124 BOTANY OF THE LIVING PLANT 



relatively minute. They were also seen to be irregular, and to circle 

 roughly round a central point (Fig. 85, bis). To these movements he 

 gave the name of circumnutation. They may be ascribed to variation 

 from time to time in the relation of turgor to resistance of the cell-wall 

 in the constituent cells. They record the sum of those variations. If 

 the aggregate of them, on all sides of the organ, over a period of time 

 such as a day, were relatively uniform, then the growth of the whole 

 part would appear to be straight forward, as in normal stems. But how- 

 ever quiescent growing shoots may look, and however straight on 

 they appear to grow, still it may be shown that they execute the slight 

 and slow, but constant, movements of circumnutation. These move- 

 ments are not dependent for their existence upon any recognised 

 external stimulus. They are therefore distin- 

 guished as autonomic. 



But if any external cause were to upset this 

 uniform aggregate of growth on all sides of the 

 organ, and the rapidity of growth became un- 

 equal, a curvature of the whole part would result, 

 and a movement of a more obvious kind than 

 FlG - 86 - those of circumnutation would follow. The 



Single node of a Grass- . ...,.,.,. 



haulm, a shows it hori- organism would thus show its irritability, being 



zontal, as it is after the . . r> i 



grass is " layed " by wind, susceptible to external stimulus. Such movements 



b shows it after unequal r . . t ... . . . Tr . 



growth stimulated by gra- of irritability are styled paratomc. If the ex- 



vity more strongly on the , , ., ., 1 r .!_- 



lower side, so that position ternal cause acts till the period of growth is 

 ended, the curvature will itself become perma- 

 nent. This is of frequent occurrence in growing plants, and it is by 

 this means that they are able, in parts that are still young, to adjust 

 themselves to their surroundings, and to recover from mechanical 

 accidents. Instances of such recovery may commonly be seen in 

 vegetation grown in the open. The most prominent example is 

 in the case of cornfields " layed " by wind and rain. In wheat 

 and oats and other Grasses the base of each leaf-sheath retains the 

 power of growth, and is susceptible to the stimulus of gravity : so that 

 the haulm can become again erect after it has been dashed to the 

 ground (Fig. 86). Reaction to the stimulus of gravity provides the best 

 illustration of irritability. Normally the plumule of the seedling grows 

 upwards, and the radicule downwards, both parts showing movements 

 of circumnutation. We have seen that this happens whatever the 

 position of the seed may have been during germination. But if after 

 this position has been assumed, the plant be placed with its shoot and 

 root horizontal, the shoot by a curvature which is strongest at the 



