64 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



region of relatively heavy rainfall, the dry season is likely to be 

 shorter and the supply will dwindle less rapidly. A soil with a 

 high water-retaining capacity will lose less through gravity. If the 

 evaporation-rate is high, more will be lost thereby, and forest 

 vegetation will extract a far greater quantity than will scrub. But 

 whatever the difference in rate due to these separate elements, the 

 depletion goes on, and the water-content approaches the wilting 

 coefficient. In some places at least, and in some years, it actually 

 drops below for a considerable period. The Jasper Ridge observa- 

 tions demonstrate this; moreover, the general conditions of moisture 

 here are much less severe than in other regions where broad- 

 sclerophylls are even more thoroughly in control. When the autumn 

 rains finally arrive there is still, in some soils at least, a period of 

 several weeks before the water forces its way into the deeper layers 

 of the air-dry soil. The importance of this delay is lessened some- 

 what by the fact that much of the root system is contained within 

 the first half-meter of depth. There is probably but small delay 

 in the production of new absorption apparatus from the old dormant 

 roots, once the moisture touches them. 



As the store of soil-moisture decreases, the evaporation-rate rises 

 and water-loss from the leaves must increase. Thus the water- 

 balance is affected unfavorably at both ends at the same time, and 

 the late summer and early autumn, therefore, constitute the time 

 of greatest danger. The deeply penetrating roots and the effectively 

 guarded leaves are the answer. 



Air-temperature and, in response, soil-temperature, at a minimum 

 during the months of abundant moisture, rise as water-content 

 decreases. This combination is altogether unfavorable for veg- 

 etative activity. When moisture is abundant, the low soil-tem- 

 perature makes absorption difficult and root-growth slow, and the 

 low air-temperature is unfavorable to photosynthesis and the other 

 growth activities. When the soil-temperature is favorable for 

 root-growth and absorption, the water-content is scanty or neg- 

 ligible. When air-temperature is favorable to rapid photosynthesis, 

 the water supply necessary to that process and to vigorous growth 

 is insufficient. High temperature is now itself a danger, in that it 

 raises the evaporation-rate and thus increases the depletion of the 

 scanty moisture supply, both in plant and soil. 



The seasonal development of the broad-sclerophylls is accurately 

 adjusted to the peculiar seasonal march of the factors just outlined. 

 With the beginning of the heavy rains (usually December, sometimes 

 earlier or later), growth starts in most of the species. The history 

 in detail presents many variations. For instance, Arctostaphylos 

 flowers immediately from buds already formed, then proceeds to 

 put forth new leaf-shoots, which terminate with next year's flower- 



