PHYSIOLOGY 469 



If the roots of a plant are brought in contact with a polished marble 

 slab, a pattern of the growing roots will be etched upon this by 

 means of C0 2 given off by the roots. Lichens and other low plants 

 growing upon rocks are able to decompose the rock, probably by the 

 excretion of organic acids similar to those developed by the roots 

 of higher plants. 



The disintegration of rocks, such as lava, begins through the 

 agency of Bacteria, and the lower green forms, like the Schizophy- 

 cese, and possibly Green Algae. Lichens soon form, if conditions 

 are suitable, and their action is very marked in the disintegration of 

 rock. The detritus is soon sufficient for the establishment of Mosses, 

 etc., which continue the process until Ferns and other vascular plants 

 can effect lodgment in the soil formed by the mixture of decomposed 

 rock and humus derived from the decay of the dead tissues of the 

 Lichens and Mosses. These changes go on most rapidly in the 

 Tropics, and are studied to especial advantage on the fresh lava 

 masses thrown out by such large volcanoes as Mauna Loa in Hawaii, 

 and some of those in the Malayan Archipelago. The reestablish- 

 ment of the flora, following the terrific eruption of Krakatoa in the 

 Strait of Sunda, in 1883, has been studied by several botanists, espe- 

 cially Treub. 



In the preparation of ordinary soils for the use of the higher 

 plants, the nitrifying Bacteria are of the first importance. Darwin 

 has also called attention to the role played by earth-worms in the 

 production of vegetable mould. The latter are probably of much 

 greater importance in England than in the hotter and dryer regions of 

 most parts of America. 



The Root System 



The roots in most vascular plants form a, complicated system of 

 branches which ramify widely through the soil, and serve both to 

 anchor the plant firmly, and to act as absorbent agents. Only the 

 younger parts of the root act in the latter capacity, and the root-hairs 

 are the direct absorbents in most instances. The great number of 

 root hairs developed from the younger parts of the roots increases 

 enormously the absorbent surface. The root-hairs apply themselves 

 closely to the solid soil particles, and exhibit extraordinary powers 

 of absorption in wresting from the soil the moisture which is so tena- 

 ciously held by the soil particles. It is the roofchairs, also, which 

 excrete the solvents of the soil components. 



MOVEMENTS OF WATER 



In ordinary terrestrial plants the water amounts to 60 per cent 

 to 90 per cent of their total weight. In order to maintain this the 



