LECTURE XVII. 

 IRRITABILITY OF GROWING ORGANS (continued}. 



IN the last lecture we considered the irritability of grow- 

 ing organs as exhibited in their response, by alterations in 

 the rate of their growth, more particularly of their growth in 

 length, to the action of various external influences. In the 

 present lecture we will further pursue the study of this subject 

 by considering the response, by alterations in the direction of 

 their growth, which growing organs give to the action of 

 external agents. 



Before, however, we can enter upon the consideration of 

 changes in the direction of growth due to the action of ex- 

 ternal stimuli, we must ascertain as far as possible what 

 internal influences there may be which contribute to determine 

 the direction of growth of an organ. We learned, in a 

 previous lecture (p. 360), that the growth in length of an 

 organ rarely, if ever, takes place in a straight line, in con- 

 sequence of spontaneous heterauxesis, but that its apex 

 nutates. The changes in the direction of growth which con- 

 stitute nutation are, however, only temporary ; as growth 

 ceases, the axis of an organ becomes approximately straight. 

 In endeavouring to account for this fact, we must remember 

 that, in nature, plant-organs are exposed to the action of 

 light, of gravity, and of other agents which tend to modify the 

 direction of their growth. We must not assume, simply 

 because we see that a stem or a branch is straight, that it has 

 an inherent tendency to grow in a straight line, for its straight- 



