110 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



stating that the abnormal leaf type is an ancestral character to which 

 the plant reverts in its juvenile stage. This in itself is begging the 

 question, and doubt is cast upon its adequacy by the fact that the 

 flat leaf sometimes appears in mature individuals in mesophytic 

 situations (plate 21a, 6). Light can not be operative here, since it 

 is at maximum intensity where sprouts and seedlings are found, and 

 observed changes due to increased intensity are invariably in the 

 opposite direction. Moisture as the governing factor seems entirely 

 adequate. New stump sprouts, even in the midst of the dry season, 

 have an exceptional advantage as to water, since they may draw 

 upon an absorbing system which formerly supplied a complete 

 mature shrub. The large size of leaves upon stump sprouts and 

 their mesophytic structure are well-known phenomena, and are 

 characteristic of other chaparral species besides Adenostoma. As 

 to seedlings, germination and early growth take place at a time of 

 year when soil-moisture is abundant and evaporation low, and the 

 plant supplements this advantage by immediately sending down a 

 taproot to a considerable depth. The occasional occurrence of the 

 flat leaf upon mature individuals in mesophytic situations is the 

 final argument. It may well be that the mesophytic leaf with its 

 distinctive form and structure points back to a mesophytic ancestry 

 for the species, which would help to bring it into harmony with the 

 majority of the members of its family, among which it is decidedly 

 an anomaly. Finally, since moisture seems adequate as a cause of 

 variation in the form and thickness of the leaf of Adenostoma and 

 in the character of its mesophyll and cuticle, it is reasonable to assume 

 that it is the controlling factor in the other species where the same 

 differences are present but less striking. 



COMPARATIVE TRANSPIRATION-RATE. 



In the summer of 1917 I made a brief study to determine the actual 

 transpiration-rate of two or three of the important broad-sclerophyll 

 species. The results were somewhat puzzling, though consistent 

 enough among themselves, and it is therefore the better part of valor 

 to withhold them in the main for further investigation. One or two 

 points were clearly brought out, however, which are worth stating 

 here. 



The study was made by the simple potometer method — the placing 

 of branches in bottles of water, which were weighed at the beginning 

 and end of the experiment. The mouths of the bottles were plugged 

 with cotton, which was kept perfectly dry. One or two which became 

 wet during the experiment were discarded. The species used were 

 Adenostoma fasciculatum , Arctostaphylos tomentosa, and Arbutus 

 menziesii. Ten shoots of each of the first two were cut in station 10 

 {Adenostoma consociation; see p. 41 for environment), placed in 



