INTRODUCTION 35 



corky or made waterproof, but consists of cellulose, 

 or is permeable to water, and mucilaginous. If soil 

 adheres to a plant above ground or above the region 

 of the root-hairs, it is easily shaken off, but in the 

 latter region this is more difficult, and the water 

 becomes slimy when a root has been vigorously 

 shaken up in it. Various organic acids and carbonic 

 acid gas are secreted by the root-hairs. 



The number of the root-hairs gives the plant a 

 great area of absorption, and if torn up a plant 

 deprived of them at first withers, and does not com- 

 pletely recover till they grow again. 



The protoplasts in the root are in the form of a 

 film, and the cell-sap charged with organic substance 

 in solution fills the rest of the cells. There is some 

 power in this membrane which enables it to reject 

 or select what substances in solution it needs from 

 the soil. 



The substances in solution enter the root-hairs 

 through the epidermal cells, pass into the cortical 

 cells, thence into the tracheids, and then ascend as 

 cell-sap. The cells are arranged so that the sap 

 already in part formed into food and the new cell- 

 sap are not intermixed. There is, in other words, 

 ascending sap and descending sap, which are set in 

 motion by different means, travelling by different 

 paths. The former, as water with mineral salts, 

 etc., later travel by xylem strands upwards, the latter 

 by the phloem downwards, as carbohydrates elabo- 

 rated from the atmosphere by the chloroplasts. 



