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. 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 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, 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 perforce obliged to cease dividing, and so the 
