224: VEGETABLE PHYSIOLOGY 



transitory store, for. the starch deposited there during 

 exposure to sunlight is removed almost as soon as dark- 

 ness supervenes. A plant which has been vigorously 

 forming starch in its chloroplasts during a summer's day, 

 will show that at evening there is a considerable amount 

 accumulated there ; if the leaves are examined again early 

 next morning, the starch will be found to have disappeared. 

 This is not brought about by its having been used in the 

 metabolism of the cells during the night, for if the path of 

 removal is obliterated, as it may be by severing the petiole 

 in the evening, the leaf is found as full as ever in the 

 morning. If a plant whose chloroplasts are charged with 

 starch grains is kept for a time in an atmosphere free from 

 carbon dioxide, the starch is gradually removed, whether it 

 is kept in light or darkness, so that the removal of the 

 starch can, and probably does, take place continuously, 

 though it cannot be easily detected so long as construction 

 is proceeding simultaneously. 



The deposition of food in such other reservoirs in trees 

 and shrubs as are not connected with the reproduction of 

 the plant is generally of a transitory character, though not 

 so markedly so as in the case of the leaves. These 

 temporary storage places are found very widely distributed, 

 and the reason for their occurrence is in each case trace- 

 able with comparative ease. A tree that has a trunk 

 and a root which are growing in thickness is in need of a 

 constant rather than an intermittent supply of food placed 

 near the actively growing regions. The growth in thick- 

 ness of such a trunk or root is brought about by the activity 

 of a layer of delicate living cells, which are constantly 

 dividing to produce new wood and new bast, and which 

 appear quite early as a ring of cambium on the exterior 

 of the woody mass (fig. 105, b). The new cells need a 

 constant supply of nutritive material, at the expense of 

 which they develop into the peculiar elements of wood and 

 bast respectively. The cambium too is in continuous need 

 of food, or it is perforce obliged to cease dividing, and so the 



