502 SECRETORY AND EXCRETORY SYSTEMS 



B. DIGESTIVE GLANDS. 



1 . Tli e digestive (/lands of carnivorous plants?* 6 



Since Charles Darwin's classical investigation of the subject, it has 

 been a matter of common knowledge that a number of species, 

 representing very diverse families of Angiosperms, have acquired a 

 highly characteristic method of nutrition, termed the " insectivorous " 

 [or " carnivorous " habit. The plants in question capture insects 

 [and other small animals] by means of specially constructed foliar 

 organs, hold them captive until they are dead, digest their car- 

 cases and absorb the soluble products of digestion. This is not the 

 place to discuss the varied structure of the organs which serve as 

 " traps." In the simplest cases they do not differ appreciably from 



ordinary leaves, while the trapping arrange- 

 ment consists of the sticky digestive glands 

 that occur in large numbers on the adaxial 

 surface of the leaf (Drosera). Where special- 

 isation and division of labour have progressed 

 a little further, special capturing hairs may 

 be developed (Pinguieida), or the leaf itself, 

 FlG - 20 - or a portion thereof, may be converted bodily 



Digestive gland of Pinguecula [ n ^ a trap (Dionaea, Neventhes, Sarracenia), 



vulgaris (vertical section). r x - 1 ' 



in which case the digestive glands serve solely 

 to produce a digestive solution (though in certain instances they 

 perhaps also assist in absorbing the soluble constituents of the carcase). 

 Morphologically considered, all digestive glands correspond to 

 trichomes of one kind or another. The leaves of Pinguieida bear 

 glands of two distinct types on their upper sides. The capturing hairs 

 are all glandular trichomes, with long stalks and discoid heads which 

 secrete a sticky, mucilaginous substance. The other glandular structures 

 are sessile, and consist of a discoid head or body, a short concavo- 

 convex stalk cell and a basal cell, both the latter being sunk below the 

 general level of the epidermis (Fig. 200). The body of these glands is 

 usually made up of eight cells, which arise from a common mother-cell 

 by means of a quadrant-division followed by the appearance of an 

 anticlinal wall in each quadrant. All the cells contain abundant 

 protoplasm, differentiated into a central mass enclosing a nucleus which 

 contains numerous protein crystals, and a peripheral region distinguished 

 by the presence of numerous vacuoles of various sizes. The lateral 

 walls of the stalk-cell are cutinised. These sessile glands are the true 

 digestive structures ; their surfaces are perfectly dry until they are 

 stimulated by contact with a dead insect [or other animal matter], 



