6 THE PHYSIOLOGY OF PLANTS 



naturally be stopped, as the root will scarcely grow at all. 

 The development of root-hairs is most abundant in a damp 

 atmosphere. 



We have already mentioned that the particles of soil cannot 

 be removed from the absorbing portion of the root, but adhere 

 to it pretty firmly. If seedlings are grown in a damp atmos- 

 phere on moderately damp sand, in a short time many root-hairs 

 will be found which have come in contact with grains of sand. 

 Where this is the case, they will flatten themselves against 

 the particle, will curve themselves round it, and may grow 

 on to repeat the process with another grain of sand. The 

 cessation of growth in length and the expansion of the point 

 of contact are not, however, the only phenomena which make 

 their appearance when a root-hair touches a particle of soil. 

 The outer wall of the root-hair will be seen to lose its sharp 

 outline and swell up into a slimy mass. It is this mucila- 

 ginous coating which fastens the grains of sand so firmly to 

 the root-hairs. Now let us imagine that instead of a grain of 

 sand, the root-hair has grown round a particle of soil which is 

 covered with a thin deposit of the salts necessary for the plant. 

 As a matter of fact, the mineral salts which are most essential 

 to plant life are absorbed by the soil, and are mechanically held 

 by each particle of soil, together with a thin pellicle of water 

 which surrounds each particle. Now when a root-hair comes 

 into close contact with such a particle, the mucilage will help 

 to dissolve the salts, and the latter pass through invisible pores 

 of the membrane or cell-wall into the root-hair (see Fig. i H), 

 saturate the liquid or sap it contains, and permeate the 

 membrane which separates it from the next cell within, belong- 

 ing to the region B. If this inner cell of the cortex is in want 

 of more nutritive matter, it will absorb some from the root-hair 

 and satisfy its own wants. But that will not conclude the 

 matter, for the neighbouring cells will in their turn absorb 

 from the cortical cell, only, however, to pass on these substances 

 to still more eager cells which lie above them. In this 

 way a passage of the nutritive substances takes place to- 

 wards the upper regions of the plant, towards the growing 

 points, which are always in want of food material, and as these 

 substances are constantly being used up in these regions, the 



