200 



BOTANY 



The absorptive power of soil for water is due to its capacity to retain water by 

 capillarity, so that it does not drain oft". Of the soils investigated by S.\< us, 

 cultivated soil retained in this way 46 per cent, loam 52 per cent, and sand only 

 21 per cent of water. 



The young roots, and especially the root -hairs, in addition 

 to the carbonic acid exhaled by them, which, no doubt, aids in 



loosening the soil, excrete other acid 

 substances which are able to dissolve 

 about the same amount from the soil as is 

 done by a 1 per cent solution of citric acid. 

 Roots growing upon a polished plate of 

 marble, dolomite, osteolith, or even ivory 

 will so corrode them that an etched pattern 

 of the course and direction of the roots is 

 thus obtained. Plants which excrete 

 acid actively (maize, rye, oats) make less 

 demands on a good worked soil than 

 those in which the excretion is less active 

 (wheat, barley) or almost absent (meadow 

 grasses). 



The nutrient water with which the 

 cell walls of the epidermal cells and root- 

 hairs first become permeated is taken 

 up by the epidermal cells, and thence 

 passes through the cortical cells and the 

 endodermis to the central cylinder of the 

 root. 



The Distribution of the Nutrient 

 Water. 1. ROOT-PRESSURE.- The causes 

 ic. is:. vigorous exudation of water which determine the direction and strength 



as the. result of root-pressure from Q{ the movement Q f the water through 

 a cut stem of DcMia mnabuu. . ,. . ,, .. , 



The smoothly cut stem is joined the living cells of the root-cortex into the 

 to the glass tube g by means of vascular bundles are not vet fully under- 

 1 stood. The fact that the water does 

 into them, and at times 

 indeed is forced into them with a con- 

 siderable pressure, may be easily de- 

 monstrated. If the stem of a strongly 

 growing plant be cut off close above the ground, and the cut 

 surface dried and then examined with a magnify ing-glass, water will, 

 in a short time, be seen to exude from the severed ends of the 

 bundles. By close inspection, it is also possible to determine that 

 the water escapes solely from the vessels and tracheides of the 

 bundles. When the soil is kept warm and moist the outflow will be 

 greater, and will often continue for several days and even months. 

 The excreted water may amount to several litres (to 1 litre in the 



W, absorbed by the roots from the 



soil, is pumped out of the vessels actually pass 



of the stem with a force sufficient 



to overcome the resistance of the 



column of mercury Q. 



