28 



GENEBAL BOTANY 



a stem usually receives the stimulus of light or of a mechanical 

 agent. The nearest approximation to sense organs in plants are 

 certain groups of cells in root tips, and in the stem tips of some 

 particular plants, which are endowed with the property of re- 

 ceiving gravity or light stimuli. In general, however, plants 

 have no special sense organs with nerve endings, like animals, 



with which to receive external 

 stimuli. 



The mechanism, or method of 

 response to stimuli, is likewise 

 very different in the higher plants 

 and the higher animals, since 

 plants have no muscles, attached 

 to a jointed skeleton, for effect- 

 ing movements. The 

 most common method 

 of response in plants 

 is that of unequal 

 growth on the two 

 sides, or on regularly 

 alternating sides, of 

 an organ which has 

 been excited by an 

 external stimulus. Thus, in the pea seedling (Fig. 13) the 

 erection of the epicotyl from its curved position was effected 

 by the greater elongation of the tissues on the concave side 

 of the epicotyl than on the convex side. The horizontal spread 

 of the leaves is likewise brought about by the more rapid 

 growth of the tissues on one side of the young leaves than 

 on the other. The curvatures of roots and stems, with which 

 we shall have to deal later, are also brought about in the same 

 manner. This method of securing movements in plants by 

 growth is necessarily slower than the corresponding move- 

 ments of animals, which are results of nerve stimulation and mus- 

 cular contraction, but it is nevertheless well suited to the nature 

 of stationary organisms in which a new set of leaves and new 

 growths of stems and roots are produced each year. 



FIG. 15. A compound leaf of the bean 

 with pulvini 



The pulvini appear at the base of each leaf- 

 let. Note also a large pulvinus at the base 

 of the main petiole. After Sachs 



