Development of Mechanical Tissue. 519 
spaces more frequent, and the supply of crystals and of starch 
is abundant. 
In the upper part of the shoot the parenchymatous character 
of the cells increases until, on the side nearest the fruit-stalk, 
very little wood is present, and such secondary wood-cells as 
are formed are not lignified. Above the lateral vegetative 
bud there is no cambium, and no secondary wood appears ; 
but here the supplementary mechanical cells, bast and scler- 
enchyma, become prominent. Groups of sclerenchyma-cells 
appear in the cortex and in the medullary rays ; the pith is 
usually lignified ; hard bast is present in great quantity and 
continues, but decreases in amount down to the base of the 
shoot, where it disappears. 
The presence and arrangement of the hard bast is interesting. 
In the fruit-bearing shoot it appears in great abundance, its 
distribution being the reverse of that of the wood, namely, 
the greatest amount at the apex, where there is least wood, 
and decreasing until there is none at the base, where the 
wood is well developed. In vegetative one-year-old shoots 
of Rhode Island Greening, I found no bast-fibres. In the 
mature fruit-stalk the greater part of the cortex as well as 
the pith becomes lignified. 
It is clear that the upper portion of the fruit-bearing shoot 
has a weak development of xylem. The wood-cells decrease 
in number and in the thickness of their walls until the xylem 
disappears entirely. This is, however, the case only in the 
upper portion of the shoot, which subsequently dries up and 
falls away. But this weakness is more than compensated for 
by the abundant supply of well lignified sclerenchyma and 
hard bast-cells. These are found in groups and bundles 
where the xylem is weak, decreasing where the latter becomes 
strong. 
The comparative development of the xylem-cylinder at the 
bases of the fruit-bearing and vegetative shoots had, of course, 
to be determined by measurements of the tissue-zones in the 
cross-sections of the two kinds of shoots. The results of these 
measurements are embodied in the tables given subsequently. 
