ee ey ee Ti 
1894. ] The Influence of Mechanical Resistance. 231 
leaf is torn and the leaf changes from the thin, sword-shape 
tothe inflated. The cast effects what darkness effects; in 
both cases the small peripheral cells remain undeveloped. 
But casts better than darkness preserve the life of the central, 
hyaline cells, since the larger etiolated leaves always undergo 
considerable peripheral extension and always show a narrow 
cavity lined by fragments of destroyed cells. That this cen- 
tral part of the leaf has not been kept alive longer than about 
two weeks is accounted for by the fact that at about that 
time the leaf, which is plano-convex, in its effort to expand 
~ inward on its flat side, thus crushing the delicate central 
cells. 
The petiole of Ricinus communis when unextended con- 
tains no central cavity, but a wide one extended. By apply- 
ing casts the death of this central mass of tissue has been 
Prevented for about three weeks longer within than outside 
the casts. 
With the rhizome of Triticum repens a similar result was 
obtained, the internodes being allowed to reach not one-half 
their normal size, and the cavity appearing above and below 
the casts before appearing within. 
Before elongation was complete, casts were put around the 
stems of Caltha palustris, Lamium garganicum, Urtica dioica 
and Vicia faba. All of these plants form central cavities 
during elongation or at its termination, partly by the schiz- 
*senous partly by the lysigenous method. All of them have 
Preserved alive the pith of those parts of the stem within the 
Seats longer than in the parts outside the casts. The period 
of this Preservation was not determined for any species; nor 
tN Matter probably of much moment since the duration 
the 
wholly lies. in which several individuals were found with pith 
n °*Y living within the casts 116 days after it would have died 
‘ormally, 
Internode u ‘ : 
usu 
half A reached nearly a meter in height and had seed-pods 
Nn. 
