CHAPTER XIX 

 THE PRIMARY STEM 



The shoot or subaerial part of the body of the higher 

 plant consists, as we have seen, of stem and leaves 

 together. The stem forms the branching axis of the 

 shoot system and bears the leaves, which grow out from 

 its sides. It thus (a) supports and displays the foliage 

 leaves in the light and air, (6) acts as the channel of 

 communication between root and leaves, and (c) supports 

 and displays the flowers, in which the reproductive cells 

 are formed and in which conjugation of the gametes 

 takes place. 



Tropisms. — The erect aerial shoot is negatively geo- 

 tropic, growing away from the centre of the earth, just 

 as the taproot is positively geotropic, growing towards 

 it. This can be seen in plants kept in the dark which 

 grow straight upwards. If the plant is laid on its side 

 in the dark, the actively elongating region of the shoot 

 bends so as to bring the apex into the vertical position 

 again. The shoot is also positively phototropic, growing 

 towards the source of maximum illumination. These 

 two tropisms normally act in the same direction and 

 cause the shoot to grow straight upwards as the opposite 

 ones cause the root to grow downwards. But if illumina- 

 tion is one-sided, the positive phototropism of the shoot 

 overcomes its negative geotropism, so that it bends 

 towards the source of light. This can be seen in the 



case of many herbaceous plants grown in pots in windows 



31S 



