Development of Mechanical Tissue . 5 1 5 
When we use the diameter of the shoot as a basis, a truer 
conception is afforded of the difference between the two 
shoots ; since the important question is not what relation the 
size of the wood-cylinder bears to that of the pith, but what 
relation it bears to the diameter of the shoot. It must be 
borne in mind, too, that, although the wood does not increase 
in the same proportion as does the cortex, yet there is 
a considerable increase in the diameter of the xylem-cylinder ; 
and it still remains to be shown that it is necessary for the 
well-being of the tree that the proportion of increase shall be 
the same in all the tissues. 
Sorauer thinks that the increase in cortex is due to the 
need for greater food-supply, and he states that the increased 
proportion of parenchymatous tissue brings with it a greater 
liability to injury by frost. Finally, he claims that absolute 
size has no value in a comparison of results. It is, he says, 
the proportionate amount of xylem that determines the 
strength of a shoot. 
In a later paper Sorauer 1 reiterates his former conclusions. 
He also attributes to over-cultivation certain swellings on 
shoots, which he finds are due to a parenchymatous change 
in some of the wood-cells. I have not found any of these 
growths, and cannot therefore treat of them in this paper. 
The inference drawn from Sorauer’s work is that cultivation, 
and more directly fruit-bearing, may become injurious to 
a tree by reason of the greater development of cortex and 
thd proportionally smaller amount of xylem produced in the 
fruit-bearing shoot, which renders it weaker mechanically and 
more liable to injury by frost. 
I shall present here a study of the effect of fruit-bearing 
on the permanent mechanical 2 tissue of the tree, in order to 
1 Nachweis der Verweichlichung der Zweige unserer Obstbaume durch die 
Cultur. Zeitsch. f. Pflanzenkrankheiten, Bd. II, 1892, pp. 66-7° an d 142-148. 
2 By ‘ mechanical tissue ’ is meant all those collections of cells having thick and 
lignified walls, and serving to give strength and firmness to the shoot. Since the 
wood-cylinder is the principal collection of such cells, and is the only one capable 
of accurate measurement, most attention will be given to it, with incidental 
reference to supplementary mechanical cells when these are of importance. 
