io6 THE PHYSIOLOGY OF PLANTS 



up water, it increases much more in length than the other 

 tissues, especially more than the woody cylinder. If we 

 remove the latter, the pith will increase still more in length, 

 which shows us that it is its attachment to the wood which 

 prevents the pith from increasing to its full extent. The 

 rapidly elongating pith, however, drags the young ring of 

 wood with it, as long as the latter is still soft and thin-walled. 

 Besides this chief function, it has a secondary one, which often 

 lasts until the stem is very old, and that is of storing away 

 the reserve substances of the tree. In the winter, either a few 

 rows of cells, or the whole of the pith, may be found to be 

 filled with starch. 



The stem, therefore, which is enormously developed in long- 

 lived plants, appears in spite of its great dimensions to have 

 only a subsidiary function in the economy of the plant. It 

 is primarily a framework which bears the organs of chief 

 activity, the leaves, the vascular bundles of which it connects 

 with the single central vascular cylinder of that organ, which 

 is second in importance to the plant — namely, the root. Like 

 all organic machines, as distinguished from mechanical ones, 

 the stem has the power of developing and completing its own 

 structure to suit new demands. By means of its pith it can 

 increase in length, and thus make room for new generations 

 of leaves ; its ring of cambium enables it to form new con- 

 ducting tissues for the increased number of leaves. The 

 conducting tissues are of two kinds. The wood contains 

 vessels which lift the nutritive solution taken up by the root 

 to the centres of assimilating activity, and carry them by 

 delicate ramifications (veins) to the most extreme points of 

 the leaves, the green cells of which have the power, under the 

 influence of light, of building up from the carbonic acid of the 

 air and from the salts of the soil new organic 'material which 

 can be used for further growth. 



Whatever material the leaf cannot itself employ at once for 

 its own growth passes into the second part of the vascular 

 bundle, the soft bast, and through the sieve-tubes of the latter 

 into the axis. There the formed food substances pass slowly 

 downwards through the bast, giving off laterally (both to the 

 pith and to the cortex) whatever may be needed by these 



