DIGESTIVE GLANDS 



503 



whereupon they at once begin to secrete a digestive enzyme. The 

 shortly-stalked shield-shaped glands of Dionaca muscipula behave in a 

 similar fashion. 



In the genus Nepenthes, the corresponding glands are sessile, 

 spherical or cake-like structures, attached by a disc-shaped base to the 

 floor of a depression with an overhanging upper margin (Fig. 201). 

 The multicellular body of the gland consists of a large number of 

 central cells surrounded by radially elongated, superficial secreting 

 elements. A delicate bundle- end terminates in a tuft of short and 

 rather wide tracheides immediately beneath each digestive gland, an 

 arrangement which suggests that the glands are well supplied with 

 water. As a matter of fact, these glands begin at an early stage of 

 development to secrete a watery, 

 mucilaginous liquid ; this collects 

 in the pitcher on which the glands 

 are borne (they are situated on its 

 inner surface near the bottom). 



The most remarkable digestive 

 glands of all are the tentacles 

 of the genus Drosera, which not 

 only serve as digestive organs 

 but also represent capturing or- 

 gans, endowed with special forms of 

 irritability, and finally carry out 

 the absorption of the soluble 

 products of digestion. The ten- 

 tacles are multicellular, glandular 

 villi which densely clothe the 

 margins and adaxial surfaces of the 

 orbicular or oblong-spathulate leaf- 

 blades. Each consists of a stalk, 



which is longest in the case of the marginal tentacles and of a 

 swollen club-shaped head or body (Fig. 201). The whole structure is 

 traversed by a bundle-end ; in the stalk this consists of a single row of 

 tracheides with closely wound spiral thickening fibres, while the head or 

 body contains a row of short tracheides. Evidently the intense secretory 

 activity of the tentacle necessitates an increase in the water-supply, 

 which is ensured by the extension of a tracheidal strand into the 

 tentacle. The large terminal tuft of tracheides is surrounded by three 

 layers of parenchymatous cells, the innermost of which the inter- 

 mediate layer (" Mittelschicht ") of Goebel resembles a typical 

 endodermis in having corrugated and suberised radial walls. The 

 actual secretory cells form the outermost layer ; they are radially 



Fig. 201. 



Digestive gland of Nepaithts Phyllamphora (ver- 

 tical section). 



