CHAPTER I. 3ENERAL PHYSIOLOGY. 683 



root to curve towards it (-positive liydrotropism) : 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. Similar phenomena of irritability 

 are exhibited by aerial roots, but with various modifications in 

 accordance with the widely different conditions. 



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 posi- 

 tion 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 

 described (pp. 40, 44). The general physiological meaning of this 

 variety of form is that different plants attain the most favourable 

 position of their foliage-leaves and reproductive organs in differ- 

 ent ways which depend upon the particular combination of ex- 

 ternal conditions under which they severally have existed. 



The internal structure of the stem varies to some extent with 

 its general habit, and mainly in the arrangement and relative 

 degree of development of the sclerenchyma ; thus, the scleren- 

 chyma is more largely developed in an erect than in a trailing 

 perennial stem. 



The arrangement of the vascular tissue, however, depends 

 essentially, at least in all those plants which have common 

 bundles, upon the arrangement of the leaves. In stems having 

 cauline bundles (p. 171) there is a more or less marked aggrega- 

 tion of the bundles towards the centre, so as to lead to the 

 formation of an axial vascular cylinder, as in roots ; this is espe- 

 cially marked in stems bearing small leaves, as in Lycopodium. 



There is one point in connexion with the relation of the vascu- 

 lar tissue to the leaves which requires more special consideration. 

 It has been pointed out (p. 191) that vascular tissue is formed 



