184 PRACTICAL COURSE IN BOTANY 
E. Lloyd of the Alabama Polytechnic Institute (Figs. 245- 
250), though obviously influenced by the sun, are not 
directed toward it as in those of truly heliotropic plants. 
These movements are common also among flowers, many 
of them having regular hours for opening and closing, as in- 
dicated by such names as ‘‘morning-glory” and “ four- 
o'clock.” In these cases, however, other causes (277, 280) 
than the light relation must be taken into account. 
201. Irritability is a general term applied to the power in 
plants of receiving and responding by spontaneous move- 
ments to impressions from without. In its widest accepta- 
tion, irritability includes, besides the various forms of 
adjustment described in this section and the next, all move- 
ments due to geotropism, those of roots seeking air and mois- 
ture, the revolution of twining stems and tendrils, the circu- 
lation of protoplasm in the cell — any movement, in short, 
that is made in response to an impression from the environ- 
ment is a manifestation of irritability. It may be of various 
degrees, but is possessed to some extent by every living vege- 
table organism. 
The term is usually applied, however, more especially to 
those obvious and pronounced responses made by plants to 
their surroundings, as exemplified in the cases just given. 
Still more marked instances are to be found in the movements 
of the tentacles of insectivorous plants, and the sensitive 
leaflets of the mimosa that close at the slightest touch. The 
tendrils of the passion flower are said to appreciate and 
respond to a pressure that cannot be distinguished even by 
the human tongue, and many plants will detect and respond 
to the ultra-violet rays of light, which are entirely invisible 
to man. 
This faculty of irritability among plants corresponds, in an 
imperfect, rudimentary way, to what we recognize in animals 
as nervous excitability. By this it is not meant to imply 
that the two things are identical in their ultimate manifes- 
tations, though we may regard them as fundamentally the 
