Cae 
Senet 
234 The Botanical Gazette. [June, 
reached their normal dimensions, and since cork-formation 
must be accomplished by extension and division of cells, it 
follows as a probability that cork-formation is delayed by 
pressure because of the mechanical resistance to cell-extension. 
But since cork-formation is attended by the death of exter- 
nally lying cells, it is easily seen that soon after the formation 
of the phellogen, an external ligature or gypsum band will 
exert no immediate pressure in those cases where the phello- 
gen is some distance within the periphery of the stem, simply 
because of the contraction of the dying tissue between the 
phellogen and the external ligature or band. Thus it seems 
probable that if external resistance is applied early enough to 
prevent the normal extension of the cells in which the phel- 
logen usually appears, the formation of cork may be indefi- 
nitely postponed. If however the cells normally giving rise 
to the phellogen attain their full dimensions before the exter- 
nal resistance is applied we may expect the phellogen to be 
formed and cork-formation to proceed more or less slowly ac- 
cording to the greater or less vitality displayed by the cells 
external to the phellogen, and also according to the depth of 
phellogen within the cortex. 
In this connection it is worth noting that Krabbe*® found 
cork to arise deep within the cortex when a very great pre 
sure was applied to the trunks of trees, this phen 
being abnormal. This is certainly aregulatory act perfor oe 
by the plant, but whether to regard it as a means of furnisi- 
ing more room for the growth of the cambium cannot be d¢- 
cided with the lack of detail touching the accompany!mg “ 
nomena as given by Krabbe. It may be mentioned petaie 
that Melianthus major, though the cork appears within r 
enveloping cast more tardily than outside of it, oven 
frees itself completely from the resistance of the cast eee m 
very act of cork-formation, the activity of the cambium 
that time forward being usually great. ; cells 
IV. Under the pressure of a mechanical Agden “ant? 
veach their permanent condition with smaller size and tht 
walls than normal. : ‘od p 
V. Cells which usually die early have their iit d ad 
longed when their full extension or that of proper ly ve oi ancl: 
Jacent cells is prevented by an external mechanical Tor a 
As appears from the narrative of the experimenta 
rene oe ee 
1®Krabbe: Ueber das Wachsthum des Verdickungsringes, et¢- es 
