13^2 



BOTANY OF THE LIVING PLANT 



or Venus' Fly-Trap [Dionaea) (Fig 92). These plants show elaborate 

 though different mechanisms for the capture of insects, which are subse- 

 quently digested, and the materials absorbed as nourishment. In the 

 former the action is slow, in the latter its success depends upon rapidity. 

 Drosera bears on its spathulate leaves numerous radiating tentacles, 

 each terminating in a spherical gland, which secretes a viscid juice. 

 (Fig. 91.) This acts like birdlime, detaining any small insect that 

 touches it. The contact-stimulus, confirmed by chemical stimulus from 

 substances dissolved, is conveyed to other tentacles than those first 

 touched, and movements of curvature result, so that the tentacles close 

 in and envelop the victim. It is thus covered over by the glandular 

 secretion, and digestion follows. Here the response is in a mature organ, 

 and it is relatively slow ; the stimulus is conveyed from the point of con- 

 tact to a distance, where the 

 reaction takes place, as atem- 

 porary curvature of the ten- 

 tacles. But the exciting cause 

 is contact^ confirmed by the pre- 

 sence of a digestible substance. 

 In the case of Dionaea, 

 rapidity of movement is the 

 chief factor in success. Each 

 of the rosette of leaves of the 

 plant bears at its distal end 

 a two-flapped mechanism, 

 like the covers of a book, 

 mubile along the median line as a hinge (Fig. 92). The flaps are 

 furnished with marginal spines, while three sensitive bristles rise erect 

 from the upper surface of each. A touch on any of these six bristles 

 sets the mechanism in motion. If conditions of temperature be 

 favourable the flaps suddenly close together. An insect touching them 

 would be captured within the trap ; and as the inner leaf-surfaces are 

 furnished with secreting glands, the digestion follows, with absorption 

 of the soluble substances of the insect-body. Here the stimulus 

 applied at the tip of the bristle is amplified by mechanical leverage, 

 which compresses certain thin-walled cells at the base of each bristle. 

 These are the actually receptive cells. From them the stimulus is 

 rapidly conveyed to the reacting tissue of the hinge. 



In all the cases described the living protoplast is the ultimate instigator 

 of the movement. It may act from inner causes, setting up movements 

 which are styled autonomic, such as those of circumnutation. Or it 



Fig. 92. 



Leaf of Dionaea in the receptive state. 



(After Darwin.) { x 4.) 



