6i8 THE STRUCTURE OF ECONOMIC PLANTS 



funnel or tactile pit, and a canal leading from it enters the lumen 

 of the cell which has a thick layer of cytoplasm containing chloro- 

 plasts and a large nucleus. (Fig. 3x1, D.) The papilla is com- 

 pletely filled with cytoplasm and a small crystal of calcium oxalate 

 usually lies in it, although sometimes the crystal may be located 

 in the canal leading to the cell. As in the cystoliths which occur 

 in the epidermal hairs of Cannabis, the function of these crystals 

 is not definitely known. They may be entirely non-functional 

 excretion products, but it is possible that they are related to the 

 irritability of the tactile cells. 



Within the epidermis is a layer of collenchyma which is variously 

 distributed in different parts of the tendril. In C. maxima and 

 C. Pepo, Penhallow (3i) found the collenchyma interrupted on 

 the adaxial surface, and on the right and left flanks by chloren- 

 chyma in which intercellular spaces subtend the stomata. Within 

 the collenchyma, there are three or four rows of large parenchym- 

 atous cells. The sclerenchyma forms a continuous ring in the 

 proximal portion of the tendril, but occurs only on the abaxial 

 side in the distal part. At the base, the five to seven vascular 

 bundles are arranged in a ring around the hollow central region. 



Miiller (p.^') found evidence of sclerenchyma in the tendrils of 

 cucurbits in the bud stage, and observed a progressive lignification 

 of this tissue and the xylem as the tendril matured. The portion 

 of the tendril which surrounds a support becomes swollen, hard, 

 and brittle owing to the enlargement and lignification of the cells 

 of the epidermis, parenchyma, and collenchyma. In contrast, 

 the portion which forms the spiral is not generally lignified except 

 the xylem and sclerenchyma; and, as a result, this part remains 

 more elastic. 



Dastur and Kapadia (7), in investigating the mechanism of 

 curvature in the tendrils of cucurbits, found that the parenchym- 

 atous cells on the convex (adaxial) side of the tendril elongate 

 and produce the curvature, while the character of the mechanical 

 tissue on the concave (abaxial) side tends to prevent elongation 

 of this surface. The abaxial surface shows the effect of pressure 

 resulting from curvature since the transverse walls are thrown into 

 folds that project into the lumina of the cells. Actual measure- 

 ments of the cells of the epidermis, and of the underlying rows 

 of cells on the adaxial or convex side, indicate an average increase 

 in length of 69.1 per cent. 



