422 SUMMARY OF CURRENT RESEARCHES RELATING TO 



BOTANY. 



GENERAL, 

 Including the Anatomy and Physiology of Seed Plants. 



Structure and Development. 

 Vegetative. 



General View of the Genus Pinus.* — M. T. Masters adds another 

 useful memoir to those which he has previously published on the 

 Conifers. He draws special attention in the present instance to histo- 

 logical characters, especially those of the leaf, and their values for 

 systematic purposes. The integrity or subdivision of the fibrovascular 

 bundle affords an excellent character. The position of the resin-canals 

 is more variable ; they are not always present, and in some cases vary 

 in position on the same branch. The difference in general form of the 

 leaf section is well known, but Dr. Masters points out that the shape of 

 the meristele does not always correspond with the general leaf -shape,, 

 tnough it nevertheless affords a good means of distinguishing certain 

 species. The number and size of the endoderm-cells were also found 

 to afford good characters. The thickness of the hypoderm or stereome 

 layers varies in different species, and even in the same species under 

 varying conditions. "When the tree is much exposed to wind, the hypo- 

 derm-cells are likely to become thicker if not more numerous than in 

 the same species growing under less exposed conditions. The occasional 

 presence or absence of a layer of thin-walled cells, presumably water- 

 containing, just beneath the epidermis, should also be noted, as well as 

 the position and number of the rows of stomata. 



The histological structure of the temporary foliar members, such as 

 the cotyledons, is less highly developed than in the more permanent 

 foliage. Certain characters which are common to all the species in 

 their juvenile state may sometimes continue in adult life with com- 

 paratively little modification. Thus the unbranched bundle, which is 

 a common characteristic of the cotyledons and the primordial leaves, in 

 some species remains in the fully developed adult foliage, while in 

 others the bundles become branched. It is probable, therefore, that 

 the species with branched bundles are derived from those of a simpler 

 form, an assumption which is supported by other characteristics, 

 such as the nature of the bud-scales and peculiarities of the cone-scales. 



There is usually no distinct palisade layer in the leaves of Pinus, 

 the structure differing in that respect from that of the leaves of Abies 

 and other flat-leaved genera. 



As regards the value of histological characters for taxonomic pur- 

 poses, the author regards them as useful but not infallible guides. For 

 the most part they are of a physiological or adaptive nature, and there- 



* Journ. Linn. Soc. (Bot.) xxxv. (1904) pp. 5G0-G59, (4 pis. and figs, in text). 



