DIGESTIVE GLANDS 



149 



fC3 



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. ^ The osmotic sub- 

 stances secreted by extrafioral 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 

 Butterwort (Pingtiicula) 

 the digestive glands are 

 found on the upper 

 surface of the leaf and 

 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 ()».), 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. 



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



Fig. 74. — Extrafloralnectary of the Bracken 

 (Pteris aquilina) (after Lloyd). A, Tlie 

 fork of a frond showing the nectary (n.). 

 B, A stoma from the nectary in surface 

 view. C, The nectary in section. 

 el., glandular cells ; St., stoma. 



