88 THE OAK 
makes of this starch, and it must suffice to say that 
the starch serves as the basis of all the constructive 
materials used by the tree. Thus it is converted into 
a soluble form, and combined with nitrogen, phosphorus, 
sulphur, &c. (obtained from the earth-salts), to make new 
protoplasmic materials, and it passes down from the 
leaves to nourish all the living cells that require it, in 
the embryonic tissue at the apex of the roots, and that 
at the apex of the stem and branches, buds, &c., and some 
of it passes to nourish the cambium cells, the developing 
flowers, acorns, &c.—in short, wherever new organic ma- 
terial is needed it is supplied from these stores formed 
by the green leaves waving in the sunshine. If we 
reflect that the little embryo in the acorn starts its life 
with only a minute store of starch and proteids in its 
cotyledon, and that all the tons of organic material 
(chiefly wood) found in an old oak-tree have been super- 
added to this by the action of the leaves—the small 
proportion of salts taken up by the roots being quite 
inconsiderable in comparison—we obtain some idea of 
the enormous gain of matter and energy from the outside 
universe which goes on each summer. 
