Development of Mechanical Tissue. 521 
wood-cells in the fruit-bearing and in the vegetative shoots, 
sections were taken near the bases of both ; camera drawings 
were made of the wood-cells in a certain area ; and the paper 
covered by the drawing was weighed. The portions repre- 
senting cell-cavities were then cut out and the remainder 
again weighed. It was found that the weight of the paper 
representing cell-wall was 63-64 per cent, of the weight of the 
entire section in the case of the vegetative shoot, and 46-72 
per cent, in the fruit-bearing shoot. This difference in the 
thickness of the walls became greater near the apex of 
the shoot, where many of the cells remain thin-walled until 
the following year, when their walls become thicker and are 
lignified. The lignification of the wood-cells proceeds from 
below upward, and from the younger secondary cells inward 
to the older primary ones. The primary vessels themselves 
are always first lignified, but the cells surrounding them and 
the secondary cells, formed early in the season, remain soft- 
walled until after the more lately formed secondary cells have 
become lignified. 
The figures given above as representing the relative thick- 
ness of the walls of the wood-cells in the two kinds of shoots 
must not be regarded as average ones, since only one test 
was made. But there is no doubt that, as a rule, the wood- 
cells in the one-year-old fruit-bearing shoot have thinner walls 
than the cells in vegetative shoots of the same age. 
The young fruit-bearing shoot must support the weight of 
the apple, but ample provision has been made for this in the 
abundant supply of hard bast and sclerenchyma. In view 
of this splendid development of supplementary mechanical 
tissue, the slightly smaller proportion of xylem in the fruit- 
bearing shoot cannot be considered a serious weakness for the 
first year. But it remains to be seen whether the relations 
existing at that time continue during the life of the shoot. 
To answer this, small branches were gathered, portions of 
which, now three and five years old, showed by their swellings 
and fruit-scars that they had borne fruit during their first 
year’s growth. Other portions of branches were gathered, of 
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