208 



MECHANICAL SYSTEM 



bundle of stereides ; in this case there is no specialised terminal barb- 

 cell, corresponding to that of Caucalis. 



Somewhat complicated appendages, bearing numerous barbs, are 

 found on the fruits of Cynoglossum chcirifolimii (Fig. 80). Each of 

 these structures takes the form of a slender cone, about "5 to "8 mm. in 

 height ; it consists of a central core of thin-walled parenchyma, sur- 

 rounded by a sheath of thicker-walled epidermal cells, the long axes of 

 which extend oblicp:iely outwards. The radial walls of each epidermal 

 cell are thrown into folds ; a small two- or many-toothed central 

 protuberance adds to the roughness of the outer wall. The appendage 

 terminates in a group of from four to six thick-walled epidermal 



elements, with short re- 

 curved distal ends and long 

 embedded basal portions. 

 The radial walls of the ter- 

 minal and sub-terminal cells 

 are not folded. The whole 

 appendage is a striking ex- 

 ample of inflexible construc- 

 tion, and altogether con- 

 stitutes a highly effective 

 organ of attachment. 



The mechanism of 

 another type of fixing organ 

 depends upon adhesion to, 

 or fusion with, the support 

 or substratum, instead of 

 upon the presence of hooks 

 or barbs ; an illustration is provided by the adhesive discs or pads 

 peculiar to the tendrils of certain Bignoniaceae and of species 

 of Ampelopsis. In the latter genus the rudiments of the adhesive 

 apparatus are present even in the unstimulated condition of the 

 tendril. The first result of contact-stimulation is the abundant 

 secretion of a mucilaginous substance, which owes its origin [in part 

 at any rate] to the gelatinisation of the entire outer epidermal wall, 

 with the exception of the cuticle and the innermost pellicle. Accord- 

 ing to Lengerken 113 , mucilage is also secreted within the cell-cavity, 

 whence it passes to the outside. Whatever its origin, the mucilage 

 ultimately exudes, after rupturing the cuticle, and glues the disc to 

 the substratum. The contact-stimulus further induces a considerable 

 hypertrophy of the epidermis, with the result that hair-like or 

 papillose protuberances penetrate into every nook and cranny in the 

 substratum, fitting themselves exactly to all the inequalities of the 



Fin. 79. 

 Distal portion of a hook from the fruit of Caucalis daucoides. 



