19° PLANT LIFE. 



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. 386, 205.) 



275. Transmission of stimuli. — 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. Much uncertainty exists as to how this is ac- 

 complished. In some cases it is doubtless done by means of 

 the connecting threads of the protoplasm from cell to cell, 

 after the analogy of a diffuse nerve. In other cases it may 

 be transmitted through certain strands of tissue by the altera- 

 tion of the hydrostatic pressure in the interior of the cells. 



The movements of plants may be conveniently considered 

 as (1) movements of locomotion by single cells; (2) move- 

 ments of protoplasm within a cell-wall; or (3) mass move- 

 ments of multicellular members of the higher plants. 



I. Locomotion of single cells. 



276. 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 or ciliary movements. Amoe- 

 boid movements are slow creeping movements brought about 

 by the protrusion of a portion of the protoplasm (a pseudo- 

 podium), toward which the remainder gradually flows (fig. 

 169). 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. 168). 

 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 withdrawn or drop off. 



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

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



