IOWA ACADEMY OF SCIENCES. 48 



material is freshly exposed, there soon appears upon the 

 surface, especially if it be not directly exposed to the sun, 

 a coating of some unicellular blue-green alga which holds 

 t-he soil particles by its gelatinous sheath. Small mosses, 

 and perhaps liverworts, such as Marchantia, follow, making 

 the anchorage still safer by their numerous rhizoids. On 

 the loess bluffs of western Iowa a Nostoc and a lichen, 

 Biatoni decipiens (Ehrh.) Fr,, commonly perform this func- 

 tion. Then grasses, weeds, various flowering herbs, estab- 

 lish themselves, and finally shrubs and trees gain a foot- 

 hold. A good illustration is furnished by the bluff at 

 Sioux City, represented in Plate VI. Here the Russian 

 thistle, various grasses, etc., represent herbs, while the 

 Cottonwood and smooth sumach are the forerunners of 

 woody plants. 



The writer has frequently observed this sequence of 

 floras on loess banks which were protected from the sun, 

 while exposed portions of the same bank were bare, 

 and more or less eroded. Wet seasons, such as that of 

 1902, may give such plants an opportunity even in com- 

 paratively exposed places. 



That trees and shrubs prevent erosion is well known, 

 and some, such as willows, are planted along water-courses 

 for this purpose. Plate VII furnishes another illustration. 

 Here the roots of a lilac bush have held a mass of loess 

 clay in a column, while the adjoining materials were 

 eroded away. An upturned tree carries with it a mass of 

 earth entangled in the meshes of the roots, and more or 

 less agglutinated by the root-hairs, and such a mass as that 

 in Plate VIII, will often resist the action of winds and rains 

 for a long time. 



In deep woods very little erosion takes place, partly 

 because of the dissipation of rain-drops by the branches 

 and leaves, and partly because the spongy leaf mould so 

 rapidly absorbs the water which falls upon the surface. 

 The result is that even in the deepest ravines with steepest 

 sides even after the heaviest rainstorms the water runs 

 clear, unclouded by the products of erosion. Once remove 



