THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 107 



species of the chaparral; in thickness of cuticle they are superior to 

 all the species of the forest and to many of the chaparral. The 

 mesophyll, however, is decidedly suggestive of mesophytic influences. 

 The habitat of these plants is far more mesophytic than that of the 

 other broad-sclerophylls, and the xeromorphic construction of their 

 leaves is a warning against the determination of the essential ecolog- 

 ical nature of a plant by one structure alone. The commonness of 

 this type of leaf in the family Ericaceae, apparently regardless of 

 habitat, is too well known to require comment. 



EFFECTS OF ENVIRONMENTAL CONDITIONS UPON LEAF STRUCTURE. 



It is commonly assumed that moisture and light influence power- 

 fully the structure of leaves, especially the thickness and character 

 of the mesophyll and epiderm. Many have essayed to separate 

 the effects of the two factors, not always with success. While it is 

 impossible in such a field study as the present to make a close analysis 

 of cause and effect, any correlation of structural differences with 

 accurately measured habitat factors is of value, and therefore the 

 following brief study is presented. 



Three stations in the series at Jasper Ridge (p. 33) were selected, 

 Nos. 4, 3, and 7 representing respectively the Adenostoma, the 

 Arctostaphylos, and the Quercus agrifolia-Arbuius communities. 

 A summary of the moisture and light conditions of each station is 

 presented in table 15. The figures are derived from the data given 

 in an earlier section (p. 42). Soil-moisture is expressed in per cent 

 of dry-soil weight, and the figure is an average of a series of six 

 monthly determinations at two depths (10 cm. and 30 to 50 cm.) 

 during the dry season from June 15 to October 31, 1913. Evapora- 

 tion is given in cubic centimeters of daily loss from a standard 

 atmometer, determined weekly, and averaged for practically the same 

 period as the soil-moisture readings. Light is expressed as a decimal 

 of the full illumination. The readings were made as close together 

 as possible in time, at midday of August 17, 1917. The species 

 chosen for study were the three most prominent chaparral shrubs of 

 the vicinity, Adenostoma fasciculatum, Arctostaphylos tomentosa, and 

 Quercus durata. The natural habitat of these species is of the character 

 of stations 4 and 3, but they grow occasionally as interlopers in the 

 forest, which is represented by station 7, and thus give opportunity 

 for comparison. The material was collected from widely separated 

 plants within a given association, preserved in formalin-alcohol and 

 cut in paraffin. In making the measurements, 10 separate leaves 

 were used and 5 measurements made upon each leaf. Every figure 

 given is therefore an average of 50 separate measurements. 



The habitat analysis shows that the stations, in the order as given, 

 are progressively more favorable as regards moisture conditions, 

 both in water-content and in evaporation, and progressively more 



