TACTILE PITS OF DROSERA 



570 



B 



perception of mechanical stimuli : finally, they absorb the soluble pro- 

 ducts of digestion. The actual perception of mechanical (and perhaps 

 also of chemical) stimuli seems to take place within minute papillose 

 protoplasmic processes, which, in Drosera rot u.ndif alia and D. longifolia, 

 are restricted to the margins of the outer wall. These processes are 

 contained in "marginal pits" (cf. above, p. 121), which arise in the 

 following manner : The radial walls of the 

 sensory cells are provided with small pro- 

 jecting ridges ; these are thickest at their 

 points of insertion on the outer wall, but 

 rapidly become narrower as they pass 

 downwards and inwards, and finally die 

 away altogether (Fig. 233 c). These ridges 

 enclose minute superficial pockets, which 

 are prolonged inwards as short, and some- 

 what obliquely directed canals. The fact 

 that these pockets contain protoplasmic 

 processes, can be very convincingly de- 

 monstrated by mounting the head of a 

 tentacle in dilute sulphuric acid for a 

 short time, and then squeezing the pre- 

 paration gently under the coverslip. By 

 this treatment, the protoplasmic contents 

 of the glandular cells are separated from 

 the disorganised cell-walls and at the same 

 time " fixed." The outer surface of the 

 protoplast of each marginal glandular cell 

 is then seen to be furnished with a 

 number of minute papillae ; in Drosera 

 rotundifolia these processes are roughly 

 isodiametric (Fig. 233 b), but in D. longi- 

 folia their height exceeds their width 

 two- to three-fold (Fig. 233 a). In the 

 palisade-shaped apical glandular cells the 



processes are usually confined to the angles of the outer face, a single 

 one being developed at each corner. 



In other carnivorous plants, where the digestive glands and absorbing 

 hairs are not sensitive to mechanical stimulation, the outer walls of 

 the glandular cells are never furnished with tactile pockets. We are 

 therefore justified in concluding that this structural peculiarity of 

 Drosera is connected with mechanical sensitiveness, and not with the 

 secretion of mucilage or enzymes, or with absorption of soluble 

 products. This view is further supported by the strong resemblances 



A. Protoplast isolated from one of 

 the marginal glandular cells of a ten- 

 tacle of Drosera longifolia. B. A similar 

 preparation from a tentacle of D. ro- 

 tundifolia. '. A few marginal glandu- 

 lar cells of D. rotundifolia seen in surface 

 view. 



