LABORATORY FOR PLANT PHYSIOLOGY. 63 



redwood and tan oak, and the ridges, running at approximately right angles 

 to the coast-line, are clothed with low chaparral of a composition depending 

 on the soil and underlying rock. The ridge lying between Palo Colorado 

 Canon and Rocky Creek is formed mainly of sandstone, its soil is a fine sand 

 very poor in organic matter, and its summit is covered with an open chaparral 

 of manzanita. The vegetation of this ridge is the most xerophytic that has 

 been observed in any habitat on the seaward slope of the Santa Lucias. 

 Instrumentation has been devoted to securing a comparison between the 

 conditions of evaporation and soil moisture on Palo Colorado Ridge and in 

 the heavy stand of redwood in Rocky Creek, three-quarters of a mile distant 

 and 500 feet below. During the course of the rainless summer the soil 

 moisture on the ridge, at a depth of 6 inches, has fallen from 4.1 per cent to 

 3.7 per cent, and in the flood-plain of the creek from 18.2 per cent to 13.0 per 

 cent. The evaporation on the ridge has varied from 15.6 to 22.9 c. c. per day, 

 and in the floor of the canon from 5.6 to 7.0 c. c. per day. 



The ratio of evaporation to soil moisture is a datum of importance in 

 indicating the conditions of a habitat with reference to the water relations of 

 plants, and its value has been determined for several localities in previous 

 work. For Palo Colorado Ridge the value of the ratio is 5.2 and for Rocky 

 Creek is 0.4. The previously determined value of this ratio for Carmel is 

 3.5, and values for four other localities in the Santa Lucias, at different 

 altitudes and farther from the sea, are all greater than the value for Palo 

 Colorado Ridge — that for the summit of Chew's Ridge, at 5,000 feet elevation 

 and 20 miles from the sea, being 14.1 (see Annual Report, Year Book, No. 19, 

 1920, p. 78). Since the ratio of evaporation to soil moisture is an index of 

 the aridity of a habitat, it is of interest to note that the value for the most 

 xerophytic habitat in close proximity to the sea is little more than one-third 

 as great as that for Chew's Ridge, where the summit is dominated by a similar 

 type of chaparral and the adjacent slopes by an open stand of Coulter pine. 

 For comparison it may be noted that the ratio at Tucson in the dry early 

 summer is 50.5, but falls to 3.3 on the adjacent mountains at 8,000-feet 

 elevation. The ratio for Rocky Creek is the lowest that has been determined 

 for any of the habitats investigated, and its value, 0.4, may be taken as 

 typical of the highly mesophytic conditions which favor the growth of the 

 redwood. Comparison with the ratio for Tucson indicates that the soil and 

 atmospheric conditions make the maintenance of a balance between water 

 income and water outgo 126 times as difficult in the habitat of the giant 

 cactus as in that of the redwood. 



Conditions influencing Soil Temperatures, by Forrest Shreve. 



Our knowledge of the temperature of the soil is very far behind our 

 knowledge of atmospheric temperature. Being outside the realm of clima- 

 tology, it has remained as one of the least investigated and most poorly 

 known of the physical conditions of biological importance. Work on certain 

 aspects of soil temperature has been continued during the past year with 

 reference to the influence exerted upon it by accompanying conditions. So 

 large is the number of these conditions that it is only by their separate inves- 

 tigation that we shall be able to understand the role of each of them in 



