siderably that a thick layer of "duff accumulates under trees. 

 The presence of duff may prevent the estabUshment of seedlings 

 simply because it is so loose and well aerated that the roots of 

 newly germinated seedlings dry out before they reach the soil 

 surface. In other instances, however, the presence of humus due 

 to sluggish decomposition favors the growth of some plants, 

 expecially saprophytic orchids and relatives of the heather fami- 

 ly. (A saprophyte is a plant which obtains much of its nutritional 

 requirements from decomposing organic material, such as dead 

 leaves or wood.) 



Another plant-plant interaction is one which has received 

 much attention in recent years. This is the phenomenon called 

 allelopathy, which can be defined fairly accurately as chemical 

 warfare among plants. Recent work on various native and in- 

 troduced California plants indicates that certain species give off 

 chemical compounds that are toxic to other plants (and in some 

 instances are toxic also to seedlings of the same species). An 

 example of a genus which shows allelopathic behavior is Salvia, 

 a member of the mint family which is commonly known as sage 

 (not sagebrush). Shrubs of Salvia produce volatile odorous 

 chemicals called terpenes. These terpenes are given off into the 

 air and also diffuse or wash into the soil and ii#iibit the estab- 

 lishment of seedlings in the immediate vicinity of Salvia shrubs. 

 As a result, stands of Salvia contain individuals that are rather 

 uniformly spaced, much as if they had been planted by man. The 

 advantage of allelopathic inhibition of other plants is probably 

 that by preventing other plants from growing in their vicinity, 

 the Salvia shrubs insure themselves access to the local ground- 

 water supply. Another plant that exhibits allelopathy is Chamise 

 (Adenostoma fasciculatum), a shrubby member of the rose 

 family that is an important constituent of Chaparral in CaUfornia. 

 There are other examples from the California flora which suggest 

 that allelopathy may be an important factor in influencing the 

 presence and distribution of various plant species in California 

 plant communities. 



Another biotic factor is the presence and activities of man and 

 other animals. Without doubt, one of the most important animal 

 effects on the California flora has been the direct or indirect 



22 



