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 surrounding medium 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, ciystalloids, &c, in thickened roots, 

 stems, and other receptacles. Part of this organic material is in a 

 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. Elaborated 

 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, 

 &c , serve as stores of reserve materials, and this is also the case 

 with the trunks of trees which shed their leaves in autumn. The 



