchungen iiber die Scheiteloffnung bei Wasser- [pp. 261, 266, 269] pflanzen. Funfstiick'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.) Weiss, F. E.) and Murray, H. j [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.] 4i2 BIBLIOGRAPHY Went, F. A. F. C. Untersuchungen ueber Podostemaceen. Verhande- (1910) lingen d. Konin. Akad. van Wetenschappen te [pp. 114, 122] 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.] Werner, E. See Magnus, W. and Werner, E. (1913). West, G. (1905) A Comparative Study of the dominant Phanerogamic [p. 287] 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.) West, G. (1908) Notes on the Aquatic Flora of the Ness Area, [pp. 287, 290] 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).] West, G. (1910) A Further Contribution to a Comparative Study of [pp. 20, 87, 145, 200, the dominant Phanerogamic and Higher Crypto- 234, 287, 299, 325] 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).] Wettstein, R. von Beobachtungen uber den Bau und die Keimung des (1888) Samens von Nelumbo nucifera Gartn. Verhandl. d. [p. 38] 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 perisperm, is figured with great clearness.) Wheldale, M. (The The Anthocyanin Pigments of Plants, x + 318 pp. Hon. Mrs Huia Cambridge, 1916. Onslow) (1916) [Chapter vi. (Physiological Conditions and Factors Influencing [p. 277] tne Formation of Anthocyanins) and Chapter vni. (The Significance of Anthocyanins), may be consulted in connexion with the red coloration so prevalent in water plants.] Wheldon, J. A.} TheFloraof West Lancashire. 511 pp., 15 pis., i map. and HI9O7) Eastbourne, 1907. Wilson, A. J (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) Nelumbium speciosum, W. Bot. Zeit. Jahrg. 29 1871 [P- 37] PP- 813-826, i text-fig. (An account of the development, morphology, anatomy and starch distribution in this member of the Nymphaeaceae.) Wight, R. (1849) Conspectus of Indian Utriculariae. Hooker's Journal [P- 99] of Botany and Kew Garden Miscellany, Vol. i. 1849, PP- 372-374- (The author records the occurrence of a whorl of floats belqw the flower in V. steUaris.) Willdenow, C. L. Determination of a new aquatic vegetable Genus, (1806) called Caulinia, with general Observations on Water- [p- 85] 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 Ccratophyllum was hydro- philous.) Willis, J. C. (1902) Studies in the Morphology and Ecology of the [Passim and Figs. 78, Podostemaceae of Ceylon and India. Ann. Roy. Bot. p. 115, 80, p. 118, 82, Gard. Peradeniya, Vol. i. 1902, pp. 267-465, 34 pis. p. I2l] (An important general work dealing with the structure and biology of this group.) Willis, J. C. (I9I41) On the Lack of Adaptation in the Tristichaceae and [pp. 112, 286, 327, Podostemaceae. Proc. Roy. Soc. Vol. 87, B. 1914, 329] 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.) Willis, J. C. (i9i42) The Endemic Flora of Ceylon, with Reference to [p- 305] 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.) Willis, J. C. (I9I51) A New Natural Family of Flowering Plants — [p. 112] 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.) Willis, J. C. (I9I52) The Origin of the Tristichaceae and Podostemaceae. [pp. 112, 327] Ann. Bot. Vol. xxix. 1915, pp. 299-306. (A reconstruction of the type of ancestor from which these groups are probably derived.) Willis, J. C. (1917) The Relative Age of Endemic Species and other [p. 306] 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.) 4i4 BIBLIOGRAPHY Willis, J. C. ) Flowers and Insects in Great Britain. Ann. Bot. and I (1895) Vol. ix. 1895, pp. 227-273. Burkill, I. H. j (This paper includes observations on the pollination of Peplis [p 230] an(l Mentha aquatica.) Wilson, A. See Wheldon, J. A. and Wilson, A. (1907) Wilson, W. (1830) Lemna gibba. Remarks on the Structure and Ger- [p. 76] mination. Hooker's Botanical Miscellany, Vol. i. 1830, pp. 145-149, i pi. (A description, with clear figures, of the seedlings of this species.) Wydler, H. (1863) 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 A lisma Plantago, L.) Wylie, R. B. (1904) The Morphology of Elodea canadensis. Bot. Gaz. [PP- 55. 57l Vol. xxxvu. 1904, pp. 1-22, 4 pis. (An account of the gametophytes, pollination, etc. in this species.) Wylie, R. B. (1912) A long-stalked Elodea flower. Bull, from the Labs, [pp. 55, 86 and Fig. of Nat. Hist. State University Iowa, Vol. vi. 1912, 35, P- 56] 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.) Wylie, R. B. (I9I71) Cleistogamy in Heteranthera dubia. Bull, from the [p. 234 and Fig. 153, Labs, of Nat. Hist. State University Iowa, Vol. vn. p. 234] No. 3, 1917. PP- 48-58, i pi. (The cleistogamy of this species, which is very thoroughly described, is considered by theauthor to be ' largely accidental.') Wylie, R. B. (I9I78) The Pollination of Vallisneria spiralis. Bot. Gaz. [P- 235] Vol. 63, 1917, pp. 135-145, i pi. 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.) Zacharias, O. (1891) 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 (1884*) • Klebahn H. (1891); Kotschy, T. (1858). Aeschynomene. Ernst, A. (1872*); Hallier, E. (1859); Jaensch, T. (1884*); Moeller, J. (1879). Aldrovandia. Aug6 de Lassu (1861) ; Caspary, R. (1858*). (1859 and 1862) ; Chatin, A. (1858*) ; 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 (I9O31); Gluck, 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). ARACEAB. 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 (1903"); Micheli, M. (1881); Sauvageau, C. (1893). Butomus. Buchenau, F. (1857); Fauth, A. (1903). CABOMBEAE. Gray, A. (1848); Raciborski, M. (1894!) and (i894«). Caldesia. Gliick, H. (1905)- Callitriche. Baillon, H. (1858); Borodin, J. (1870); Brown, R. (1814); Fauth, A. (1903); Frank, A. B. (1872); Hegelmaier, F. (1864); Irmisch, T. (i859»); JCnsson, B. (1883-1884); Lebel, E. (1863); Ludwig, F. (1881); Magnus, P. (1871); Mer, E. (1881); Richard, L. C. (1808). 4i6 INDEX TO BIBLIOGRAPHY Caltha. Geneau de Lamarliere, L. (1906). CAPPARIDACEAE. Schenck, H. (1889). Cardamine. Schenck, H. (1884). CARYOPHYLLACEAE. Cambessedes, J. (1829). Castalia. (See Nymphaea.) Caulinia. Willdenow, C. L. (1806). CERATOPHYLLACEAE. (See Ceratophyllum.) 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, E. (I8771) and (1877"); Roze, E. (1892); Sanio, C. (1865); Schleiden, M. J. (1837) and (18382); Strasburger, E. (1902); Willdenow, C. L. (1806). Coleanthus. Duval-Jouve, J. (1864). Comarum. Irmisch, T. (1861). Cotula. Hutchinson, J. (1916). Crassula. Magnus, P. (1871). Cymodocea. Agardh, C. A. (1821) ; Bornet, E. (1864) ; Cavolini, F. (i7922) ; 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"), (iSgo1) and (iSgi3); Tepper, J. G. O. (1882). Cynomorium. Juel, O. (1910). CYPERACEAE. Ascherson, P. (1883); Esenbeck, E. (1914). Damasonium. Gliick, H. (1905). Desmanthus. Rosanoff, S. (1871). Diplanthera. (See Halodule.) Echinodorus. Gliick, H. (1905). Eichhornia. Boresch, K. (1912); Muller, F. (1883); Wachter, W. (1897!); Webber, H. J. (1897). ELATINACEAE. Cambessedes, J. (1829). Elatine. Cambessedes, J. (1829); Caspary, R. (1847); Muller, F. (1877); Raciborski, M. (1893). Eleocharis. Paillieux, A. and Bois, D. (1888). Elisma. Fauth, A. (1903); Gliick, H. (1905). Elodea, Bolle, C. (1865) and (1867); Brown, W. H. (1913); Caspary, R. (iSsS1), (i8582) and (iSsS3); Douglas, D. (1880); Geneau de Lamarliere, L. (1906); Hauman-Merck, L. (1913*) ; 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). Enhalus. Cunnington, H. M. (1912); Delpino, F. and Ascherson, P. (1871); Sauvageau, C. (1890*). Epilobium. Batten, L. (1918); Lewakoffski, N. (i873a). Equisetum. Geneau de Lamarliere, L. (1906). Erigeron. Hutchinson, J. (1916). EUPHORBIACEAE. Baillon, H. (1858); Hegelmaier, F. (1864); Richard, L. C (1808); Schenck, H. (1889). Euryale. Anon. (1895); Treviranus, L. C. (18482) and (1853). Genlisea. Warming, E. (1874). Glyceria. Geneau de Lamarliere, L. (1906). Gunner a. MacCaughey, V. (1917). GUNNEREAE. 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. (1890*). 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!) and (1884*) ; 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. (iSss1); 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. (1858*); Seehaus, C. (1860). HYDRILLEAE. Caspary, R. (iSsS1) and (1858*). Hydrocharis. Frank, A. B. (1872) ; Griset, H. E. (1894) ; Irmisch, T. (iSsg1) 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), (i8$8l) and (iSsS1); Gliick, H. (1901); Montesantos, N. (1913); Rohrbach. P. (1873); Sauvageau, C. (1890*); Solereder, H. (1913). Hydrocleis. Buchenau, F. (igos2); Ernst, A. (1872*); 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. (i88o«); Scott. J. (1869). JUNCAGINACEAE. Buchenau, F. (1882); Micheli, M. (1881); Planchon, J. E. (1844). Jussiaea. Chatin, A. (iSss1); Martins, C. (1866). LABIATAE. Schenck, H. (1889). Lacis. Brown, C. Barrington (1876). Lagarosiphon. Caspary, R. (1858*). LEGUMINOSAE. Ernst, A. (1872*); 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. (1894*); 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. Fauth, A. (1903); Goebel. K. (1891); Wagner, R. (1895). Limnobium. Montesantos, N. (1913)- A. w. P. 2? 4i8 INDEX TO BIBLIOGRAPHY Limnocharis. Ernst, A. (1872*) ; Hall, J. G. (1902). Limnophila. Goebel, K. (1908). Limosella. Hooker, J. D. (1847); Schlechtendal, D. F. L. von (1854). Littorella. 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). LUDWIGIBAE. Parmentier, P. (1897). Lycopus. Lewakoffski, N. (I8731). Lysimachia. Irmisch, T. (1861). LYTHRACEAE. Gin, A. (1909); Schenck, H. (1889); Schrenk, J. (1889). Lythrum. Lewakoffski, N. (1873!). Marsilea. Hildebrand, F. (1870); Karsten, G. (1888). Mayaca. Ludwig, F. (1886). MELASTOMACEAE. Schenck, H. (1889). Mentha. Willis, J. C. and Burkill, I. H. (1895). Menyanthes. Fauth, A. (1903); Irmisch, T. (1861). M Crimea. Cambessedes, J. (1829). Mimosa. Humboldt, A. de and Bonpland, A. (1808). Montia. Focke, W. O. (i8g3l). Mourera. Aublet, F. (1775); Went, F. A. F. C. (1910). Myriophyllum. Bokorny, T. (1890); Borodin, J. (1870); Fauth, A. (1903); Geneau de Lamarliere, L. (1906) ; Goebel, K. (1908) ; Irmisch, T. (I8591); Knupp, N. D. (1911); Ludwig, F. (1881); Magnus, P. (1871) ; Perrot, £. (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. (i88gl); 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. (I8701); Desmoulins, C. (1849); Fries, E. (1858); Geneau de Lamarliere, L. (1906); Hausleutner, (iSso2); 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') i 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 l) 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. (1792*). 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. (1854*). Aublet, F. (1775) ; Brown, C. Barrington (1876) ; Gardner, G. (1847) ; Goebel, K. (1889*) 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. (I8971); Warming, E. (1881, 1882, 1888, 1891) and (1883*); Weddell, H. A. (1872) ; Went, F. A. F. C. (1910) ; Willis, J. C. (1902), (I9I41). (I9I51). (I9I52) and (191?)- 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. (I7921); Delpino, F. and Ascherson, P. (1871); Sauvageau, C. (1889*) 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 (1859*); Lundstrom, A. N. (1888); Mer, E. (1882*); Raunkiaer. C. (1903); Sanio, C. (1865); Sauvageau. C. (1889*) 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)- 27—2 Oenanthe. Oenone. OENOTHERACEAE. ONAGRACEAE. OROBANCHACEAE. Ottelia. Ouvirandra. Paradoxocarpus. Pectis. PEDALINEAE. Peplis. Phragmites. Phyllocactus. Phyllospadix. Piaropus. Pinguicula. Pistia. PODOSTEMACEAE . Polygonum. Pontederia. PONTEDERIACEAE. PORTULACACEAE. Posidonia. Potamogeton. 420 INDEX TO BIBLIOGRAPHY POTAMOGETONACEAE. Ascherson, P. (1867) and (1875) ; Ascherson, P. and Graebner, P. (1907); Chrysler, M. A. (1907); Fischer, G. (1907); Gliick, H. (1901); Irmisch, T. (i8583); Sauvageau, C. (iSgi1); Schumann, K. (1892). Proserpinaca. Burns, G. P. (1904); McCallum, W. B. (1902). Ranunculus. 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). RHINANTHACEAE. Hovelacque, M. (1888). Rubus. Lewakoffski, N. (i8732). Ruppia. Chrysler, M. A. (1907) ; Delpino, F. and Ascherson, P. (1871) ; Gaudichaud, C. (1826); Hofmeister, W. (1852); Irmisch, T. (1858°). Sagittaria. Anon., (1895) ; Arber, A. (1918) ; Bauhin, G. (1596) and (1620) ; Blanc, M. le (1912); Bolle, C. (1861-1862); Buchenau, F. (1857); Costantin, J. (1885 2); 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. (1897!) ; Walter, F. (1842). Salix. Lewakoffski, N. (1877). Saururus. Planchon, J. E. (1844). Schizotheca. Ascherson, P. (1870). Scirpus. Anon., (1895); Desmoulins, C. (1849); Esenbeck, E. (1914); Kirschleger, F. (1856) and (1857); Scheuchzerus, J. (1719); Snow, L. M. (1914). Sesbania. Hallier, E. (1859); Jaensch, T. (1884*); Scott, D. H. and Wager, H. (1888). Sisymbrium. Chatin, A. (iSsS1). Sium. Shull, G. H. (1905). Solanum. Klebahn, H. (1891). Sparganium. Kirschleger, F. (1856). Spirodela. Hegelmaier, F. (1871); Micheli, P. A. (1729). Stratiotes. 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). Subularia. Hiltner, L. (1886). Terniola. Goebel, K. (i8893). Thalassia. Sauvageau, C. (1890*). Tillaea. Caspary, R. (1860). Trapa. Anon., (1828); Anon., (1895); Areschoug, F. W. C. (I8731) and (1873*); 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). Trapella. Anon., (1828); Oliver, F. W. (1888) and (1889). Tristicha. Cario, R. (1881) ; Lister, G. (1903) ; Went, F. A. F. C. (1910). TRISTICHACEAE. Willis, J. C. (ig^1), (igis1), (1915*) and (1917). Udora. (See also Elodea.) Marshall, W. (1852). INDEX TO BIBLIOGRAPHY 421 Utricularia. 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. (1893*); Gardner, G. (1846); Gluck, H. (1902), (1906) and (1913); Goebel, K. (i88gl), (1889*), (1891), (1891-1893) and (1904); Goppert, H. R. (1847); Im Thurn, E. F. and Oliver, D. (1887); Irmisch, T. (iSsS1); KamieAski, 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 (1869*) ; Treviranus, L. C. (1848*) ; Warming. E. (1874); Wight, R. (1849). UTRICULARIACEAE. Benjamin, L. (1848); Hovelacque, M. (1888). Vallisneria. Chatin, A. (1855*); 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. (1917"). Victoria. Blake, J. H. (1887) ; Caspary, R. (1856*) ; Henfrey, A. (1852) ; Im Thurn, E. F. (1883); Knoch, E. (1899); Seidel, C. F. (1869); Trecul, A. (1854); Unger, F. (1854*). Weddellina. Wachter, W. (i8g7l). Wolffia. Hegelmaier, F. (1885); Micheli, P. A. (1729); Milde, (1853); Weddell, H. A. (1849). Zannichellia. Campbell, D. H. (1897) ; Chrysler, M. A. (1907) ; Hochreutiner, G. (1896); Hofmeister, W. (1858); Irmisch, T. (1858*); Roze. E. (1887). ZANNICHELLIACEAE. Fischer, G. (1907) ; Prillieux, E. (1864) ; Schumann, K. (1892). Zostera. Cavolini, F. (I7921) and (1792*); Chrysler, M. A. (1907); Clavaud, A. (1878); Delpino, F. and Ascherson, P. (1871); Duchartre, P. (1872) ; Engler, A. (1879) ; Gronland, J. (1851) ; Hofmeister, W. (1852) ; Jussieu, A. L. de (1789) ; Martens. G. von (1824); Ostenfeld, C. H. (1908); Sauvageau. C. (1889'), (iSgo1), (1891*); Walsingham, Lord, and Payne-Gallwey, R. (1886); Warming. E. (1871). 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 A chillea 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-ni, 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, 20,0, 295, 298, 312, 313 American Indians, 17, 118 Ammania, 303 Amphibious plants, effect of water upon, 201, 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; Zoster a, 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, , 3It 3I5 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 Utricularia 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. 313 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 parnassijolia, 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 Callilriche stagnalis, 176 (Fig. 114), 208, 209 (Fig. 138), 271 Callitriche verna, 6, 146. 147 (Fig. 94), 163, 1 66, 169, 170 (Fig. ni), 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 Elodta 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) 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. ii), 39, 217; seedlings, 28 (Fig. 13), 29; seeds, 302; stipules, 25, 26 (Fig. n); submerged leaves, 29, 146, 159; terrestrial form, 32 Castalia alba, 24, 25, 26 (Fig. 11), 27, 28 (Fig. 13), 29-31 (Fig. 15), 32, 243, 276, 302 Castalia flava, 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, 168, dimensionsof, 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- cularia, 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 tomentoswn, 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 Utricularia, 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), I3I. 33J» chlorophyll in epidermis, 164 ; flowers, 1 26 ; 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 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, in Diplanthera, 123 Distribution, geographical, 73, 112, 295-307 Dock, 271 Dodder. See Cuscuta alba Dodoens' Histoire des Planles, 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. '34. 236; root-hairs, 208; vegetative re- production, 55, 210-213; wintering shoots, 55 (Fig. 34), 219 Elodea callitrichoides, 55, 56, 236 4*5 Elodea canadensis, 6, 7, 55 (Fie 34) 57 165 (Fig. 106). 169, 173. 210-213, 219; K 253. 254. 265, 266 Elodea densa, 57, 236 Elodea ioensis, 55, 56 (Fig. 35) Embryo, macropodous, 246 (Fie. 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-anthpstrobilus," 315 Eu-callitriche, 236. 237, 306, 307, 330 Eucalyptus Preissiana, 256 Eitpatorium cannabinum, 188 Euphorbiaceae, 311 Europe, 290, 295 Euryale, affinities, 38; cleistogamy, 34, 234 Euryale ferox, 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, 102 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 INDEX Geophytes, aquatic, 217, 323 Geotropism, 281, 282 Germany, 17, 53, 290, 299 Germination, delayed, 36, 71, 72, 243, 244; in situ, 80, 127, 246, 248; of various genera, see heading, Seedlings Giant Waterlily. See Victoria regia Gill-tufts of Oenone, 1 18,119 (Fig. 8 1), 255 Glechoma hederacea, 5 Glyceria, aquatic grass, 317, 318; dis- tributed by birds, 301 Glyceria aquatica, 317, 318 Glyceria fluitans, 301, 317 "Gramen bulbosum aquaticum," n Gramineae, 317, 318, 326 Grass-wrack. See Zostera Grasses in Lake Ellis, 286 Grew, Nehemiah, on flower colour of aquatics, 230; on heterophylly, 154, !55 Griffithiella Hookeriana, 114 Ground Ivy. See Glechoma hederacea Guiana, British, 109, 118, 119, 300 Guiana, French, 113 Gunner a, gigantic herb, 181; polystely, 180-182, 346 Gunner a scabra, 181 Gunnereae, 312 Hairs, absence in submerged leaves, 165, 1 66 ; absence in water forms of amphi- bious plants, 151, 152 (Fig. 99); change in character of hairs of Rubus when submerged, 200 ; growth of hairs due to wounding of Waterlily petiole, 258 ; loss of hairs of Mentha when sub- merged, 201 ; mucilage-containing hairs of Ceratophyllum, 86, 87, 272; mucilage-secreting hairs of Nymphae- aceae, 38 (Fig. 20), 272 ; Myriophyllum trichomes, 168, 169, 170 (Fig. no); Polygonum amphibium, hairs on air leaves only, 151, 152 (Fig. 99); Utricu- laria, hairs of bladders, 92 (Fig. 60), 93 (Fig. 61), 94, 95 (Fig. 62); Utricularia, protective hairs of turions, 101, 102, 220 (Fig. 143 A ) ; sensitive hairs of A l- drovandia, in; stipular hairs of Nym- phaea lutea, 26; see also under Root- hairs and Squamulae intravaginales Halodule, anatomy of vegetative organs, 132 (Fig. 88), 331; marine Angio- sperm, 123 Halodule uninervis, 132 (Fig. 88) Halophila, anatomy, 131, 169; in bio- logical classification, 6; leaf form, 124; marine Angiosperm, 57, 123; pollen-grains in strings, 130; pollina- tion, hydrophilous, 130, 236 ; structure and life-history, 129, 130 (Fig. 87); styles filiform, 130 Halophila ovalis, 129, 130 (Fig. 87) Halophila ovata, 129 Halophila stipulacea, 129, 130 Haloragaceae, 180, 205, 311, 312 Haloragideae, 312 Haptera of Podostemaceae, 114; of Tristichaceae, 113 Hawthorn, pigmented variety, 276 Heliotropism, 281 Helobieae, 52, 123, 124, 245, 248, 313, 314, 318, 319, 320, 321, 325, 326 Herbarium material, use in anatomical work, 331 Herbarum vivae eicones of Brunfels, 27 and Frontispiece Herminiera elaphroxylon, 192 Heteranthera, cleistogamy, 234 (Fig. Z53); phyllodic leaf anatomy, 342 (Fig. 170), 343, 344; root differentia- tion, 207; support of inflorescence, 228; water pores and apical opening, 268 Heteranthera dubia, 234 (Fig. 153) Heteranthera renifotmis, 342 (Fig. 170), 343. 344 Heteranthera zosterae folia, 207, 228, 268, 342 (Fig. 170), 343, 344 Heterophylly, 143-162, et passim Himanthalia lorea, 114 Hippuridaceae, 312 Hippuris vulgaris, affinities, 311, 312; altitude, 290; anemophily, 230, 232; diaphragms of stem, 184 (Fig. 119), 257; flowers, 230, 231 (Fig. 151); fruit dispersal, 297 ; heterophylly, 141, 146, 147 (Fig. 95), 148 (Fig. 96), 231 (Fig. 151); in biological classification, 6; nutlets, 242; perenniation, 215; re- duction of primary root, 244 ; rhizome, 173 (Fig. 112); root-hairs from "collet," 245; stem, 172, 173; stem anatomy, 175-178 (Fig. 115), 181, 184 (Fig. 119 ), 185 (Fig. 120); sto- mates, 166; tenderness of leaves, 163 Holland, 303 Horned Pondweed. See Zannichellia Hornwort. See Ceratophyllum demersum Horse Chestnut, submerged germina- tion, 199 Hottonia palustris, affinities, 312, 318; in biological classification, 6, 7 ; land and water forms, 197 (Fig. 127); leaf anatomy, 169; non-cleistogamic, 233; polystely, 181 ; ripening of fruit in air, 239; sinking of seeds, 297; support of inflorescence, 228; vegetative repro- duction, 216 Hyacinth, Water. See Eichhornia speciosa Hydrilla, in Britain, 54, 55; leaves, 57; spathe. 315; tendril roots, 205 ( Fig. 136) Hydrillaverticillata, 54, 55, 205 (Fig. 136) Hydrilleae, 175 Hydrobryum, 114, 115 (Fig. 76) Hydrocaryaceae, 311 INDEX 427 Hydrocharis, anthocyanin, 276; buds, summer, 43 (Fig. 24); buds, winter, 47 (Fig. 29), 48, 49 (Fig. 30); de- hiscence, 47, 241 ; dioecism or monoe- cism, 46; flowers, 46; fruits, 46, 47, 241; freezing, 220; germination of turions, 48, 49 (Fig. 30), 280; in biological classification, 7; inverted bundles of leaf, 46 (Fig. 28) ; land form, 42, 49; leaf anatomy, 44 (Fig. 25), 45 (Figs. 26, 27), 46 (Fig. 28); light, effect of, 280; petiole length, experi- ments on, 283, 284; pollination, 236; root-hairs, 42, 43; roots, 42, 43, 244; stipules, 43 (Fig. 24), 44; stomates, 45 (Fig. 26) ; submerged form, 45 ; turions, 47 (Fig. 29), 48, 49 (Fig. 30); winter- buds, 47 (Fig. 29), 48, 49 (Fig. 30) Hydrocharis asiatica, 42 Hydrocharis Morsus-ranae, 7, 32, 42, 43 (Fig. 24), 44 (Fig. 25), 45 (Figs. 26, 27), 46 (Fig. 28), 47 (Fig. 29), 48, 49 (Fig. 30), 53, 54. 57, 166, 195, 215, 219, 220, 236, 241, 280, 283 Hydrocharis parnassifolia, 42 Hydrocharitaceae, fresh-water, 42-57, 84, 151, 157, 248; marine, 123, 129- 131, 133, 134; other references, 169, 205, 235, 236, 239, 314, 340 Hydrocleis, apical cavity of leaf, 269, 270 (Fig. 164); heterophylly, 157 Hydrocleis nymphoides, 157, 269, 270 (Fig. 164) Hydrocotylevulgaris, 200, 201 (Fig. 132) Hydromystria, 57 Hydrophilous pollination. See Pollina- tion, hydrophilous Hydrothrix, cleistogamy, 234; phyllode leaf, 344 Hydrothrix Gardneri, 234, 344 Hydrotriche, affinities, 313, 318; hetero- phylly, 151 Hydrotriche hottoniaefolia, 151 Illecebraceae, 311 Illecebrum, affinities, 311; cleistogamy, 234 Illecebrum verticillatum, 234 Illumination, 157, 278-280 "Imbibition theory," 174 Indehiscent fruits of aquatics, 241-2/14 India, 112, 191, 291, 305 Infusoria, as food of Utricularia, 94 Inn, River, 228 Ireland, 210 Iridaceae, 326 Iris, phyllodic anatomy, 340 Iris Pseudacorus, 199 Isoetes, altitude, 291; in peaty water, 287; in mountain lochs and heath pools, 290; ousted by Potamogeton, 333; replacement of sporangia by plantlets, 225 Isoetes amazonica, 291 Isoetes echinospora, 225 Isoetes lacustris, 225 Italy, 303 Ivy-leaved Duckweed. See Lemna trisulca Japan, 17 Juncaginaceae, 248, 313, 314 Juncus, 299, 309 Juncus conglomerate, 299 Jura lakes, 279, 287, 290, 323 Jussiaea, aerenchyma from phellogen, 189, 190 (Fig. 122); affinities, 311, 318; breathing roots, 189; replace- ment of cork by aerenchyma, 188, 189; roots not floats, 192, 193; sub- mersed leaves whorled, 230 Jussiaea amazonica, 230 Jussiaea grandiflpra, 189 Jussiaea peruviana, 189, 190 (Fig. 122) Jussiaea repens, 189, 193 Kaieteur, 109 Kerguelen's Land, 233 " Kiemenbuschel " (gill-tufts) of Oenone, 118, 1 19 (Fig. 81), 255 Kingston, 273 Kurdestan, 303 Laboul, 291 Lace-plant of Madagascar. See Apono- geton fenestralis Lads alata, 120 "Lady-Smocks," Grew on heterophylly in, 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; Callitriche, 195; Cardamine, 202 (Fig. 133); Hottonia, 197 (Fig. 127); Hydrocharis, 42, 49, 195; Lemnaceae, 77, 78; Limnan- themum, 195 ; Limosella, 198 ; Litlorella 198 (Fig. 128); Myriophyllum, 195, 223 (Fig. 146); Nymphaeaceae, 32, 195; Polygonum, 152 (Figs. 99 and i oo), 197, 198; Potamogeton, 195. i?6 (Fig. 12 5); 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," 33<>, 347 "Law of Loss," 182, 336-347 Latvia, 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), 14 (Fig. 5), 19, 20, 22, 23, 140, 141, 343, 344, etc. Leaves, floating, 30-32, 44-46, et passim 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, 140 Deduction of cuticle, 163 ; 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-4±(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 heterophylla, 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, J55 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, 168; 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 May oca 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 trifoliata, 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 Monster a, 142, 314 Montia, affinities, 310; biennial and perennial forms, 216; submerged xerpphyte, 310 Montia fontana, 216, 310 Mosses, 113 M our era, anthocyanin, 113; flowering, 120; haptera, 114 Mourera fluviatilis, 113, "4- I2° 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. 1 10) ; turions, 219, 220, 221 (Fig. 144), 222 (Fig. 145), 223 (Fig. 146), 224; wave motion, 289 Myriophyllum alter ni folium, 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. 1 75 ; 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. 14°). 2I5 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- 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 lutea 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 Aponogeton 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 Phragmites, 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 Ulricularia, in ; polystely, 181 Pinguicula vulgaris, 181 INDEX Pistia Stratiotes, air tissue of leaves, 82, J54. 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 A lisma Plantago Podostemaceae (including Tristicha- ceae), 112-122, 327-333; affinities, 310, 319; anatomy, 117, n8(Fig. 80); ancient aquatics, 321; and natural selection, 327-333; and wading birds, 300; anemophily, 120, 121 ; anthocya- nin, ii2, 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), H7;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. 34$ Polygonaceae, 311 Polygonum, affinities, 311, 318; colonis- ing new waters, 289; land and water forms, 150, 151, 152 (Figs. 99, 100), 197. 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, loo), 197, 198, 215. 271, 289, 311, 3i8 Polygonum vivtparum, 225 Polypetalae, 309-312, 319 Polystely, 37, 180-182, 346 Pondweed. See Potamogeton 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, 160, 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 (F«g- i°3). 157- »38 (Fig. 104). 159 (Fig. 105), 3.vj( 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), 340 432 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- 4°)' I^4, 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. 107), 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 per foliatus, 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 pusillus, 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, 337 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, i76(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 kederaceus, 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, -zoo 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 fruticosus, 200 Ruppia, in brackish water, 134; macro- podous embryo, 319 (Fig. 166); pollination, 70; reduction of primary root, 244 Ruppia br achy pus, 319 (Fig. 166) Sacred Lotus. See Nelumbo Nelumbo Sagittaria, band or ribbon leaves, 9, II (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. b), 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, 340, 344. 345 (Fig. 171); squamulae intravaginales, 13, 15; stolons, 15, 16 (Fig. 6), 18 (Fig. 7); tubers, n (Fig. 3), 13 (Fig. 4), 15, 16 (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-. 171) Sagittaria sagittifoha i. vallisneriifolia, II, 12 Sagittaria teres, 7, 22 Sagittaria 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 lacustris, 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); Elatine, 245 (Fig. 158); Hippuris, 245 ; Lawia, HT,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 Stum 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 A chillea ptarmica Solanum Dulcamara, 198 Soldier, Water. See Stratiotes aloides Sparganium, fruits eaten by wild ducks, 301; resemblance to Pandanaceae, 317 Sparganium natans, 317 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, 131. 33°, 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. I4L *57. l69I 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 aloid.es, 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 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 Syrcpetalae, 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 7>a/>e//a,cleistogamy,234 ; heterophylly, 151 Trapella sinensis, 234 Trianea, 45 Tri folium, sleep habits of, 161 Trifolium resupinatum, 199 (Fig. 131) Tristicha, dorsi ventral 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) ; of Hydrocharis, 47 (Fig. 29), 48, 49 (Fig. 30), 219; of Lemnaceae, 75—77, 219; of Myriophyttum, 219, 220, 221 (Fig. I44), 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,27o 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), 1 68, 299 Utricularia neglecta, 92 (Fig. 59), 93, 97, 99 (Fig. 66), 104 Utricularia nelumbifolia, 108, 109 Utricularia ochroleuca, 97 Utricularia quinqueradiata, 99 Utricularia stellaris, 99 Utricularia vulgaris, 91, 93, 94, 97, 98 (Fig. 65), zoo (Fig. 67), loi, 102, 104 (Fig. 70), 105 (Fig. 71), 106 (Fig. 72), 107 (Fig. 73), 215, 220, 227 Valerian, Grew on heterophylly in, 155 Vallisnena, 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 Vallisnena 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; evoiution 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 Limnanthcmum 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. aquatilis, 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 latifolium 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 Callitriche 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 Woljfia, 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 Zannichellia, 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, Zannichellia palustris, 134, 173, 205, thread-like, 125; pollination, 127, 206 (Fig. 137), 215, 282 129, 237 Zannichellia 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 oceanica." 125 CAMBRIDGE: PRINTED BY j. B. PEACE, M.A., AT THE UNIVERSITY PRESS HY JFORNM LES 105 A6bw UC SOUTHERN REGIONAL LIBRARY FACILITY Ill ill INI III II I!