66 THE BIOLOGY OF THE SEASONS 



not by the bark is proved by the fact that the leaves of 

 a tree from which a ring of bark has been removed are 

 still able to flourish. That the path is not by the central 

 wood is plainly shown by the fact that a tree whose heart 

 has rotted out may still bear fresh and vigorous leaves. 

 Thus we may at once assert that the upward path of the 

 sap is by the outer or younger wood. 



As to the use of the ascending current, the answer is 

 complex. It forms the whole food-supply of the plant 

 except the gases absorbed by the leaves from the air. 

 The water enters into the formation of starch and proteids, 

 and the salts also help more or less directly. The water 

 also serves to maintain the vigour of the protoplasm, for 

 abundance of water is almost always a condition of activity 

 in plant cells. Finally, no small part of the water makes 

 good the loss which leaves are always suffering from 

 radiation. 



The factors are (i) the difference of density between 

 root cells and soil water ; (2) the suction from the leaves 

 which obviates the dilution of the whole plant ; (3) the action 

 of capillary forces ; and (4) the special activity of certain 

 cells on the upward path. 



UNPACKING OF BUDS 



Comparable in many ways to the germination of seeds 

 is the unpacking of buds, which is equally characteristic 

 of the Spring. Like the seeds, the buds were made in the 

 abundance of a past summer ; they express the super- 

 abundance of vegetative life. Like the seeds, the buds 

 are adapted to contain much within small compass, and 

 are protected by scales which correspond to husks. Like 

 the seeds, the buds have a period of quiescence after the 

 period of formation, for many of them remain nine months 

 or so in a dormant state, 



