414 New combe. — On the Cause and Conditions 
Cepa preserve all their cells alive for a considerable period 
after normal leaves become hollow. But it is also true that 
in such etiolated leaves the central parenchyma finally dies 
before the leaves die, and dies without collapsing. Again, the 
intercarinal canals in Equisetum limosum , though their forma- 
tion has been delayed for a time by gypsum-casts which 
prevented the young internodes from elongating, finally 
appear within the limits of the casts as well as outside. In 
Caltha palustris , in whose stem a large lysigenous cavity 
appears during primary growth, the death of the pith is 
delayed by the use of gypsum-casts, but not altogether 
prevented, unless the cast is put about the stem when the 
latter is so young that the great prolongation of the life of the 
pith must, as will be shown later, be looked upon as a regulatory 
process. In the stems of this plant the tension is so much 
reduced by the use of the casts that the pith does not collapse 
when it dies, and we have the same condition as normally 
occurs in Sambucus and other plants. 
In L annum garganicum , Myrrhis odor at a, A rchangehca 
sativa and Melianthus major , precisely similar results have 
been obtained as with Caltha palustris. All of these plants 
normally form a lysigenous cavity in the pith during primary 
growth : all of them had casts applied about their stems 
before any pith-cells died, yet not so early as to prevent the 
pith-cells from attaining or nearly attaining their full size. 
In all such cases the life of the pith was prolonged from one 
to several weeks, but the pith then died zvithout collapsing. 
It is, of course, to be understood that to obtain these 
results, and to be able to make the statements given, very 
numerous experiments have been performed. Some of these 
experiments under somewhat different conditions have given 
results which are not stated in this place because they will 
best be considered later under another heading. The one fact 
which now demands special attention is the result that the 
pith died in all these plants but a short time later than 
normally and without being torn as it is usually ; and this 
leads to the conclusion that such cells would not live much 
