ARNOLD ARBORETUM HARVARD UNIVERSITY BULLETIN OF POPULAR INFORMATION SERIES 3 VOLUME V 1931 Digitized by the Internet Archive in 2017 with funding from BHL-SIL-FEDLINK https://archive.org/details/bulletinofpopula03arno_3 ILLUSTRATIONS Budding a Lilac, 51 Cones of the Dunkeld Hybrid Larch and of its parent species, (plate) facing page 66 Davidia involucrata Vilmoriniana, 27 Diospyros virginiana, 43 Ficus aurea, 38 Ficus aurea, (plate) facing page 38 Graft-blight of Lilac, 7 Japanese Blossoms and Pond in the Arnold Arboretum, 57 Juniperus virginiana, 63 Koelreuteria paniculata, 47 Lonicera amoena arnoldiana, 35 Malus ioensis plena, 31 Pinetum, Highland Park. Rochester, N. Y., 55 Primus inandshurica, 11 Prumis serrulata sachalinensis “Fugenzo”, 15 Rhododendron yedoense poukhanense, 23 Sorbopyrus auricularis hulhiformis, 19 Viburnum fragrans, 3 ARNOLD ARBORETUM HARVARD UNIVERSITY BULLETIN OF POPULAR INFORMATION SERIES 3. VOL. V APRIL 23, 1931 NO. 1 Effect of Cold on Flower Buds of Trees and Shrubs. As a rule our native plants are so accustomed to the rigors and vagaries of our New England winters that few of them show serious injuries from the varying degrees of cold or heat affecting them during the months when they are dormant. It is true that the exposed catkins of Alders and Hazels may be wholly or partially destroyed and the scales covering the flower buds of our Flowering Dogwood {Cornus florida) may be- come so dried and stunted that they lose their accrescent power or vi- tality, and do not develop into the beautiful snowy flowerlike organs call- ing attention to the small inconspicuous blossoms produced by this tree, which reaches its natural northern limit of growth in eastern Massa- chusetts. Such winter injuries are not uniform and may be dependent upon the conditions of growth and maturing of tissues during the pre- vious season or upon local environment. In shelter of woods the Flow- ering Dogwood may develop beautiful large white showy bud scales when those on exposed trees are stunted and make little growth. Of course many of our really hardy native plants may be heavily damaged by unusual freezing temperatures late in the season, so that a crop of flowers or fruits may be lost for that year and even young growing shoots may be lost and the tree be forced to develop a new growth from latent or suppressed buds. As gardeners and horticulturists we have come to judge the severity of our winters largely by the behavior or condition of plants which have been brought to us from the Old World or, more rarely, those which have been introduced from other parts of our own country, chiefly from regions enjoying a milder climate than the average winter temperature of central and southern New England. Aside from the records left by thermometers, and judging merely by the general condition in the Ar- boretum, the past winter was, on the whole, fairly mild and so favor- able as to warrant our expecting a good season for flowers and fruits. This is true of many species which are in situations less favorable than others of the same kinds. For example, we do not expect exotic 1 2 vines, shrubs or small trees to show the same degree of winter cold endurance in the low ground, near the Forest Hill gate, where the gen- eral shrub collection is located, as we would expect to find in the same species at the top or near the top of the adjacent Bussey Hill. While better soil drainage conditions may be found on the hill, a factor con- ducive to better ripening of wood in the autumn, the chief factor is the better air drainage or air flow which prevails on the higher land. The low ground in which the shrub collection is located is nearly surrounded by low hills or ridges forming a sort of lake into which the cold air settles and is pocketed, forming a basic stratum which may be several degrees colder than the zone of air a few feet higher. In seasons when the winter temperature does not go much below zero (Fahrenheit) the effect of the difference in zones on certain plants may not be very apparent but if the recorded temperature falls to 12 or 15 below zero (Fahrenheit) serious damage to plant flowering may result, although leaves, twigs and branches may appear uninjured. This is well illustrated in the varieties of Peach, a native of China. In common with all early flowering trees and shrubs the flower buds are developed during the preceding summer. While some varieties of Peach have been developed in which the flower buds are considered more hardy than others a fall of temperature to 12 or 15 below zero is considered fatal to most of them, and 20 below zero is cer- tainly so in virtually all cases. That the embryo flowers are dead may readily be ascertained by splitting open some of the blossom buds a day or two after the freeze. The hearts of the buds, represent- ing the blossom portion, will be found dead and brown instead of be- ing bright clear green as they should be if free from injury. Prac- tical orchardists in the northern limits of peach growing recognize the advantage of planting Peaches where there is good air drainage as well as planting, if possible, on northerly slopes where spring growth would naturally be retarded and where there would be less danger of late frosts hurting the open flowers. Peaches and allied species of trees promise a good full bloom this spring, and this applies to Japanese Cherries in general, as well as to the garden varieties of our common Sweet Cherry {Prunus avium) from Europe and Western Asia. The flower buds of P. avium appear nearly as tender as those of the Peach, this being one reason why this plant is rarely cultivated and seldom produces fruit in northern parts of New England and other cold sections of our country where varieties of the Sour Cherry {Prunus Cei'asus) thrive. The early spring months show a good deal of variation in the time of flowering of many of the precocious or very early flowering species, dependent on the number of warm days and degrees of temperature which prevail in any particular season. It may be interesting to note some of the species which have already blossomed or which are now in blossom in the Arboretum. As usual, the White or Silver Maple is the first of the large trees to flower. This spring it blossomed during the last two weeks of Viburnum fragrans Photograp)hed in the Arnold Arboretum, April 15, 1931 by Professor Oakes Ames 4 March and early days of April and it is usually at about that season we should expect the flowers to appear. But we have records of well developed flowers appearing on January 24th, 1913, on February 23rd, 1915, and February 1st. 1916, these records usually being taken from the same tree. At the present time the Red Maple (Acer rubrum) is in blossom, displaying shades of color from honey yellow to deep red in different individuals. Our American Elm, too, is in flower and the flow- ers of some of our F’oplars are passing or gone. '1 he spring flower- ing Witchhazels are now past their blossoming stage, the earliest being the southern Hamamelis verrmlis, some of the flowers of which may open during warm days in January. It is not so beautiful and inter- esting in blossom as the Chinese Hamamelis mollis which is so precoc- ious that the flowers are occasionally severely damaged or are destroyed by hard frosts. An interesting shrub, though not especially beautiful, is the Leatherwood [Dirca palustris) native in northeastern America. Given proper chance for development it may form a short trunk branching to the ground or near it and producing pretty little bell- like yellow flowers scattered over the slender, tough barked, .leafless twigs. It requires no special care in cultivation except freedom from too close crowding by other shrubs or vegetation or too much shading by trees. Among the most interesting of the newer shrubs in full flower (April 15th) in the Arboretum collection, is Viburnum fragrans. This spec- imen is now 6 or 7 feet high, the leafless branches well covered with the small panicles of fragrant, salver-shaped flowers which are pink in bud but white or light pinkish white when open. Although first de- scribed by the botanist Bunge nearly 100 years ago (1835) this species appears not to have been introduced into cultivation in Europe or America until about twenty years ago when seeds were collected in northern China by the late William Purdom for this Arboretum and for the nursery firm of Veitch, in England. The late Reginald Farrer collected seeds in 1914 which were also sent to England, producing plants which were soon distributed in English gardens. It is said to be a shrub attaining 8 or 9 feet in height and is described by Farrer as bearing “profuse trusses of pearl-pink flowers in spring like lilac deliciously smelling of heliotrope”. He also described it as “rare as a wild plant but generally cultivated in Kansu (China) for its loveli- ness and fragrance”. This species is apparently going to rank with our earliest flowering shrubs. In England it has been called the Win- ter-flowering Viburnum and in the southern part of the British Isles it is stated that the bushes flower more or less throughout the winter, some blossoms attempting to open before the leaves fall from the branches. Farrer described the fruits as shining scarlet and of good flavor. J. G. J. The subscription to this Bulletin is $1.00 per year. ARNOLD ARBORETUM HARVARD UNIVERSITY BULLETIN OF POPULAR INFORMATION SERIES 3. VOL. V MAY 2, 1931 NO. 2 Graft-Blight of Lilac. The result of the discovery of the use of Privet in the propagation of Lilacs has been that to-day practically half of the Lilacs grown in America are privet-grafted. The remaining half of the Lilacs in this country are grown by various own-root methods. As might well be expected, when one considers the various methods of propagation employed, there has been a certain amount of contro- versy among the nurserymen as to the most satisfactory method of li- lac propagation. In fact while nurserymen in general almost univer- sally condemn the grafting of Lilac upon common Lilac, there are two distinct schools of opinion among them supporting respectively the own- root methods and the methods involving the use of Privet. Each of these schools of opinion has backed its contentions with numerous asser- tions as to the relative superiority of Lilacs propagated by one of the methods considered, but neither group has subjected its beliefs to sci- entific analysis. The extensive lilac collection in the Arnold Arboretum is constantly being added to by new plants received from a variety of nurseries as well as by plants propagated by cuttings in the Arnold Arboretum greenhouses. During comparatively recent years it has become increas- ingly evident that many of these newer plants were manifesting sym- ptoms of disease, and the diseased plants eventually became so numer- ous and unsightly as to require a thorough investigation for the purpose of determining and eliminating the cause of the diseased condition. The diseased Lilacs in question showed symptoms of chronic nutritional deficiency. The plants were very small and their growth exceedingly limited. When fifty diseased plants were measured during their sixth and seventh years of growth it was found that their average growth during the seventh year was an inch per plant. Practically none had blossomed even after nearly a decade of culture in some cases. When the leaves appeared in the spring they were usually very small and dark in comparison with leaves of normal Lilacs, and were gathered in little tufts at the tips of the thin, frail twigs. As the summer wore on the leaves became curled and very yellow, this yellowing beginning at the 5 6 tip and between the larger veins and proceeding eventually to involve the w'hole leaf. The leaves during this yellow statze w'ere notably thick- er than normal and markedly brittle. Leaf-cast began early during the summer and the plants usually w'ere devoid of leaves by midsummer, although in some cases the dead leaves actually persisted on the twigs long after normal leaf-fall. The new grow'th in a given season was very limited. The buds w'ere w'eak, the twigs frail, and the plants in an obviously hazardous condition to meet the exigencies of an unfavor- able season. The accompanying illustration is a photograph of a Lilac showing the disease in typical form. The curled, pale leaves are seen in their characteristic diseased condition, and the severity of the disease is very evident. Early in the investigation of these diseased Lilacs it became evident that all of the plants in question had been propagated by grafting up- on Privet. In fact the distribution of the disease in the Arnold Arbo- retum almost exactly coincided with the distribution of the privet- grafted lilac plants which had been received in recent years. It was soon observed and demonstrated that the disease w’as not contagious, that it W’as not due to unfavorable soil or moisture conditions, and that it W’as not restricted to certain lilac varieties, but that it was evident on old, long-proven varieties as w’ell as on the newer French hybrid Lilacs. Since the disease under consideration w’as always associated w’ith the grafting of Lilac upon Privet, and since it has been found that the disease is caused by such grafting, it has received the name “Graft-blight of Lilac”. On investigation it w’as found that graft-blight was not limited in its distribution to the Arnold Arboretum. It was seen in typical form in numerous nurseries and private collections of Lilacs in New’ Eng- land, New’ York State, New Jersey, Pennsylvania, and Ontario. Canada, while what w’as unmistakably the same disease was reported by cor- respondents in Oregon, Ohio, Colorado, and Germany. That large num- bers of Lilacs w’ere involved w’as evident both from the observations and from the reports. In order to investigate the effect of grafting Lilacs upon Privet, a set of experiments in lilac grafting w’ere planned and executed in the Arnold Arboretum greenhouses and nursery beds. Lilac scions of one of the most vigorous varieties in culture (Andenken an Ludwig Spath) W’ere grafted upon a number of species of Privet, Ash, Forsythia, Chi- onanthus, and Lilac. These w’ere compared in their development w’ith the normal health of sister lilac scions grafted upon common Lilac, rooted in the soil directly, and growing on the parent plants. The grafts of Lilac upon Ash, Forsythia, and Chionanthus proved unsuccessful, those upon common Lilac were entirely successful, w’hile those graft- ed upon the various species of Privet show’ed in characteristic form the same condition as the graft-blighted Lilacs in the Arnold Arbore- tum ornamental collection described above. The lilac scions grafted upon the Calfornia Privet {Ligustrum ovali- folium) and the Amur Privet {Ligustrum amurense) w’ere especially Graft-blight of Lilac. The horizontal line approximately divides the privet section of the root-system from the adventitious lilac roots. The healthy appearance of the sucker is explained in the text. 8 significant. In the former case the grafted plants showed precisely the same diseased condition as Lilacs in the field suffering from graft- blight, while on the other hand, the Amur Privet grafts showed in a single season the whole course of symptoms, stunting, yellowing, leaf- curl, leaf-cast, and death, evidenced by California Privet grafts during the course of several years. Meanwhile the Lilac-on common-Lilac grafts and the own-rooted scions remained strong and healthy. A con- firmation of these experiments resulted from examinations of many own-rooted and grafted Lilacs in nurseries. Here the symptoms of graft-blight were always associated with privet grafting. Finally fur- ther evidence resulted from the observation that even in privet-graft- ed Lilacs, occasionally a lilac sucker found to be on its own roots, is seen in perfect health although attached to a severly blighted plant. Hence it was plainly evident from these experiments and observa- tions that the cause of the disease observed in the Arnold Arboretum lilac collection lay in the practice of grafting the Lilac upon Privet, particularly upon the California Privet. To obtain further information a questionnaire was submitted to the majority of the larger lilac produc- ers in America and to some in Europe. This investigation yielded a number of interesting and significant facts regarding the commercial practices of lilac propagation. Since it was discovered by this means that half the Lilacs in culture in Am- erica were propagated by own-root methods, the practicability of own- root lilac propagation was at once evident. But it was found that privet-grafted Lilacs are much quicker in attaining marketable size than Lilacs grown from cuttings. This fact implies that the privet- grafted Lilac may be sold at a somewhat lower price than the owm-root Lilac and still afford a reasonable profit on the nurseryman’s invest- ments. Hence the problem is plainly an economic one, and the solution lies in the hands of the purchaser of Lilacs. The nurseryman who uses the privet-grafting method of propagat- ing Lilacs does so, in most cases at least, with the conscious belief that such Lilacs will soon become own-rooted and throw off the privet stock. Many observations, however, show that this does not invariably happen. In fact, in the majority of cases privet-grafted lilacs are still dependent on their privet roots years after the most careful propaga- tion on Privet. Graft-blight results, but since the symptoms are seen in the collection of the purchaser and not in the nursery of the prop- agator there is a tendency for the nurseryman to be unaware of the diseased condition or to attribute it to unfavorable soil or culture. The purchaser of Lilacs is anxious to obtain permanent, long-lived, healthy plants. The additional cost of own-rooted Lilacs is but a small fraction of the original cost of the plants and a still smaller fraction of their value as ornamentals. Moreover there are nurserymen in every section of the United States who can provide the finer named varieties of Lilacs propagated by own-root methods. The nurseryman will produce what the purchaser demands. Hence the connoisseur of Lilacs will toler- ate within his ornamental planting only own-rooted Lilacs, thereby escap- ing at once the winter-killing of privet-grafted Lilacs, the danger of pre- dominance of privet suckers, and the penalty of graft-blight. K. S. Chester. ARNOLD ARBORETUM HARVARD UNIVERSITY BULLETIN OF POPULAR INFORMATION SERIES 3. VOL. V MAY 6, 1931 NO. 3 Forsythias. Probably no hardy shrubs which have been introduced from the Old World into our gardens can equal or surpass the Forsy- thias in cheerful beauty, which is developed at a season when there are comparatively few other competitors so attractive. The beautiful gol- den glow of their early bloom attracts and pleases almost everyone who is acquainted with them, and they are destined to have wide popularity because of their attractive flowers, generally clean foliage, and com- parative freedom from insect or fungous diseases. More important still as a circumstance in favor of popularity and wide distribution is the fact that the Forsythias may be propagated by division with greater ease or facility than most other trees or shrubs cultivated in our gar- dens. Least trouble will be found by propagating from cuttings of the ripe wood taken in fall or winter or very early spring and planted in the open ground. Of course where the ground freezes in winter and cuttings are taken at that season they may be kept fresh by heeling in or burying in moist earth in a cellar or pit until seasonable time to plant out. Good sturdy well rooted plants should develop before the following autumn. Propagation may also be effected by taking green cuttings in summer and growing them in a frame, a method entailing more trouble or labor than when cuttings of ripe wood are used. As in nature’s methods propagation may be easily brought about by the process of layering, and it will be found that a slender arching spe- cies, like Forsythia suspensa, commonly forms roots on the stems or tips, where they touch the ground, in this way forming new plants on the periphery of the original stem. Thus a single plant of an arch- ing or trailing species may in time cover a large area if no competing plants interfere. The facility with which this genus of plants may be propagated makes their dissemination or distribution almost indepen- dent of the commercial nurseryman, excepting in the case of newly in- troduced species or improved forms which are new or little known. f^orsythias require little pruning except to cut away weak or old stems. It should be remembered that flower buds are formed during 9 10 the preceding summer, so that cutting back of the stems should be done immediately after flowering in spring. Pruning in autumn or winter involves the destruction of some of the best flowering branch- es for the next spring. If it is desired to raise plants from seed it must be borne in mind that the resulting plants are not always just like the parent and some may be inferior. The seeds of Forsythias are large, flat and somewhat winged and are probably only disseminated by wind agency, or by be- ing carried by water or moving soil during freshets. All of the known species of Forsythia have yellow flowers, varying somewhat in size, form, and shade of color of blossoms in different species and varieties. To the botanist and close observer of Nature the flowers have peculiar interest in the fact that where grown from seed some plants have short styles (microstyle) while in others the styles are longer and more prominent (macrostyle). This is a provision of Nature to insure cross pollination of the species. The general hab- it of growth of the various types is one of the most important fea- tures to be considered by the planter. The Forsythias have been described as hardy but there is a limit to their endurance of such cold as we often have in northern New Eng- land, While the plants may endure lower temperatures, in some cases the flower buds are very likely to be destroyed if subjected to a tem- perature of 15 or 20 degrees below zero (Fahrenheit), In a climate like that of the interior of northern New England, or that which pre- vails at Montreal or Ottawa, Canada, the Forsythias may be classed as only half hardy, being killed to snow line, at least, in winter. Occasion- ally the flower buds have been killed in the Arboretum while the plants have shown little injury. What we call hardiness may in some meas- ure be affected by location, drainage, maturity of wood, length of sea- son and other factors. The earliest species to flower this season was Forsythia orata which has been described as the hardiest of the genus. It is not yet well known in cultivation and whether the plants and flower buds will with- stand 15 or 20 degrees below zero or lower remains to be proved by thorough test. As growing at the Arboretum it is a rather compact upright or arching bush, 6 to 8 feet high. Its habit is much like that of F. intermedia already well known and superior to F. ovata by rea- son of its larger, deeper yellow and more profusely produced flowers which, however, may be a week or ten days later, Forsythia intermedia itself is not considered as a true species but is a hybrid of the rigid upright growing F. viridissima and the more free growing, slender, long-stemmed F. susperisa. Of this hybrid there are several cultivated forms, one of the best and most florifer- ous being that known as Forsythia intermedia, spectahilis. It usually bears an abundance of rich golden yellow flowers. Another form, known as F. intermedia primulina, bears flowers of a pale yellow or primrose color. It is generally considered less desirable than specta- bilis except by those who prefer the lighter shade of yellow color. Prunus mandshurica (Height 20 feet, spread 30 feet) Photographed in the Arnold Arboretum, April 17, 1927, by Dr. E. H. Wilson 12 Those desiring a slender branched or trailing form of Forsythia should select F. suffpensa Sieboldii which is stated to have been in- troduced into Europe from Japanese gardens nearly one hundred years ago (1833). There are several forms or varieties of this species in cul- tivation, a good and common one being known as F. suspensa Fortu- nei {F. Fortunei of some catalogues). But, although more floriferous, it lacks the peculiar slender vine-like effect of F. suspensa Sieboldii which may be used as a covering for arbors, trellises or as an effective pend- ulous covering over rocks, walls, etc. In such situations it may grow many yards in length, forming a graceful network or screen. Early Flowering Apricots and Cherries. Altogether these early flowering trees and shrubs passed through the winter with very little injury from winter cold, although there were exceptions. In the Ar- boretum the Manchurian Apricot, Prumis mandshurica, flowered well this year, an event which is by no means annual as the flower buds are so often destroyed by winter freezes which kill the blossoms in Peach and some of the other species of Prunus. The flower buds are pink, the blossoms pale pinkish or white, the fruit yellow and about an inch in diameter. While attractive in flower this species also ap- pears to be one with great possibilities for improvement in its fruit which may become a valuable addition to those we have already do- mesticated. Its habit of flowering decidedly earlier than the Peach is a factor against it in northern climates, although the tree itself appears larger and much more rugged than the average Peach. It was in full bloom in the Arboretum on April 18th this season. Some of the flow- er buds were destroyed during the past winter but not enough to af- fect the general beauty of bloom. Such injury may be ascribed to the fact that our tree under observation is located upon low ground where it may have been subjected to several degrees below zero (Fahrenheit). It is about 25 feet high with a diameter spread of branches of about 35 feet. This season the Sargent Cherry, Prnnns serrulata sack aline n sis, might have been registered as in fullest bloom about April 22nd, the same date as recorded for full inflorescence of the same tree in 1927. The pink flowers of this species are rather fugacious and their beauty is past within three or four days if the weather is warm. It is a mis- take to plant this tree in a small garden, with only eight or ten feet allowed for spread, because if given favorable conditions it may be- come a tree at least 40 or 50 feet high with an equal spread of bran- ches which may be produced near the ground if allowed to do so or if the tree is not crowded by other competing plants. While the flowers have usually been described as pink there appears to be a good deal of variation in individuals raised from seed, some having flowers al- most white, while in others the blossoms may have a rather unattrac- tive light pinkish color. Reproduction for assurance of any particular shade of flowers would appear to require grafting or budding, though such plants may never attain the symmetry or proportions of those raised from seed. A good plan is to secure several seedlings where one is wanted and to select the most desirable one when they flower, dispos- ing of the others. J. G. J. ARNOLD ARBORETUM HARVARD UNiVERSITY BULLETIN OF POPULAR INFORMATION SERIES 3. VOL. V MAY 9. 1931 NO. 4 Flowering Cherries. Primus subhirtella, which probably attracts more attention in early spring than any other Cherry in the Arboretum collection has often been referred to in these Bulletins. It was at its best on April 27th. Although really in attractive flower several days earlier it was kept in cheek by the cool weather. P. subhirtella is a tree for which enthusiasts often allow a space of perhaps ten feet in their gardens, whereas a much greater area should be allowed for its full ultimate development. The largest plant near the Forest Hills Gate is now 25 or 30 feet high and has a spread of branches of 50 feet in diameter near the ground. This is another plant which does not always come ideally true from seed. Seedlings may produce plants of erect, spreading, or more or less pendulous habit and may bear flowers of poor form and substance compared with the parent, so that if such plants are used it is wise to have several of them from which to select the most desirable individual. The plants usually flower early so that selec- tion may be made while they are small. Otherwise the best forms must be propagated by division, as by grafting and budding. This species may also be grown from cuttings but plants are not easily propagated in this way. In making selections of forms or varieties of this species there are, besides the type, two of outstanding interest or attractiveness. The more common one, Prunus subhirtella pendula, is a favorite and best known in gardens, while Prunus subhirtella autumnalis is less known but is equally worthy of cultivation. Its pink flowers appear at about the same time and are inclined to be semi-double instead of single. It bears beau- tiful and abundant bloom in spring, but also has the habit of bearing a good many flowers in autumn, hence the varietal name which has been given to it. Its flower buds appear to be more easily damaged by winter cold than those of the typical P. subhirtella. Our notes taken in 1923 showed about 75^0 of the buds destroyed in winter while those of the type, growing by its side, appeared to be uninjured. 13 14 It is an interesting fact that practically all of the double-flowered forms of Cherries are appreciably later in flowering than the single flowered progenitors, so that no common date can be given which would cover the flowering period of all the forms related to a single species. Prunus yedoensis, Yoshino Cherry, has had a very full load of its white blossoms this season and, on account of several days of cold weather, it has remained in good attractive flower longer than usual, notwithstanding winds and rainstorms, eight or ten days intervening between the opening of the first flowers about April 25th and the first dropping of petals. Hot days, such as sometimes occur at this season, would cause a preliminary dropping of petals within two or three days of flower opening. This species has been described as the dominant shade and flowering Cherry planted in and about Tokio, Japan, for the celebrated cherry blossom festivals which are annual events there. It is a sturdy species, forming a broad, horizontal branched, rounded top which may easily spread 50 feet or more across although it takes many years to reach such proportions. The origin of this tree is unknown, but by some specialists it has been called a possible hybrid between two Japanese species, a supposition which is hard to prove. It is a curious fact that while descriptions and illustrations of Cher- ries in and about Tokio usually state or show the blossoms as pink, ex- perience with the species, if it is a species, in America is that the color is white or has a very light pinkish trace in the petals when expanded, the buds being decidedly pink as seen from the backs of the petals. The famous Cherry plantation about the Potomac Basin in Washington, which is usually in good bloom about the second week in April, is apparently mainly the Yoshino Cherry. Its flowers are white or with but a faint trace of pink, as are those at the Arboretum. Our foremost authority (Rehder) in America, in his “Manual”, describes the flowers as “white to pink”, coinciding with the description of other students and writers in this country. The description of the Yoshino Cherry as given in one of the best known Japanese nursery catalogues is “single light pink of medium sized flower, best kinds for avenue planting and park for its quick growth and beautiful blossom and no park without this tree seems perfect, grows most freely in any soil”. This question of the differences in color as portrayed in most Jap- anese pictures and as actually developed on plants in this country is one which is puzzling and disconcerting to the average planter and it remains one which requires careful study and investigation. The blossoms of the single flowered Cherries are fast fading away and as they lose their attractiveness the double-flowered forms, or hybrids, come into their most showy condition. This year that period appears to be during the second week of May. There are many named forms of these beautiful plants, some derived from species, others orig- inating from hybrids, all grafted or budded and all probably much slower growing and never attaining the size of their naturally single flowered progenitors. They should generally be considered as short lived as compared with the types, but this is not always the case. Primus serrulata sachalinensis “F'ugenzo” Photographed in the Arnold Arboretum, June 9, 1924, by Dr. E. H. Wilson 16 Besides local conditions much depends upon the stocks upon which they are budded or grafted, a subject which may be referred to in later issues of these Bulletins. A beautiful double flowered species is the well known and often planted Prunus avium plena, which is often listed as Primus (Cerasus) avium multiplex. Originally derived from the Sweet or Mazzard Cherry, of Europe and Western Asia, this form has developed flowers which have the aspect of very small fully double white roses borne in umbels of several clusters. For those who wish for a suggestion of a selection of several of the best and hardiest of the Japanese double forms the following may be mentioned as very satisfactory. They are regarded as forms of the fairly hardy Prunus serrulata. Some forms of Prunus Lannesi ana may perhaps be regarded as more beautiful but they are not so hardy as P. serrulata. Of this species there are now a good many forms offered in American nurseries, six of the best may be found under the following names: Albo-rosea, pink in bud changing to white; Fugenzo, rose pink; Hisakura, pale pink; Horinji, pale pink, clustered; Kirin, rose colored, large flowered, late; Shogetsu ( = P. serrulata sachalinensis superhn) pale pink, large, late. The Japanese name of the last is said to sig- nify “moonlight through Fine branches”. There are many others which may be grown in our climate, though with less confidence in their immunity from winter injury, particularly those belonging to the Prunus Lannesiana group, of which the double or semi-double form known as P. Lannesiana sirotae, sold by some Japanese nurseries as a '"Mount Fuji”, with large, fragrant, pure white blossoms, is probably the finest known of all double Cherries. Recently we received an inquiry from a correspondent who wishes to know the species and varities of choice Japanese Cherries which will grow at an altitude of 1500 or 2000 feet in northern New York State. The answer to such a question is that probably none of them would live and thrive and blossom in that latitude and that altitude, where the temperature frequently falls to from 10 to 20 below zero (Fahrenheit). If the trees were not killed or badly injured, their flower buds would almost always certainly be destroyed every winter unless the season hap- pened to be exceptionally mild for the region. Possibly the more north- erly forms of such a species of variety as the Sargent Cherry, Prunus serrulata sac/za/inensis, which some botanists think should be regarded as a distinct species {Prunus Sargentii), might be able to withstand the rig- ors of our cold north country but such particularly hardy plants remain to be introduced and proved. While a few of the double flowered Cherries are ’to be seen near the Forest Hills Gate the best specimens are to be found in a group on top of Bussey Hill. The Japanese or Flowering Quince, Chaenomeles lagenaria and various forms, often known as Pyrus japonica in nurseries and persistently called Cydonia japonica in “Standardized Plant Names” are in flower now in the shrub collection. J. G. J. ARNOLD ARBORETUM HARVARD UNIVERSITY BULLETIN OF POPULAR INFORMATION SERIES 3. VOL. V MAY 15, 1931 NO. 5 Plant Hybrids. Many of our most valuable ornamental shrubs have been produced by natural or artificial hybridization. Among the best examples are the hybrid Lilacs and Rhododendrons. Most of the numerous varieties of Lilacs have been produced by hybridization and selection by Lemoine in France. These hybrids are for the most part confined to crosses within the species Syringa vulgaris. Hybrids can- not be made between the Villosae and Vulgares Lilacs and neither of these groups will cross with the Tree Lilacs. Within the Vulgares group hybrids are known between 5. vulgaris and S', oblata and be- tween S. persica and S. vulgaris. The latter hybrid, S. chmensis, first originated spontaneously but was subsequently produced by crossing the parental species. Rhododendron hybrids were produced for the most part by English plant breeders, although many crosses between species have occurred spontaneously in cultivation. The best known varieties of Rhododendron have been produced by crossing the Ameri- can R. catawbiense with European or Asiatic species. Most of the species of Rhododendrons are interfertile and in some cases hybrids have been made between the true Rhododendrons and the Azaleas. In the genus Philadelphus we have numerous examples of hybrids betv/een species and it seems probable that all species in this genus are interfertile. Several hybrids are known between American and Asiatic species. These parental species must have been separated long before the Glacial Period, yet when they are brought together and crossed they produce perfectly fertile hybrids. Similar cases are known in the Rhododendrons and Apples. The Arnold Arboretum offers unusual opportunities for the plant breeder because of the extensive collection of both American and Oriental species. Many of these have never before been brought to- gether in the same collection so that they have never had an oppor- tunity to cross naturally or to be combined by the plant breeder. In many cases it is impossible to cross distinct species but in certain genera or subfamilies crosses between species are easily obtained and in certain cases even generic hybrids are possible. 17 IS In the Pomoideae a number of generic hybrids are known to occur. Near the Forest Hills Gate is a large tree of Sorhopyrus which is a hybrid between Sorbus and Pyrus. Along the Willow path there is a small specimen of Amelasorbus, a natural hybrid between Amelan- chier and Sorbus, which was discovered in Idaho by J. G. Jack. Another generic hybrid in this group of plants is Sorbaronia, a hybrid between Sorbus and Aronia. This hybrid is in the Shrub Collection. It has been found growing spontaneously in several regions in North America. Hybrids have also originated spontaneously between Cratae- gus and Mespilus and between Pyms and Cydonia. In this subfamily there are many ornamental trees and shrubs, as Apples, Pears, Flowering Quince, Sorbus, Amelanchier, Coton- easter, and Crataegus. It is possible that additional hybrids can be made between some of these genera. This spring a number of crosses have been made between Amelanchier and Pyrus, the Japanese Quince and the Pear, between Cotoneaster and Crataegus. It is to be expected that many of these crosses will be unsuccessful, but if a few hybrids are obtained they should be of considerable interest and probably of some ornamental value. The Lilacs offer some interesting opportunities for the plant breeder. Most of the earlier breeding work has been confined to the species S. vulgaris. There are, however, two other species which are very valuable, especially if they can be combined with the flowering habit of the common Lilac. Syringa pubescens is the most fragrant of all Lilacs but the flowers are not showy and the shrub is not grace- ful in its habit of growth. If this species could be combined with S. vulgaris it should give us an entirely new type of Lilac. Syringa pinnatifolia is one of the most attractive Lilacs in its habit of growth but the flowers are borne in small clusters. This species combined with either S. pubescens or N. vidgaris should produce some interesting hybrids. The Persian Lilac is also a valuable species for the plant breeder although when crossed with N. vulgaris it produces a sterile hybrid so that the cross cannot be carried beyond the first generation. Many of these sterile hybrids are valuable, however, be- cause they flower freely and although they set no seed they can be propagated by grafting or by cuttings. The Technique of Crossing. The methods used in making crosses between species or varieties are dependent on the natural method of pollination. Plants are chiefly insect pollinated, wind pollinated, or self pollinated. Most plants with conspicuous flowers are insect pollinated while those with inconspicuous flowers are usually wind pollinated. Among the former are the Apples and related genera, while the wind pollinated plants include the Conifers, the Oaks, and most of the grasses. All grasses are not wind pollinated, contrary to the statement in botanical textbooks, and among the economic genera Wheat, Oats and Barley are all normally self pollinated. SorhopyruH auricularis var. hulbiformis Photographed in the Arnold Arboretum, May 24, 1924, by E. H. Wilson 20 In order to cross two species or varieties it is first necessary to prevent the normal pollination. With insect pollinated plants this is easily done by removing the anthers and petals just before the flower opens. The insects will not usually visit a flower which has the petals removed so that there is only a slight chance for pollination to occur. Apple flowers for instance are emasculated by removing the calyx together with the petals and anthers just before the flower opens leav- ing the stigmas exposed. Pollen from the male parent is then applied to the stigmas and the cross is made. It is advisable to leave a check branch on which the flowers have been emasculated and not pollinated to see if seeds set, due to insect or wind pollination. If the check branches do set fruit there is no assurance that the pollinated flowers will produce hybrids. This method of emasculation is known to be satisfactory with Apples and related genera, but it may not be ap- plicable in all cases. If the stigmas are large and moist there is a chance that smaller insects will visit them and thus effect pollination. For such cases the flowers should be covered with a paper bag after emasculation and kept covered until the petals would normally drop. In the case of wind pollinated flowers, it is necessary to cover the female flower before the stigma is receptive. For instance, in corn the ear is covered with a bag before the silks appear. When the silk is developed the bag is removed just long enough to dust on the pollen which has been collected and then covered again until all chance of wind pollination has passed. In the Pine the small cones are covered in the same way and when they are receptive the pollen from another species is applied and the cone again covered. Lilacs are easily crossed by pulling off the corolla to which the anthers are attached and brushing the exposed stigma with a freshly opened anther from a mature flower of the other parental species or variety. Lilies are also easy to cross although many species seem to be intersterile. The anthers and petals are removed from the female parent just before the flower opens and the stigma covered with pollen from the male parent. In the case of the Lily it is prob- ably best to cover the flower with a bag since small insects may carry the pollen to the stigma even when the petals are removed. A number of crosses have been made at the Arboretum using E. H. Wilson’s Regal Lily as one of the parents, but since it takes from 3 to 6 years for a Lily to flower from seed it is too early to know the results of this work. At best the breeding of new varieties of shrubs and trees is a long process. The hybrids cannot be expected to flower for several years at least and in some cases one must wait ten years or more to determine the flowering characters of the hybrid. Often a second generation is necessary in order to obtain the desired combination of characters and another five or ten years must elapse before the results are known. Nevertheless, it is a fascinating occupation and full of interesting surprises. One great advantage of working with trees and shrubs is that any hybrid obtained can be propagated by cuttings or by grafting even if it does not breed true from seeds or is sterile. Karl Sax. This Bulletin to replace No. 5 previously issued ARNOLD ARBORETUM HARVARD UNIVERSITY BULLETIN OF POPULAR INFORMATION SERIES 3. VOL. V MAY 20, 1931 NO. 6 Various Shrubs. With the distribution of this Bulletin special at- tention should be called to various shrubs now passing or still in good flowering condition. Our native Hobble-bush, Viburnum olnifolium, has been in blossom for two weeks and has been conspicuous with its white, bract-like, sterile flowers. This shy, northern, shade-loving, woodland shrub may be induced to grow under cultivation if given a cool and somewhat shaded location such as it has near the Birch group in the Arboretum. Near it, in the Viburnum group, the visitor will find the much and deservedly advertised Viburnum Carlesii in passing bloom with its clusters of pinkish white, fragrant flowers. V. bitchiu- ense, close by, has similar flowers but they are borne in more open cymes, the petals being wider spreading. The plant is less compact and less attractive than V. Ca/rlesii. The flowers of these two Vi- burnums, especially those of V. Carlesii, suggest Daphne Cneorum by their delightful odor and appearance, except that the flowers of the Daphne are of a deeper color. A fine group of this pretty, evergreen, little shrub is in full inflorescence along the roadside near the Hick- ory and Chestnut group, between the Centre Street gate and Hem- lock Hill. Farther along this same road may be seen the purple bloom of Rhododendron canadense, more familiarly known as Rhodora. In the same family, Ericaceae, belongs an interesting little Heather, Eri- ca darleyensis, with rosy-red flowers and dark red anthers. It is pass- ing but has been flowering for several weeks past and, indeed, .‘