ETIOLATION AS A PHENOMENON OF ADAPTATION. 349 
noted that, according to what I have called the adaptation 
hypothesis, darkness does not act swiijly as a disturber of nutri- 
tion^ hut indirectly as a signal or stimulus : as long as the 
darkness-signal is continued the plant responds with exaggera- 
tion of internode and dwarfing of leaf ; daylight, on the other 
hand, is a signal to the plant that it may slow down internodal 
growth and begin to develop its assimilating machinery. 
It is worth inquiry how far other details in the form of 
etiolated plants can be explained on a similar principle. The 
shoot or plumule of a seedling Phaseolus is well known to end 
in a hook, and to make its way through the resisting soil by 
means of the smooth convex side of the arch. This is believed 
to be an adaptation by which the delicate tip of the plumule 
is protected from injury as it makes its way up through the 
earth. As soon as the plumule is above ground and free from 
the resistance of the soil, it can safely become straight. We 
might, therefore, expect that the absence of resistance would ba- 
the signal which would tell the plumule to become straight. But 
this is not so ; the signal for which it is waiting is daylight, and 
if kept in the dark it remains arched as though still under- 
ground.* The interest of this case is that the plant is guided 
by the absence or presence of light in its reaction to a part of 
its environment which has no essential relation to light. The 
arch in the plumule of the etiolated Phaseolus seedling i& 
therefore a case of a response to a signal of a more complex 
nature than the production of long internodes and dwarf leaves. 
Precisely similar cases are familiar to botanists. The flowers 
of the Crocus and Tulip are singularly sensitive to changes of 
temperature, and can be made to open by a rise of a few degrees 
and close again by a corresponding degree of cooling. These 
movements of the perianth are believed to be serviceable in pro- 
tecting the pollen, not from heat or cold, but from wet. When 
a bright day is overcast and rain threatens, the Crocus gets a 
hint of what is coming in the falling temperature, while other 
flowers make their weather forecast from the darkened sky 
rather than from the chilling of the air. 
I have dwelt on these cases because they throw light on a 
well-known form of etiolation occurring among fungi.f The 
* Wortmann, Bot. Zeitimg, 1882, p. 915. 
t Brefeld, Bot. Zeitwig, 1877, p. 386. 
