I908] T RAN SEAU— PLANT SOCIETIES AND EVAPORATION 



231 



part of seedlings takes place during the first stages. This greatly aids 

 in accounting for the well-known fact that the development toward a 

 closed association proceeds w^ith such increasing rapidity when once a 

 few plants gain a foothold. Attention has been frequently called to 

 the importance of pioneers as shade-producers, while their effective- 

 ness in reducing transpiration has been underestimated. 



The fact that the weekly evaporation rate for a beach covered with 

 xerophytes should be less than that for a garden may seem anomalous. 

 But when the diminished ground-water supply, the sterility of the soil, 

 and the probable high rate of evaporation during short periods are 



Per cent, of standard 



Sta. 



II. Salt marsh, outer margin. . 



Sta. 3. Gravel slide, open 



Sta. I. Carnegie garden, standard. 



Sta. 9 and 10. Upper beach 



Sta. 



12. 



Salt marsh, inner margin 



Sta. 2. Garden, high level 



Sta, 



4. Gravel slide, partly invaded 



Sta. 5, Forest, open 



Sta. 13, Fresh-water marsh 



Sta. 6. Forest, typical mesophytic 



Sta. 7. Forest, ra^-ine tj-pe 



Sta. 8. Forest, swamp type 



Fig. 9. — Diagram sho-vv-ing comparative evaporation rate in the lowest stratum 

 of the several plant societies. 



surprising. 



elimination of more mesoph}tic species 



o 



further 



figures 



o 



standing of the structure of these plant 



marsh helps in the under 



ipon the rate of evaporation at different levels withii 

 greatly in estimating the factors involved in compe 



trees 



For a complete imderstanding of 1 

 seems essential that an instrument 



^^ecord 



X. III. 



