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 b}^ 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 i)lants 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 



