IRRITABILITY. 507 



of the long axis is, whether it be horizontal or vertical, so 

 long as the vertical position of the surfaces is secured. 



We have now fully discussed the various influences which 

 determine the position and the direction of growth of shoots, 

 roots, and leaves, in general, and it now only remains to 

 consider some special cases. 



In speaking of the direction of growth of plant-organs, 

 we have tacitly assumed, so far, that the rigidity of the organ 

 is sufficiently great to prevent any very considerable modifi- 

 cation of the direction of its growth in consequence merely of 

 its own weight. But it not infrequently happens that the stems 

 of plants are not sufficiently rigid to support their own weight 

 and that of their foliage. When this is the case the stem 

 may simply trail along the ground, or it may in some way 

 attach itself to external objects and thus grow upwards 

 into the air. The mode of attachment, in the latter case, is 

 different in different plants. Some, such as the Brambles, are 

 simply hooked on by the prickles with which their stems are 

 provided (hook-climbers) ; others attach themselves by means 

 of tendrils (tendril-climbers), or by leaf-stalks which act like 

 tendrils (leaf-climbers); others again, like the Ivy, attach 

 themselves by roots (root-climbers); others, finally, have 

 twining stems. It is to these last that we will specially direct 

 our attention with the view of ascertaining the various factors 

 which determine this peculiar mode of growth. 



A twining shoot, at its first development is straight, but, 

 after it has come to consist of two or three internodes, its 

 apex hangs over on one side, and it then exhibits in a marked 

 manner that circumnutation which we have discussed in a 

 previous lecture (p. 363). This hanging-over of the apex is 

 due to the fact that the shoot is now no longer able to main- 

 tain itself in a vertical position. The twining shoot at this 

 stage exhibits, in fact, that excessive elongation of its inter- 

 nodes which is so often to be found in etiolated plants, and it 

 is for this reason that Sachs has termed these twining shoots 

 "normally etiolated shoots" (p. 385). If, now, the shoot 

 comes into contact with or more or less nearly vertical 

 support of appropriate thickness, it twines round it. 



