106 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



several layers deep. The epiderm is nearly always single, and often 

 papillate. The cuticle is very thick, the upper averaging 5.69 

 microns as against 1.58 microns in the deciduous species. In the 

 case of the lower the difference is still greater: 4.43 microns and 

 0.91 microns. The stomata in a large majority are on the lower 

 surface only. Many special features occur in their structure and 

 distribution that are effective in decreasing water-loss. Tannin is 

 abundant and widespread in bundle-sheath, mesophyll, and epiderm. 

 Divergences from Schimper's generalizations, quoted at the 

 beginning of the chapter, are unimportant. The average Californian 

 leaf is smaller than his statement implies; it tends toward the oval 

 rather than the lanceolate in form, and vertical placement is less 

 pronounced. Many special features are here described which do 

 not appear in his account. 



Comparing the two communities, we find that the broad-sclerophyll 

 forest has uniformly much larger leaves, which are less often entire. 



The climax chaparral leaf averages 

 50 per cent thicker than the forest 

 leaf; it includes all the isolateral 

 and the majority of the imper- 

 fectly bifacial leaves, and its pali- 

 sade tissue is more prominent, 

 averaging 4 to 5 layers and 66 per 

 cent of the mesophyll as against 

 2 to 3 layers and 43 per cent in 

 the forest species. The chaparral 

 leaves have a much thicker cuticle 

 than those of the forest — nearly 

 100 per cent greater on the upper 

 side and more than 150 per cent 

 greater on the lower. The species with stomata on both surfaces, 

 which are those with isolateral vertical leaves, are all of the chaparral. 

 Special features protective against water-loss are almost entirely 

 confined to the chaparral. 



We see thus that the broad-sclerophylls, as compared with decid- 

 uous species, have a wealth of features of the kind that have been 

 assumed to be a response to the moisture conditions of the habitat, 

 and which are certainly effective in decreasing water-loss. Certain 

 of the characteristics, such as spiny teeth, papillate epiderm, and 

 presence of tannin, have no obvious relation to this or to any other 

 habitat factor, either as to cause and effect or advantage. We see 

 further that the chaparral species are in every way more fully provided 

 with transpiration-decreasing features than are the forest species. 

 The group from the redwood undergrowth is rather puzzling. In 

 thickness of leaf the two species studied are surpassed by but three 



Fig. 39. — Vaccinium ovatum: section of leaf. 

 X125. 



