96 



Absorption 

 and Ascent 

 of Wtaer in 

 Plarts. 



Root- 

 Pressure. 



experiments, are not essential for their growth and de\ elopment. 

 These substances, however, may often be indirectly useful ;. 

 thus silica is often taken up in large quantities and in many 

 plants is deposited in the epidermal cell-walls. This increases 

 the hardness and rigidity of the plant and may aid it 

 considerably in the struggle for existence by rendering it nn~ 

 palatable to grazing animals. In this way the existence of 

 a plant, in its natural surroundings, may depend on the 

 presence of substances which appear, at first sight, to be un- 

 necessary for its healthy development. 



90. Substances in solution pass 



into the root-hairs by osmosis as in the case of the algal 

 cell, but, unlike the latter, an individual root-hair does not 

 remain in a continual state of turgidity. The protoplasm 

 of the turgid root-hair after a time appears to relax its 

 resistance to the outward pressure of the water and sub- 

 stances in solution which have accumulated in the cell-sap 

 and this absorbed water with substances in solution conse- 

 quently filters back out of the cell under a pressure corre- 

 sponding to that previously exercised by the protoplasm sup- 

 ported by the cell- wall by means of which the state of tur- 

 gescence was attained. That this pressure can be very consi- 

 derable is indicated by the fact that, when a young root pene- 

 trates a crack in a rock, or masonry wall, as the root increases 

 in size, the pressure exercised by its cells, in their efforts to 

 become turgid and grow, may suffice to split the rock, or wall, 

 as the case may be. 



The water, however, which is thus pressed out of the root- 

 hairs, instead of passing out into the soil and thus being lost to 

 the plant, finds its way from cell to cell of the fundamental 

 tissue of the root and eventually passes into the vessels and 

 tracheids of the vascular bundles. 



That water with salts in solution is thus pressed by the 

 roots into the vessels and tracheids, and often with consider- 

 able force, can be seen by felling a tree when the so-called 

 "sap" is rising, i.e. just before the leaves appear and when the 

 roots have commenced to actively absorb water from the soil. 

 The water can be plainly seen to exude from the vessels and 

 tracheids on the cut section and if analysed it is found to contain 

 various substances in solution. By attaching a bent tube con- 

 taining mercury to the rooted stumps of cut plants the pres- 

 sure with which this water is pumped up can be measured, and 

 it is found to frequently exceed that of one atmosphere. 



