1 66 OUTLINES OF PLANT LIFE. 



234. Localization of irritability. — In multicellular plants 

 irritability to certain stimuli is usually localized in certain 

 organs, and often in special parts of these organs. In many 

 tendrils, for example, the free end is curved and only the 

 concave side is irritable to contact. In the Venus fly-trap, 

 although the whole leaf moves at the contact, only the three 

 hairs upon the upper face of each lobe are sensitive to a 

 touch. (See figs. 224, 137.) 



235. Transmission of impulse. — In these cases, as in many 

 others, the effect of the stimulus must be transmitted in some 

 way from the point of application to the cells which produce 

 movement. At present it is not known how this is accom- 

 plished. 



The movements of plants may be conveniently considered 

 as (i) movements of protoplasm itself; or, (2) mass move- 

 ments of multicellular members of the higher plants. 



I. Movements of protoplasm. 



236. Naked cells. — Plants which consist of a single cell 

 may be either naked or furnished with a cell wall. If naked, 

 they may exhibit either amoeboid ox: cz'/wrj/ movements. Amoe- 

 boid movements are slow creeping movements brought about 

 by the protrusion of a portion of the protoplasm toward 

 which the remainder gradually flows (fig. no). Ciliary 

 movements are due to the extension of one or more very 

 slender threads, called cilia, whose rapid bending in different 

 directions propels the organism (fig. 109). According to 

 the nature of the movements, the course will be zigzag or 

 steady, accompanied by the rotation of the cell on its axis. 

 When the cell comes to rest the cilia are either withdrawn or 

 drop off. 



237. Cells with a wall. — Movements of locomotion in 

 plants possessed of a cell wall are either ciliary or creeping. 



