86 Proceedings of the Royal Jiish Academi). 



anatomy of great service in establishing tlie distinctive characters of the 

 European and Japanese species of Larch and their hybrid ; and one of the 

 characters investigated, the papillate epidermal cells, seemed to explain the 

 great capacity of the Japanese Larch in bearing shade. The adaptation of 

 species to their en\'ironment may evidently be elucidated by a study of the 

 comparative anatomy of the leaves, which are the organs of photosynthesis 

 and transpiration — functions that are considerably afi'ected by climate. 



This is well seen in the various species of Douglas Fir, which inhabit 

 regions characterized by great diversity in the humidity of the air, the 

 quantity of sunlight, and other climatic factors. In fact, no two species agree 

 in the structure and shape of the leaf. M'Nab was the first to investigate 

 these characters in the American Douglas Fir, only one species of which was 

 recognized at the time, and conjectured from the sections of the leaves at his 

 disposal that there wei-e two distinct species. • These two species, the 

 Oregon and Colorado Douglas Firs, indeed reflect in the structure of their 

 leaves the great dissimilarity of the climates of the Pacific Coast and Eocky 

 Mountains regions. 



All the species, e.xcept the Formosan Douglas Fir, have now been 

 examined ; and transverse sections of the leaves under the microscope show 

 considerable difTerences, which will now be pointed out. 



\. Shape. The leaves of the different species vary in the relative pro- 

 portion of their dimensions in thickness and width. Thick leaves present less 

 surface to evaporation, and are characteristic of xeropliytic conditions. The 

 Colorado Douglas Fir has very thick leaves, convex on the lower surface, and 

 glaucous above and beneath owing to a thin film of wax, which is protective 

 against heat and drought All these characters indicate a dry, sunny, hot 

 climate. Most of the other species have thin leaves, flat beneath. 



2. Pajnllne. In P. glatica and P. macrocarjia all the epidermal cells are 

 papillate, but in the other sjiecies the epidennal cells of only the lower surface 

 bear papillae. Various explanations of the functions of these papillae have 

 been given, notably Professor H. H. Dixon's, that they allow more light to 

 enter into the leaf, where it is available for photosynthesis. Leaves with all 

 the epidermal cells papillate are probably able to bear shade well. This 

 would be an advantage in regions subject to long periods of drought, as such 

 leaves persist long on the branches, forming on the tree a thick crown of 

 foliage that protects the soil from evaporation and keeps it moist. It woidd 

 be of interest to ascertain whether P. glauca and P. viacrocarpa bear dense 



' M'Xah, in Proc. Boy. Iriih Acad., ii, 703, plate 49 (1876). Fig. 32 reiiresents 

 P. Douglatii. Figs. 32a and 32b represent P. glauca. 



