41] CHAPTER I. GEXERAL PHYSIOLOGY. 169 



root-hairs are usually not developed by aquatic plants, the roots of 

 which, at least, are habitually immersed in water ; nor by plants 

 in which the transpiring surface is relatively small in proportion 

 to the root-system (e.g. small-leaved Conifers ; saprophytes, such 

 as Monotropa and Neottia). The root-hairs not only promote the 

 absorption of water, but also the absorption of salts from the soil, 

 coming, as they do, into very intimate relation with the minute 

 particles of the soil. They thus give the root a firmer hold on the 

 soil, and render it more serviceable as an organ of attachment. 



In many cases the root becomes adapted to serve as a depository 

 of reserve plastic materials : such are the tuberous roots (p. 45) 

 of various plants, in which secondary growth in thickness (see 

 pp. 140, 143) produces a large amount of parenchymatous tissue, in 

 the cells of which the plastic substances (starch, etc.) are deposited. 



The physiological adaptation of the root is even more remark- 

 able in its properties than in its structure, as is shown by its 

 irritability to the action of various stimuli. Thus the action of 

 the force of gravity causes roots (at least primary roots) to grow 

 towards the centre of the earth (positive geotropisni) : the action 

 of light, as a rule, causes the growing root to curve away from the 

 source of light (negative heliotropism) : a moist body causes the 

 root to curve towards it (positive hydrotropisni) : contact with hard 

 substances produces curvatures by which the direction of growth 

 of the root is altered. 



These various kinds of irritability are of great importance in 

 ensuring the due performance of its functions by the subterranean 

 root. Positive geotropism causes it to penetrate into the soil, and 

 this is also promoted by negative heliotropism : positive hydro- 

 tropism causes it to grow towards the moister parts of the soil, 

 and thus tends to ensure an adequate supply of water. Its sen- 

 sitiveness to contact enables the root to get round obstacles which 

 it may meet with in the soil. 



b. THE STEM. The function of the stem is essentially this : to 

 bear the foliage-leaves and the reproductive organs, and to bear 

 them in such a way that they shall occupy the most favourable 

 position for the performance of their respective functions. Further, 

 it is the means of communication between the roots and the leaves. 

 Occasionally it is specially modified to subserve other functions. 



It has been already pointed out that the form of the stem varies 

 widely in plants, and the most characteristic forms have been de- 

 scribed (p. 27). The general physiological meaning of this variety 



