TACTILE HAIRS OF SENSITIVE LEAVES 



589 



of protection against crawling insects, the above described distribution 

 of the tactile hairs is obviously advantageous. 



The histological structure of the tactile hairs of Biophytum is 

 exceedingly interesting. The hair proper (Fig. 240) consists of a single 

 very thick-walled and lignified cell, *9 to 1/2 mm. in length, and 

 tapering to a sharp point. The low T er end of this cell is slightly 

 curved and obliquely truncated. While the convex side of the basal 

 curved portion is freely exposed, the concave side rests against a 

 projecting cell-cushion, which also envelops the two flanks of the hair- 

 cell. This cushion projects far above the general level of the 

 epidermis, and, in addition, fits into 

 a shallow depression on the con- 

 cave side of the hair ; the wall 

 of the hair is usually specially 

 thickened at this point. As far 

 as the hairs on the rhachis are 

 concerned, the cushion is entirely 

 made up of several superimposed 

 tiers of tabular epidermal cells. 

 These cells, which are thinner- 

 walled and more abundantly pro- 

 vided with protoplasmic contents 

 than the ordinary epidermal ele- 

 ments, undoubtedly represent the 

 sensory portion of the tactile organ. 

 The hairs on the pinnae are some- 

 what more complex in structure ; 

 here a few palisade-cells, forming a 

 conical prolongation of the photosynthetic tissue, are also included in 

 the cushion ; but there is no reason to suppose that these accessory 

 cells are possessed of any perceptive capacity. 



The stiff hair-cells act as levers ; when they are bent downwards 

 the sensitive basal cushion is forcibly compressed, and hence undergoes 

 powerful stimulation. The mechanism is comparable to that of the 

 simple apparatus used in laboratories for compressing corks. It is 

 clear that stimulation will also take place if the hairs are bent back- 

 wards, so that the cushion is subjected to tension. Similarly con- 

 structed hairs have been observed by the author in Biophytum 

 prolifervm. 



In Mimosa pudica the sensitive lower half of the main pulvinus 

 bears from ten to twenty-five upwardly directed tactile bristles. 

 Each bristle is from 1 to 2'5 mm. in length and ends in a conical 

 point ; it consists of a bundle of more or less thick-walled lignified 



Median L.S. through one of the tactile hairs of 

 the rhachis of Biophytum sensilivum ; only the 

 basal portion of the hair proper (which acts as a 

 stimulator) is shown. 



