576 



SENSORY SYSTEM 



In Gucurbita l\'po, Cucumis sativa, Lagenaria vulgaris, Gyclanthera 

 cxplodens, etc., the outer wall of each epidermal sensory cell is furnished 

 with a single approximately central pit. The pit-cavity is nearly 

 circular or elliptical in cross-section, and, as a rule, expands towards its 

 outer end in a funnel- or basin-shaped manner. The diameter of these 

 pits varies between 1*5 and Q/u. In Cyclanthera explodens there is no 

 funnel-shaped expansion, and the pit-cavity consists of a shallow 

 circular trough. The thin membrane which forms the roof of the 

 cavity is flat, or bulges very slightly outwards {Cyclanthera explodens) ; 



in Gucurbita Pepo it is only "6 to '8/x in 

 thickness. The cuticle is no thinner 

 over the pits than elsewhere ; it is 

 underlain by a very thin cellulose-layer, 

 which in Gucurbita is slightly cutinised. 



The living contents of the sensory 

 cells consist of a relatively massive 

 peripheral layer of protoplasm and a 

 large nucleus. As a rule, every tactile 

 pit is completely filled with cytoplasm. 

 The author has observed that in Gucur- 

 bita Pcfo and C. Melopepo there are 

 usually one or more minute crystals (? of 

 calcium oxalate) embedded in the proto- 

 plasm of the pit (Fig. 232 a). It is 

 probably not going too far to assume 

 that these crystals form part of the 

 perceptive mechanism. Any deformation 

 produced by sudden pressure upon the 

 end of the protoplasmic process in the 

 pit, will necessarily be accentuated by the sharp angles and edges of 

 the crystal, and the stimulation of the protoplasm will be corre- 

 spondingly intensified ; this argument is based upon the assumption 

 that the sensory ectoplast actually comes into contact with the crystal, 

 in which case one of the edges or angles of the latter will be driven 

 into the protoplasm like a wedge, when stimulation takes place. 



In Bryonia alba, each sensory cell on the lower surface of the 

 tendril is provided with several tactile pits, which are of two kinds ; 

 the basin-shaped principal pits, which vary in number between one 

 and three per cell, are accompanied by two or three accessory pits in 

 the shape of minute transverse slits. All the pits in any one cell are 

 arranged in an irregular row, with the principal pits in the middle. 



Hitherto, tactile pits, like those of the Cucurbitaceae, have not 

 been discovered in any other family. In a solitary instance, however" 



Fig. 232. 



Tactile pits in the outer epidermal 

 walls of Cucurbitaceous tendrils. A. 

 Cucurbita Mrlopepo ; pits in vertical 

 section. B. C. Pepo ; epidermal cell in 

 surface view, showing a tactile pit in 

 the outer wall. (All preparations from 

 spirit-material.) 



