70 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



study. The stalks were cleared from the vicinity of the instruments 

 and the effects of vegetation were thus eliminated. The atmometers 

 were installed July 1, and were visited at approximately weekly 

 intervals until August 26. Unfortunately, three readings at station 

 1 were lost through interference by cattle, which did not respect 

 the chicken-wire defenses. Except for one reading in stations 3 

 and 4, the south-facing slopes show the higher evaporation-rate. 

 The reversal on July 29 is hard to understand, but may possibly be 

 due to experimental error. Omitting the figures in station 2, which 

 have no corresponding south-facing slope values, we find the following 

 average evaporation-rates : 



For the first pair, south-facing : north-facing : : 100 : 85. 



For the second pair, south-facing : north-facing : : 100 : 79. 



For the 13 complete readings afforded by the two, south-facing : north-facing : : 100:82 



There is, then, a distinct difference in evaporation-rate between 

 opposing north and south slopes, when uncomplicated by vegetation. 

 In these open ravines with comparatively gentle slopes the average 

 rates were as 4 to 5. If they had been as steep-sided as certain ravines 

 at Jasper Ridge, the contrast would doubtless have been greater. 



We have here sufficient data to explain the greater size and 

 luxuriance of the plants growing upon the north-facing slopes, and 

 the absence of the more mesophytic species from the south-facing 

 exposures. Many factors are indirectly involved, but fundamentally 

 the problem is one of water-balance. During the height of the 

 rainy season there is in both habitats an abundance of soil-moisture 

 and a minimum of evaporation. But the air and soil temperatures 

 are low, and there is consequently little growth. As the temperature 

 rises, the rate of growth increases accordingly, but at the same time 

 the rains are diminishing in frequency and amount, finally ceasing 

 entirely, and the evaporation-rate is going up. It has already been 

 stated that the period when most of the growth occurs is in the 

 spring months, culminating in April. The amount of actual growth 

 during this time, other things being equal, will depend upon the 

 ratio of water-supply to water-loss, and plainly this water-balance 

 is more favorable upon the north-facing slope inhabited by forest. 

 The size of the full-grown plant of a given species depends largely 

 upon the turgor of the growing cells, and a healthy condition of 

 turgor is conditioned upon a water-supply that will more than 

 compensate for the loss by evaporation. If the supply is so scant 

 or the loss so severe that the water-balance is barely maintained at 

 equality, the plant and its organs will be small. If either member of 

 the system varies in such a way that the supply is well in excess of 

 the loss, rapid growth will result. The ever useful example of the 

 stump sprout illustrates this point. The water-supply which 

 formerly fed a full-sized shrub is concentrated upon a few small 



