212 Elementary Biology. 



notice are some of the general characters of growing parts, 

 and their behaviour in response to certain changes in 

 external conditions. 



Growing parts, e.g. the tip of the shoot, root, &c., are 

 remarkable for possessing an extreme flexibility though 

 they have small power of elastic recoil. If a young shoot 

 or branch be twisted it frequently remains so deformed 

 during its entire life ; a fact that is well known and even 

 embodied in a proverb. Again, all growing parts are 

 extremely succulent. This might be expected, since a 

 great mass of the food supply comes to the growing parts in 

 the form of a watery solution. The abundance of water in- 

 deed makes the cells turgid and tense ; a condition in itself 

 favourable to assimilation and growth. The growing parts 

 are, moreover, very extensible. As the very tip (punc- 

 tum vegetationis) is the region of most active cell division, 

 so the region behind the tip is that of maximum growth. 



The greatest amount of growth takes place during ths 

 night. In the dark, when assimilation is in abeyance, cell 

 division is at a maximum. So in the same way the shaded 

 side of a plant grows more than the illuminated side, with 

 the result that the shaded side gradually bends round to 

 the light. This phenomenon is known as heliotropism. 

 Many parts of plants have, however, a tendency to bend 

 away from the light. Such parts may be termed aphelio- 

 tropic. The term geotropism has been applied to the 

 tendency which roots have to bend towards the earth's 

 centre. Although it is probable that gravitation has not a 

 little influence in producing this movement, yet the fact 

 that the shoot grows upwards, in opposition to the same 

 force, seems to throw some doubt on the assertion that this 

 is the only influence at work. In addition to these move- 

 ments growing parts seem also to be subject to a movement 

 termed nutation, due to unequal growth of different parts. 

 Examples of this movement are universally found in the 

 growing shoot and radicle, while modifications of the same 



