444 The Botanical Gazette. [October, 
growth. 
Thus the pressure of the cortex of woody trees is shown to 
exert little or no influence on the growth of the cambium; 
and this assumed support for the mechanical theory falls. 
With it also falls the time honored explanation of the cause 
of annual rings. 
That the first assumption in support of the mechanical 
theory of growth—that extension is mere stretching—does 
not rest upon fact was demonstrated by Pfeffer. ® 
This author by an ingenious method was able to determine 
that cell membranes will extend till they feel no internal 
stretching force. Now if extension were the result of stretch- 
ing alone, this condition could not be brought about, for the 
membrane would then be always in a state of tension. 
If now growth is not controlled in a merely mechanical way 
by the pressure of tissue, we must look for some other ex- 
planation. Without attempting a discussion of the broad sub- 
ject of growth, it has been and can be shown that growth 
especially of mechanical tissues is very often the result of 
self-regulation. It is true that the last statement does not 
propose the real cause; it is a confession of ignorance. But 
it removes the growth referred to from a mechanical phenom- 
enon to one of irritability. This is the argument which I am 
making. 
Let us now in the light of what has last been said examine 
some of the phenomena on which the founders of the mechan- 
ical theory based their argument, not forgetting however that 
the falsity of the theory has already been shown. 
De Vries’ experiments in placing ligatures about branches 
of trees gave the effect of unnatural pressure, not at all com- 
®Pfeffer, Druck- und A beitelei , iia. 
Wiss. 20: —. 1893. rbeitsleistung. Abhandl. d. k, sachs. Gese 
