8 



NATURE AND RELATIONS OF AN ORGANISM 



or region which is designed to make the adjustments in response 

 to such stimuli. The irritable system of the plant may be said to 

 be reflective. 



13. Sensory Organs and Zones. Ra- 

 diant forces may penetrate the body of 

 the plant easily and reach internal cells 

 almost as readily as external ones. As 

 a consequence of this fact no plants are 

 known which have developed special 

 organs or cells for the reception of stimuli 

 of this character and of gravitation, al- 

 though the last named force is supposed 

 to act as a stimulus only upon certain 

 embryonic cells in the tips of roots while 

 certain similar specializations of photo- 

 tropic action are shown. The reception 

 of chemical and mechanical stimuli how- 

 ever, can be accomplished only by peri- 

 pheral protoplasts, and in some species in 

 which instant perception of the stimulus 

 and rapid reaction are of advantage the 

 sensory cells and the motor mechanisms 

 are highly developed with great mor- 

 phological differentiation. This is to be 

 seen in the tentacular formations on the 

 leaves of Drosera, and the epidermal cells 

 of tendrils. Furthermore the cytoplas- 

 mic layer of the cell is probably the 

 functional organ in such action since its 

 position is undisturbed by developmental 

 changes. 



14. Transmission of Impulses. The 

 action of a stimulating force upon the 

 sensory elements may give rise to a new 

 molecular motion the effects of which 



Fig. I. Diagrammatic rep- 

 resentation of the course of 

 the fibrilte supposed to form 

 the path of transmission in the 

 plerome of roots of, Allium 

 cepa. The thickness of the 

 walls has been exaggerated to 

 bring out the interprotoplastic 

 threads. After Nemec. 



