504 



SECRETORY AND EXCRETORY SYSTEMS 



elongated, especially at the apex of the gland, and their cuticle is very 

 pervious. Both this superficial layer and the layer next to it contain 

 a reddish-purple cell-sap. In the case of the marginal tentacles the 

 preceding description only applies to the upper side, the lower surface 

 of the gland being covered by an epidermis of the ordinary type. 



In the allied genus Drosophyllum it can be shown by direct 

 observation, that the remarkable permeability of the cuticle which 

 covers the tentacles is due to the presence of innumerable microscopic 



pores. If the protoplasmic contents of the super- 

 ficial cells are dissolved by treatment with Eau de 

 Javelle, the cuticle appears minutely punctate in 

 surface view, while in section it seems to be marked 

 with delicate transverse striae. 



The liquid secreted by the digestive glands [of 

 carnivorous plants] contains a proteolytic enzyme, 

 which, in its properties, closely resembles the pepsine 

 of gastric juice, together with a certain amount of 

 free organic acid (propionic, formic, acetic and medic 

 acids) which assists the enzyme in its work of 

 dissolving the protein constituents of the carcase ; 

 in every case this process of solution results in 

 the formation of peptones. In certain cases the 

 secretion of digestive juica is an autonomous process, 

 while in others it only takes place after chemical 

 or mechanical stimulation. 



From the phylogenetic point of view, the 

 digestive glands of carnivorous plants may be 

 regarded as modified trichome-hydathodes. This 

 view, first put forward by Goebel, has been definitely established, 

 as regards Pinguicula, by the author himself, who has also adduced 

 strong evidence in its favour in the case of Nepenthes. In Pinguicula 

 the lower surface of the leaf bears typical hydathodes, which agree 

 closely in structure with the sessile digestive glands of the upper side. 

 It seems probable, therefore, that the non-carnivorous ancestors of 

 Pinguicula had both sides of their leaves furnished with water-secreting 

 and -absorbing glandular hairs, with a structure approximating to that 

 of the hydathodes on the lower leaf-surface of the living species of 

 Pinguicula. For a reason which has been suggested by Goebel, the 

 adaxial glands probably soon acquired the habit of secreting a somewhat 

 mucilaginous fluid. A mucilaginous secretion adheres better to the 

 leaf-surface and, perhaps, also evaporates more slowly, both of which 

 properties obviously facilitate its reabsorption [at a later stage]. In 

 this way the capture of insects became a possibility, though at first this 



Fig. 202. 



Digestive gland (ten- 

 tacle) of Vrosera rotundi- 

 folia (L.S.) ; ss, endoder- 

 mis-like bundle-sheath. 

 X145. (From De Bary, 

 Comp. Anat.) 



