42 THE FLOWERING PLANT. 
to another. Some crude sap also passes through the parenchy- 
matous parts of the stem by means of osmosis. The whole of the 
wood in annual stems can conduct water, but in perennials possess- 
ing sap-wood and heart-wood only the former can do so. This is 
shown by an old experiment, in which a ring of tissue was removed 
from the stem of an oak, exposing the duramen. The result was 
that the leaves quickly withered. Since removal of a ring of bark 
alone did not cause such an effect, it was concluded that the 
outer or sap-wood conducted water upwards. This is confirmed 
by the fact that trees can flourish without pith and duramen, 
as shown by hollow specimens. The leaf-protoplasm, aided by 
chlorophyll (see p. 10), forms organic matter from the crude sap 
and the carbon dioxide of the air. A solution of this organic 
matter, the elaborated sap, travels from the leaves to all parts 
of the body which contain protoplasm, compensating waste and 
rendering growth possible. In many plants it is stored up as 
reserve materials, e.g., starch, crystalloids, &c., in thickened roots, 
stems, and other receptacles. Most of this organic material is ina 
soluble diffusible form. This can travel through the parenchyma. 
Another part, consisting of proteids, travels by means of the 
sieve tubes, which form a continuous series of canals. Hlaborated 
sap, then, traverses the outer part of the stem. The experiment 
of removing a ring of bark conclusively proves this, for no growth 
takes place below the wound, since the supply of nutriment is 
cut off, while, on the other hand, increase goes on as usual above 
the wound. ‘Trees are not infrequently seen in country places 
with the lower part of the trunk comparatively small, and abruptly 
succeeded by a considerable bulge. This is generally due to a metal 
hoop having been placed round the stem years previously, which, 
as increase in size took place, first became very tight, and then 
cut through the bark. One or two facts in practical gardening 
illustrate the same point. It is sometimes required to hasten the 
ripening of fruit on some special branch of a tree. ‘This is effected 
by ‘‘ringing” the branch, when the elaborated sap formed in its 
leaves cannot pass beyond the wound, and is employed in building 
up the fruit. Again, on a plum tree for example, the fruits will 
not ripen unless they have at least one leafy shoot beyond them. 
If this were not the case, they would not be in the course of the 
descending current of elaborated sap. 
Where the stem contains chlorophyll, it assists the leaves in 
the formation of organic matter, and this function is mostly or 
solely carried on by the stem in plants which possess phylloclades. 
As we have seen, thickened stems, such as corms, rhizomes, 
&e , serve as stores of reserve materials, and this is also the case 
with the trunks of trees which shed their leaves in autumn. The 
