OF THE GENUS PINUS. 565 
by Mr. Worsdell, Mr. Fraser, and others, to whom my acknow- 
ledgments are due, as also to Mr. A. E. Smith for his 
photographic reproductions of several slides. , 
The integrity or subdivision of the fibro-vascular bundle has 
already been commented on as furnishing an excellent character, 
as indicated by Coulter & Rose and Koehne. 
The position of the resin-canals is more variable; they are 
not always present, and in some cases (as in P. Lambertiana), 
on the same branch, they may vary in position. 
The general form of the leaf, whether three-sided (Pl. 20. fig. 3) 
or semicylindric (Pl. 20. fig. 1), with the upper side concave or 
flattish, has, of course, attracted general attention. This form is 
well seen when transverse sections of the leaf are made. But the 
shape of the central half-cylinder or “ meristele,” when similarly 
cut across, does not always correspond to the general shape of 
the leaf, and has not, to my knowledge, been specially noticed. 
Nevertheless it affords a very good means of differentiating 
certain species. The “ meristele,” thus seen in section, may be 
circular, as might be expected in P. monophylla (Pl. 21. fig. 4), 
which has cylindrically-shaped leaves; but it occurs also in 
numerous species, such as P. edulis, in which the leaves are 
three-sided. The meristele has an elliptic section in many 
species of those groups which have from 3 to 5 leaves, and also 
constantly in the 2-leaved section (Pl. 20. fig. 1). A third 
variation exists in those species where the meristele is three- 
sided in section, as in P. filifolia (PI. 21. figs. 5 & 6). This tri- 
angular form is generally very distinct, butit occasionally passes 
gradually into the oblong or elliptic form *. 
The number and the size of the endoderm-cells (see Pl. 20. 
figs. 1, 2, 3, etc.) offer, as has been said, good characters, and will, 
it is hoped, attract the attention of students with the necessary 
patience and leisure. 
The thickness of the hypoderm or stereome layers varies in 
different species, and even in the same species under varying 
conditions. It is usually in one or two layers, with an extra 
number in the corners. It is uniform, or in some species it 
Knowledge of some North-American Conifer’ (1897). A few plans of leaf- 
sections of Pines are given by Prof. Tschirch of Berne, Schweiz. Wochenschrift 
fiir Chemie und Pharmacie (1903), no. 22, 
* Care must also be taken that the section be made accurately in a hori- 
zontal direction across the centre of the adult leaf. 
