SEAT OF THE PERCEPTIVE FACULTY 573 



structures in plants, especially as the latter often exhibit a close 

 resemblance in plan to some of the perceptive organs of animals.-'''' 



So far as is known at present, plants have only developed sense- 

 organs in relation to a few of the various types of external stimulation, 

 namely, to the stimuli of contact and shock (mechanical stimuli, in the 

 narrow sense), the gravitational stimulus, and, finally, photic stimula- 

 tion. It is true that plants are also capable of perceiving chemical 

 stimuli ; but special sense-organs comparable to the organs of taste and 

 smell in animals have not as yet been discovered in the vegetable 

 kingdom. Special arrangements for the perception of thermal stimuli 

 likewise seem to be lacking. 



In considering the general principles governing the construction of 

 the sense-organs of plants, it is necessary to bear in mind that the real 

 act of perception always takes place within the living substance. Noll 

 was the first to point out, that the perceptive faculty cannot be dis- 

 tributed throughout the protoplasm of a sensitive cell, and, in particular, 

 that it cannot be located in the streaming portion of the cytoplasm. Let 

 us, for example, consider the case of mechanical stimulation, the per- 

 ception of which depends upon deformation of the sensitive protoplasm. 

 In this instance, it is cpiite obvious that the streaming granular cyto- 

 plasm (polioplasm) must be left out of account ; for, since it is con- 

 stantly undergoing "spontaneous" [i.e. internally regulated] deformation, 

 it cannot possibly be stimulated by deformations of external origin. 

 By the same line of argument, we arrive at the conclusion that the 

 rotating or circulating portion of the cytoplasm cannot possibly be 

 entrusted with the duty of perceiving the direction of gravitation, or 

 of the incident illumination. We are thus forced to assume, with Noll, 

 that the perceptive faculties of a cell are mainly, if not entirely, vested 

 in the stationary and relatively solid ectoplast. If the internal 

 plasmatic membranes (vacuolar membranes or tonoplasts) should 

 happen to be so firm, as to be able to resist displacement by the 

 streaming protoplasm, they would also, of course, be able to receive 

 the impress of external stimuli. The same argument applies to 

 any other cytoplasmic structures that maintain a fixed position in 

 the cell. 280 



The most characteristic structural feature of sense-omans in 

 general, is the presence of arrangements which suitably direct and 

 control the incidence of external stimuli upon the sensitive portions of 

 the protoplasm ; it is these anatomical features that alone are acces- 

 sible to direct observation. Hence, in attempting to discover the 

 connection between the construction of any sense-organ ami its 

 function, we must always confine our attention to those structures and 

 arrangements which facilitate or intensify the actual stimulation of the 



