584 SENSORY SYSTEM 



filament (Fig. 236 c). The saucer-shaped outer walls of these cells 

 are covered by an unusually thick cuticle, which is thrown into 

 numerous longitudinal folds. The cuticle becomes much thinner 

 towards the margins of each cell ; since the cellulose layers of the 

 outer wall are not very thick, this thinning down of the cuticle 

 reduces the total thickness of the membrane very considerably. The 

 thinnest portion of the outer wall of each cell is that which immediately 

 adjoins the lateral walls ; strictly speaking, this thin strip does not 

 form part of the papilla. As in other cases, the sensory epidermal 

 elements are distinguished by the density of their protoplasmic 

 contents. 289 



After the preceding description of the structure of the sensory cells 

 of Berberis, the actual process of stimulation in Abutilon scarcely 

 requires explanation. The protruding portion of the outer wall, 

 rendered thick-walled by the massive development of the cuticle, forms 

 a comparatively rigid cover, which is attached to the lateral wall by 

 means of a continuous hinge-membrane. This hinge is most perfectly 

 differentiated at the acroscopic end of each cell. The pressure of a 

 solid body upon this rigid cover produces a characteristic deforma- 

 tion of the hinge-area (Fig. 236 c). Owing to the more or less 

 asymmetrical shape of the cover, its centre being slightly displaced 

 towards the basiscopic end of the cell the mode of deformation differs 

 at the two opposite extremities of each sensory element. At the 

 acroscopic end, the hinge membrane becomes sharply folded inwards, 

 whereas at the other extremity it bulges distinctly outwards. The 

 underlying ectoplast accordingly suffers violent extension in the 

 acroscopic portion of the hinge, and less pronounced compression in 

 the basiscopic region. 



In all the Malvaceae with insensitive stamens that have been 

 examined by the author, the epidermal cells of the filaments were 

 found to be quite flat and smooth- walled ; the outer walls (inclusive of 

 the cuticle) were always moderately thick-walled throughout, the 

 marginal region being in no way distinguishable from the rest of the 

 wall. 



Comparatively few tendrils are furnished with tactile papillae. 

 On the lower and more sensitive side of the irritable petioles of 

 Adlumia cirrhosa, Gorydalis claviculata and Clematis Viticclla, the 

 ordinary narrow, elongated, flat-walled epidermal elements are accom- 

 panied by a considerable number of shorter and broader cells, with 

 thin outer walls which protrude to a considerable extent. These 

 projecting cells are particularly well adapted for the perception of 

 mechanical stimuli. Clematis Vilalba is likewise provided with sensory 

 cells which are isodiametric, papillose and specially thin-walled, and 



