THE CONTENTS OF THE SOIL 



31 



worn away by denudation. See Sir John Lubbock, "Scenery of 

 Switzerland," Chaps. Hi. and iv. 



29a. Even hard surfaces of rock often support lichens, 

 mosses, and other humble plants. "The plant is co-partner 

 with the weather in the building of the primal soils. The lichen 

 spreads its thin substance over the rock, sending its fibers into 

 the crevices and filling the chinks, 

 as they enlarge, with the decay 

 of its own structure ; and finally 

 the rock is fit for the moss or 

 fern or creeping vine, each new- 

 comer leaving its impress by which 

 some later newcomer may profit. 

 Finally the rock is disintegrated 

 and comminuted, and is ready to 

 he still further elaborated by corn 

 and ragweed. Nature intends to 

 leave no vacant or bare places. 

 She providently covers the rail- 

 way embankment with quack -grass 

 or willows, and she scatters daisies 

 in the old meadows where the land 

 has grown sick and tired of grass." 

 Principles of Fruit -Growing, 176. 



30o. It is interesting to consider the general reasons for the 

 evolution of the root. Plants were at first aquatic, and probably 

 absorbed food from the water on all their surfaces. They may 

 not have been attached to the earth. As they were driven into 

 a more or less terrestrial life by the receding of the waters and 

 as a result of the struggle for existence, they developed parts 

 which penetrated the earth. These parts were probably only 

 hold-fasts at first, as the roots of many seaweeds are at the pres- 

 ent time. But as it became less and less possible for the general 

 surface of the plant to absorb food, the hold-fast gradually be- 

 came a food -gathering or feeding member. See Survival of the 

 Unlike, pp. 41-4.1. 



30&. If the pupil has access to ledges of rock on which trees 



Fig. 2. The halves of a rock forced 

 apart by the growth of a tree. 



