1894.] The Influence of Mechanical Resistance. 155 
sclerenchyma sheaths have, by the employment of casts 
around the stem, been kept thin-walled for thirty-seven days, 
in Caltha palustris for fifty-two days, though these cells re- 
mained alive and in the same stems above and below the 
casts passed into their thick-walled condition. 
_ Other plants in which the outer zone of pith-cells normally 
becomes thick-walled have served still better to illustrate this 
principle. Numerous examples of Vicia faba have shown 
that the outer pith-cells begin to thicken their membranes two 
or three weeks after their internode is fully elongated. If, 
however, a cast is laid around a very young internode, the 
thickening of the membranes of the pith-cells will be delayed 
for weeks after it has begun in the internodes of the same 
stem above and below the cast. Thus in a stem that had 
grown to the height of ten internodes, whose third internode 
above the cotyledons had been encased in gypsum before 
elongation was complete, the subsequent period of growth be- 
ing thirty-two days, the pith-cells beyond the limits of the 
cast were becoming thick-walled, while within the cast they 
retained their thin-walled condition. Other plants of the 
Same species similarly treated but allowed to grow twelve 
days longer, at which time they had added to their height 
eta to blossom, showed within the limits of the casts 
eat Preparations, but twelve days older, and consequently 
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