1894. ] The Influence of Mechanical Resistance. 199 
they pass into mechanical tissue. In no plant have these 
cells shown thicker walls within than without the casts, and 
where it can be determined that they have reached their de- 
finitive condition they are thinner walled within the casts 
than normally. Vicia faba and Melianthus major have 
illustrated the truth of the last statement. In these two 
within gypsum, was released and grew for weeks afterward. 
When sections were made the outer pith-cells were found 
thinner-walled than normal, though they had reached their 
definite condition. That they had reached their definite con- 
dition was determined from the examination of several indi- 
viduals at different ages, and thus the time when develop- 
ment ceased could be inferred. 
That the xylem elements remain definitely thinner-walled 
When gtowing under pressure was determined by Krabbe 
(1c) for trees. None of my plants have indicated a con- 
trary result and the two species in which it could be certain 
that the definitive condition had been reached corroborated 
Arabbe’s view. Melianthus major and Vicia faba have each 
several instances been released from casts and allowed 
“i stow Subsequently. The xylem elements that had been 
*rmed while the casts were present could be easily distin- 
guished by their thinner walls from those formed subse- 
iuent to the removal of the casts. 
Uni. ; (To be concluded.) 
"versity of Michigan, Ann Arbor. 
