1894.] The Influence of Mechanical Résistance. 195 
The explanation for these changes is not difficult to find. 
The pith is the nearest of any tissue to the close of its growth 
and has the smallest turgor. The cortex is at the stage of 
its most rapid extension, and every cell dilates in the direc- 
tion of least resistance. The young bundles grow also, but 
the amount of their extension is much less at this time than 
that of the cortex, consequently they do not check the inward 
pushing of the latter but are carried by it in against the pith 
whose cells are thus crowded closely together. This move- 
ment continues till the intercellular spaces are filled, or till 
the pre-existing pith-cavity is closed up, when an equilibrium 
ls established and the cortex grows no farther. Displace- 
ment does not always end here, however, for the force of 
growth in the primary or secondary meristem of the vascular 
ming May prove sufficient to crowd back the cortical cells, 
often causing them to show wavy walls and in some plants, 
as Ricinus communis and Eryngium planum, actually crush- 
ing them. 
ager than in normal growth their capability for extension, 
me oar of necessity reach a time when that capability is 
found ¢ ith a stem within a cast the pith-cells generally are 
furnish ae the least of any tissue the room for extension 
attain : ” the intercellular spaces. The size which they 
Which ss ative to their normal size is very nearly the same 
hus j r had When the gypsum was laid around the stem. 
youn = tchangelica sativa, which had been encased when 
ter a th € pith-cells were found to have one-half the diame- 
Cells i. above and below the cast, though none of these 
ing pro = ided meanwhile. A similar result with a vary- 
Bittner, See in the size was obtained in all the plants ex- 
recisel ath, ten species in all. 
tex, a Y similar results were found in the cells of the cor- 
Cast a that they nearly always expand more within the 
Power my to the pith-cells. In the cortex, however, the 
80 Much — is retained both normally and within ae 
the seh nger than in the pith that there were but few 0 
their defi under experiment that showed cortical cells in 
itive condition. It is true, nevertheless, that the 
