DIGESTIVE GLANDS 



149 



stances on the surface (largely sugars) which initiates the exuda- 

 tion of liquid, and the mechanism can be well imitated by placing 

 sugar in a hollow scooped out of a Potato. 1 The osmotic sub- 

 stances secreted by extrafloral nectaries tend to withdraw water 

 from the plant when it is turgid, and these organs are indeed 

 most active in a moist warm atmosphere. They may well serve 

 primarily, therefore, to fulfil the same purpose as hydathodes. 

 Floral nectaries, however, have another more important function, 

 namely the attraction of 

 insects. 



Glands whose liquid 

 secretion is a digestive 

 fluid containing proteo- 

 lytic and other enzymes 

 are present on the leaves 

 of most insectivorous 

 plants. By their agency 

 the nitrogenous 

 materials of the cap- 

 tured insects are gradu- 

 ally dissolved and 

 rendered available for 



absorption. In the FlG 74 ._ E xtrafloral nectary of the Bracken 

 Butterwort (Pinguicula) (Pteris aquilina) (after Lloyd). .A, The 



the digestive glands are f ork of a frond showing the nectary (w.). 



found on the upper B, A stoma from the nectary in surface 



rj view. C, The nectary in section, 



surface of the leaf and g i t glandular cells ; St., stoma. 



take the form of small 



-hairs (Fig. 75, d.) composed of a basal cell, a short stalk-cell, 

 and a usually eight-celled head. Similar glands occur also on 

 the under-surface, but these are probably mainly concerned in 

 the removal of water. The digestive glands are accompanied 

 by other long-stalked glands (m.), whose umbrella-like heads 

 secrete the sticky mucilaginous matter to which the insects 

 adhere. In the Pitcher-plant (Nepenthes) the glands occur 

 on the inner surface of the pitcher ; they are short -stalked 

 structures, with a very robust oval head of palisade-like secreting 

 cells. 



i Cf. F. & S., p. 257. 



