THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 109 



Evidence that this assumption is well founded may be obtained 

 from that unusually interesting plant Adenostoma fasciculalum, in 

 the remarkable type of leaf that is found upon seedlings and stump 

 sprouts and to a slight degree on mature plants growing in meso- 

 phytic situations. Prints of these, with the ordinary type for com- 

 parison, are given in plate 21b. The differences in structure are no 

 less striking than in form (plate 20, b, c, d). The sprout or seedling 



Fio. 40. — Arctostaphyloa lomentosa: section of leaf from a xerophytic habitat (station 4, 



Jasper Ridge). X125. 

 Fio. 41. — Arctostaphylos tomentosa: section of leaf from a mesophytic habitat (station 7, 



Jasper Ridge). X125. 

 Fio. 42. — Adenostoma fasciculalum: stoma of normal xerophytic specimen. X376. 

 Fio. 43. — Adenostoma fasciculalum: stoma of leaf from stump sprout. X375. 



leaf is a flat isolateral structure, averaging 300 microns in thickness, 

 with the veins in a single plane after the manner of ordinary leaves. 

 The mesophyll is loose, especially the central sponge, and both 

 palisade and sponge cells are short and rounded. The epidermal 

 cells are very large, thin-walled, and turgid. The cuticle is very 

 thin — 1.5 microns as contrasted with 9.73 microns in the ordinary 

 leaf of the species. The stomata occur on both surfaces. They still 

 possess the exterior chamber, but this is very shallow and open- 

 mouthed (figs. 42, 43). 



We may explain these remarkable structural changes — broadening 

 of the leaf to form a flat blade, loosening of the mesophyll, loss of 

 character in the palisade, reduction of cuticle — after a fashion by 



