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OF THE GENUS PINTS. 067 
flat-leaved genera. In some forms of P. Pinaster, however 
(Pl. 23. fig.11), a palisade-layer is developed. The leaves of Pinus 
Bungeana (P1. 28. fig. 10) are interesting as showing a radiating 
arrangement of the mesophyll-cells in contact with the endoderm. 
In this place I may be permitted to say something as to the 
value, for taxonomic purposes, of the histological characters. 
Briefly, they have no greater intrinsic value than any other 
characters. They are useful, but they are not infallible guides. 
For the most part they are of a physiological or adaptive nature, 
and therefore likely to vary according to varying conditions to a 
much greater extent than other characters less dependent on 
existing circumstances and more “fixed”? by long hereditary 
descent. 
It must also be remembered that whereas the student of the 
outer morphology has generally the opportunity of examining a 
number of specimens from various localities, and of comparing 
the characters as grown under varying conditions, the histological 
student is generally, of necessity, confined to the examination of 
a relatively small number of sections, taken, it may be, from the 
same specimen. Histological characters, moreover, are not con- 
veniently observed in the forest or the garden, where some more 
ready means of discriminating are necessitated. 
In the present communication I have bestowed attention 
almost exclusively on the taxonomic characters afforded by the 
inner structure of the leaf. For the structural details of other 
organs reference may be made to the papers of Van Tieghem, 
“Sur la Structure primaire et les Affinités des Pins,” Journ. 
de Bot. t. vi. (1891) p. 265; and Penhallow, in Trans. Roy. Soc. 
Canada, vol. ii. (1896), sect. iv. p. 54, who shows that the genus is 
in regard to anatomical characters a well-defined group impossible 
to confound with any other. He divides the genus, so far as the 
N. American species are concerned, into two groups: Ist, those 
in which pits occur on the tangential walls of the summer wood, 
as in section Strobus; and 2ndly, those in which the pits are 
wanting in that situation. Radais, in Ann. Sc. Nat. sér. 7, t. xix. 
(1894) p. 297, tab. 8, arrives at similar conclusions. See also 
Dangeard, “ Recherches sur les Plantules des Coniféres,” Le 
Botaniste, sér. 3, p. 191; Bertrand, “Anat. compar. des Coniferes,” 
Ann. Sc. Nat. sér. 5, t. xx. (1874), tab. 12, p. 5; and numerous 
papers by Van Tieghem, Baillon, and others on the structure of 
the female flowers. 
