230 VEGETABLE PHYSIOLOGY 



of small granules of starch. These afford us an instance 

 of a very transitory store, for the starch deposited there 

 during exposure to sunlight is removed almost as soon as 

 darkness 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 morn- 

 ing. 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 proceed- 

 ing 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 pre found very widely distributed, and the 

 reason for their occurrence is in each case traceable with 

 comparative ease. A tree that has a trunk arid 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 thickness 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, fe). 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 



