HOW PLANTS ARE INFLUENCED 359 



It is a very exact index of its surroundings, so that a 

 skilful woodsman knows at once, on looking at it, the 

 conditions under which it has grown. The influence of 

 light, water, prevailing winds, have all left their marks 

 on it. Its position, with reference to the compass, can 

 be told by examining it. For this reason, in transplant- 

 ing a tree, we should be careful to set it so that the 

 same side faces the north after, as before its removal. 



These facts serve to show that the form of the plant 

 is largely determined by its surroundings. It may also 

 be greatly changed by diseases, especially the attacks 

 of fungi, to which are due the remarkable growths 

 known as "witches' brooms," etc., as well as by in- 

 sects, which not only cause the formation of all sorts 

 of curious galls, but even in some cases produce the 

 greening of flowers, in which the petals, anthers, etc., 

 are transformed into green, leaf -like bodies. 



Moreover, the development of each part of the plant 

 is influenced by that of every other, as is demonstrated 

 every day by the experience of making cuttings and 

 of pruning (see pages 79, 86, 257 and 263). If the tip 

 of the main axis of a Pine be removed, the branches 

 just below it begin to straighten up to take its place; 

 one finally does so and the rest then fall back to their 

 original positions; the one which becomes upright 

 changes its structure and becomes like the tip of the 

 main trunk whose place it takes, e. g., it becomes 

 alike on all sides (arid the internal structure changer, 



