February 18, 1891.] 



Garden and Forest. 



75 



ward of 800 species of flowering plants, belonging to very 

 nearly 500 different genera and no natural orders — propor- 

 tions only paralleled in some insular floras. It is not sup- 

 posed, however, that these figures represent the true propor- 

 tions of species, genera and orders in the whole flora of the 

 Shan Hills, though the only kind of selection exercised in 

 forming the collection was the exclusion of common and 

 familiar weeds. 

 Kew. W- Botting Hemsley. 



Notes on North American Trees. — XXIII. 



45, Rhamnus Caroliniana. — This name of the arborescent 

 Rhamnus of the eastern states was published in 1787. 

 Miller, in the eighth edition of his "Gardeners' Dictionary," 

 published in 1768, describes a Frangula Americana, which 

 is perhaps this plant. Miller's description, however, can 

 as well be applied to Rhamnus alnifolia, to which De Can- 

 dolle (" Prod." ii., 25) doubtfully referred it. Unless, there- 

 fore, the identity of Frangula Americana with Rhamnus 

 Caroliniana can be satisfactorily established, Walter's name 

 should be retained. 



47, Rhamnus Purshiana. — The black-fruited Buckthorns 

 of the Pacific coast have lately been critically studied by 

 Professor H. H. Rusby, of the College of Pharmacy, New 

 York, who has contributed an important paper on the sub- 

 ject to the Druggist Bulletin for October, 1890; and by Mrs. 

 M. K. Brandegee, the curator of the botanical department 

 of the California Academy of Science, who has had excel- 

 lent opportunities to study these plants in the field, and 

 who has most obligingly communicated to me a large 

 number of specimens collected in different parts of Cali- 

 fornia and Oregon. The two working independently 

 arrive at quite different conclusions with regard to the 

 specific rank of the two plants known as Rhamnus Purshiana 

 and R. Californica. Professor Rusby finds sufficient ground 

 for keeping these two plants specifically distinct, and later 

 investigations lead him to consider not only these two 

 distinct, but incline him to give specific rank also to the 

 pubescent form of the Mexican boundary region to the 

 mountain form characterized by Mr. Greene as Rhamnus 

 rubra, and to what seems a small-leaved form of the 

 northern Rhamnus Purshiana distributed several years ago 

 as R. occiden/alis by Mr. Thomas Howell, of Oregon. The 

 principal differences Professor Rusby finds ill these plants 

 are : the ordinarily greater size of R. Purshiana ; the length 

 of the petioles, which "are commonly two or three times 

 as long in R. Purshiana/' and in the leaves, in which he 

 finds the chief distinguishing characters. "Those of R. 

 Purshiana" he describes as " from two to six inches long, 

 broadly elliptical, the base occasionally slightly cordate, 

 but generally rounded or very slightly produced, and the 

 apex rounded or very shortly pointed. The margin varies 

 from nearly entire to slightly crenate and finally serrate. 

 They are deciduous, thin and veiny ; green on both sides, 

 but somewhat paler below. The lower surface is densely 

 and softly hairy, the veins hirsute. The upper surface is 

 softly hairy when young, but becomes smooth, or nearly 

 so, except within the strong channel along the midrib, 

 which remains reddish or whitish-hirsute," while the leaves 

 of Rhamnus Caroliniana " are only half the length of the 

 last, elliptical-oblong or obovate, rounded or slightly heart- 

 shaped at the base and occasionally short-pointed. They 

 are often rounded at the apex, the margins varying from 

 densely serrate to almost entire. They are thick and per- 

 sistent, and generally evergreen. The under surface is 

 strongly reticulate, reddish-veined, and the primary veins 

 not nearly as numerous, straight or fine as those of Rhamnus 

 Purshiana. Above the channel of the midrib is shallow 

 and inconspicuous and smooth within, or there may be 

 none. In this channel of the midrib I seem to have found 

 an excellent characteristic." But it is in the structure of 

 the bark of the two plants revealed by the microscope that 

 Professor Rusby finds the unfailing guide by which they 

 may be distinguished. "The characters and arrangement 

 of the bast bundles, resin areas and medullary rays form 



quite as good evidence as do the leaf characters above de- 

 tailed, and guide us to the recognition of the barks alone, 

 which the leaf characters cannot do. The general structure 

 of both exhibits medullary rays supported on either side 

 with small bast bundles, circumferentially elongated, of an 

 elliptical form. Of these there are sometimes two side by 

 side between two medullary rays, while at other times a 

 single large one reaches from one medullary ray to the 

 other. In Rhamnus Californica they are more likely to 

 occur in pairs. At the region where the medullary rays 

 terminate, and extending from there to the corky layer, 

 begins a zone of very large resin spaces of variable size 

 and form, but many times larger than the bast bundles, 

 which in appearance resemble them very closely indeed." 



Mrs. Brandegee (Zoe., i., 240), who passes over the 

 characters based on the nature and the arrangement of the 

 bast bundles, resin areas and medullary rays of the bark, 

 with the remark "that their value in classification is yet to 

 be tested, even if they hold good through large series of 

 specimens, as in those of the writer, at least, they fail to 

 do, and that there is no reason why these characters should 

 not vary with age, exposure, climate and luxuriance in the 

 same manner as the leaves do," finds that the only differ- 

 ences in different individuals from different parts of Cali- 

 fornia consist in the form, texture and coating of the leaves, 

 and that connecting the extreme forms are many interme- 

 diate ones, that the number of the nutlets (three in R. 

 Purshiana and two in R. Caroliniana) which has always 

 been depended on to separate these plants is of little value. 

 These species, certainly, like all others of the genus, have 

 sometimes two and sometimes three-seeded fruit. Mrs. 

 Brandegee points out also that the red color of the bark of 

 the branches, a character partly relied on by Professor 

 Greene to distinguish his R. rubra (Pitionia, i., 68), is by no 

 means unusual, plants not otherwise distinguishable being 

 found in the same neighborhood, some with red and some 

 with green-barked twigs. Of the southern tomentose 

 form (R. lomentella), which Professor Rusby considers spe- 

 cifically distinct, Mrs. Brandegee says it "furnishes one of 

 the strongest arguments for the specific identity of Rhamnus 

 Caroliniana and Rhamnus Purshiana. It runs in the northern 

 part of Lake and Collusa Counties into broad-leaved forms, 

 which apparently bear the same relation to Rhamnus 

 Purshiana that the southern R. lomentella does to R. Califor- 

 nica. It can hardly be supposed that if the species are 

 distinct each can have a variety lomentella varying in the 

 same manner as the species." 



I have examined a large number of specimens gathered 

 in all the great region where these plants are found, from 

 the shores of Flat Head Lake, in Montana, through all the 

 Pacific coast region to Mexico, and I confess, although it 

 is several years since I have seen the plants growing, 

 except in cultivation, I can find no characters which can 

 be depended on to distinguish the described species. In 

 all the essential characters upon which botanists depend to 

 separate one species from another, Rhamnus Purshiana and 

 Rhamnus Californica are the same — in external appearance 

 of bark and in wood, in flowers and fruit, and in the color of 

 the branches. This last is a character of little value, as in 

 Oregon as well as in California it is sometimes red and 

 sometimes green. 



The only variation recorded in the structure of the flower is 

 that noticed by Mr. Greene, in his Rhamnus rubra, the petals 

 of which he describes as " externally setulose-hairy below 

 the middle," a peculiarity which we cannot detect in the 

 dried flowers he has sent to me ; while the filaments de- 

 scribed by him as deltoid are not apparently thickened at 

 the base more than those of other flowers from California 

 and from Oregon. Professor Rusby's character, based on 

 the structure of the bark, would seem to require confirma- 

 tion by the examination of a very large number of speci- 

 mens gathered in all parts of the region occupied by this 

 plant and from individuals growing under different condi- 

 tions of soil and exposure. Until this has been done it 

 hardly seems safe to depend upon it to separate species 



