1 84 



EXCHANGES AND MOVEMENTS OF FLUIDS 



257. Absorbing Organs. The lower forms absorb liquids over 

 their entire surfaces, but the higher have differentiated the roots 

 as special fixing and absorbing organs. The outer cells of the 

 roots are generally extended in the form of long tube-like exten- 

 sions which increase the surface capable of carrying on osmotic 

 absorption many fold. The root hairs penetrate between the 

 finer particles of the soil, coming into direct contact with the thin 

 layer of hygroscopic water surrounding each particle. The hy- 

 groscopic water contains some of the mineral salts of the soil in 

 solution, and a selection of these with the water is drawn into the 



hair by the osmotic attrac- 



^J^~)^ _- H~M4ll@^^°'l^r— I } ^ on °*" tne P roto plasm 



and solutions in the vacu- 

 oles. The concentration, 

 or proportions of the mole- 

 cules of the salts and water 

 are also determined by the 

 cells, irrespective of the 

 strength of the solution in 

 which they may be found. 

 The absorption of water 

 from the soil continues 

 until the osmotic attraction 

 of the substances in the 

 root-hairs is equal to the surface tension of the thin layer of water 

 coating the soil particles. If this is not renewed, there will come 

 a stage in the procedure when the plant will not be able to ob- 

 tain a further supply from the soil, although it contains an ap- 

 preciable quantity which may be driven off by heat amounting 

 from from 1.5 to 8 per cent, of its total weight in different soils. 



258. Structure of an Absorbing Root. Cut a thin cross-section 

 of a monocotyledonous root of any convenient plant and note the 

 form of the root-hairs, the cortex, endodermis if present, and the 

 central cylinder. Ascertain if, passage cells are present in endo- 

 dermis. 



iffy 



Fig. 89. Diagram of radial longitudinal sec 

 tion of young root, a, epidermal layer with root 

 hairs. 6, external cortex, c, internal cortex. 

 d, endodermis. /, pericycle. g-h, vessels, i, 

 pith. Fluids absorbed move in directions shown 

 by arrows. After Belzung. 



