TACTILE PAPILLAE OF TENDRILS 



585 



which thus differ markedly from the ordinary elongated epidermal 

 elements. In the last mentioned plant, the sensory cells may be 

 solitary or associated in groups to form prominent tactile cushions; 

 eacli cushion is accompanied by a thick-walled and comparatively rigid 

 unicellular hair, situated at its acroscopic end, which probably represents 

 a "stimulator" (cf. below, p. 586). 290 



Very interesting and remarkable are the papillose tactile cells 

 which occur on the repeatedly dichotomous foliar tendrils of a Chilian 

 Bignoniaceous climber, Eccrcmocarpiis scaler. Here the primary, 

 secondary and tertiary branches of the tendril, and even its short 

 terminal hooks, are much more sensitive to contact on the abaxial 



Pig. 23V. 



Tactile papillae of the tendrils of Eccremocarpus scaber. Above, in vertical section ; 



below, in surface view. 



than they are on the adaxial side. The numerous tactile papillae 

 are likewise most abundant on that side ; on the terminal hooks, 

 indeed, they are always confined to the abaxial surface. These 

 papillae represent entire cells, and take the form of hemispherical or 

 lenticular outgrowths attached to the flattened central areas of the outer 

 walls of ordinary elongated epidermal cells (Fig. 237). While the ten- 

 drils are still young and highly sensitive, the .smooth outer walls of the 

 papillae are distinguished by their extremely thin-walled character ; 

 their thickness only amounts to between 1*5 and 2 //, while the walls 

 of the adjoining cells are about twice as thick. The protoplasts of 

 these cells consist of a well developed peripheral layer of cytoplasm, 

 and a nucleus which invariably lies in contact with the middle of the 

 inner wall. 



In all the preceding cases, the tactile papillae or cushions arc 

 specially adapted, on account of their exposed situation and their 

 thin-walled character, to act as perceptive organs, even supposing that 



