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. Ifa 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 ag 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 traceable 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 thickness of such 
a trunk or root is brought about by the activity of a layer 
of delicate livmg 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, 6). 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 
