THE METABOLISM OF PLANTS. 247 



nectary has essentially the same structure as the water-gland 

 described above. It consists of a group of glandular cells 

 situated in close relation to the terminations of one or more 

 fibrovascular bundles, the only important difference in struc- 

 ture being this, that whereas the water-gland is sunk in the 

 tissue and is covered by the epidermis, the nectary has a 

 large free surface, so that the nectar is at once poured out on 

 the surface. The difference in function, to which attention 

 has already been directed (p. 101), is this, that the excretion 

 of the nectar is independent of the root-pressure, whereas 

 the excretion by the water-gland can only take place under 

 the influence of the root-pressure. 



Another instance of an excretion of this kind is afforded 

 by the carnivorous plants. The glands of the leaves of these 

 plants excrete a watery liquid which holds in solution a 

 peptic ferment and one or more organic acids. The structure 

 of the glands is different in different plants. In Drosera the 

 gland is borne at the end of a filament (tentacle). It consists 

 (Fig. 33) of a group of tracheides which are connected with 

 the fibrovascular tissue of the leaf by a bundle which runs up 

 the filament : the group of trachei'des is surrounded by one or 

 two layers of parenchymatous cells, and over these is the epi- 

 dermis. In Dionaea, Pinguicula, the gland is a modified hair 

 consisting of a group of cells borne upon a short stalk; in 

 Nepenthes it is sessile (Kurtz, Wunschmann). In Darlingtonia 

 and Sarracenia there are, according to Batalin, no specialised 

 glands, but the effect of the contact of organic matter 

 (insects, meat, etc.) with the cells of the lower part of the 

 pitcher is to cause the excretion of some substance (probably 

 the digestive excretion) between the cuticular and the deeper 

 layers of the cell- wall of the cells which have been touched, 

 and this is followed by the rupture of the cuticular layer. 

 This rupture has the effect not only of bringing the excretion 

 into relation with the introduced organic matter, but also 

 of enabling the cells which have thus lost their cuticle to 

 absorb the organic matter. 



The use of the nectar is to attract insects, and thus to secure cross- 

 fertilisation. The position of the nectary in a flower is usually such 



