i9i 2] STARR— ANATOMY OF DUNE PLANTS 267 



gather force as they sweep across the lake, and when they reach the 

 shore they gather up sand and carry it along with a force that 

 carves and scars the bark of the trees on the windward side or 

 completely wears it away, as in the case of Cornus stolonifera. 



Soil. — The soil is chiefly quartz sand, the particles being 

 relatively large, so that it is extremely porous, which has a great 

 influence on the water and heat relations. As a rule sandy soils 

 are poor in nutrient food materials, nor do they rapidly develop a 

 rich humus because of the rapid oxidation of the organic matter. 



Water. — The surface layer of soil is very dry, as the capillarity 

 of sand is less than that of other soils, evaporation from a sandy 

 surface is rapid, and precipitated water percolates quickly, the 

 water capacity of sand being slight. On the other hand, a sandy 

 soil yields its water to plants more freely than other soils, and below 

 the superficial layer of dry sand there is always a surprising amount 



Of Water. FlTLT/RT? ll*\ VmQ fnnnH 



this to be more 



wilting coefficient 



BlOTIC FACTORS. 



marked 



the 



they are once established, humus and shade. Humus 



the 



number of soil organisms, toxicity, and aeration. Shade influences 

 the germination of seeds and increases the accumulation of humus 



humid 

 n the < 



Fuller 



pine 



and in the beech-maple forest 8 cc, a descending scale from the 

 pioneer formation to the climax forest. 



Description of the plants 



I. Xerophytic forms 



Herbs 

 Cakile edentula. — A small, very succulent annual. Leaves 

 smooth and thick ; outer walls of epidermis 4 /* ; several rows of 

 Palisade on each side with a narrow zone of sponge in the center; 

 w ater-storage tissue about the bundles ; stomata on both surfaces ; 

 conductive elements not well developed. Stem with epidermal 

 walls thickened all around, the outer 10 /*. 



