472 A TEXTBOOK OF THEORETICAL BOTANY 



choline. The former causes itching and the combination produces the 

 prolonged burning sensation. Acetyl choline alone is without effect. 

 Urtica stimulans, the Java Nettle, causes serious illness, comparable in 

 severity to a snake bite. 



The glands produced on the leaves of the Insectivorous Plants are generally 

 glandular hairs and they function both for the capture and the digestion of 

 the insect prey. They will be discussed in detail under Bionomics in 

 Volume IV. 



Superficial glands, if not formed from hairs, may be simply modified 

 patches of epidermis, as, for instance, in the nectaries, found in most 



Epithem 



Vascular supply of 

 nectary 



Petiolar bundle 



Fig. 461. — Prumis cerasus. Garden hybrid. 

 Transverse section of petiole showing two 

 extra-floral nectaries, the one on the right 

 cut obliquely. 



flowers and sometimes also on leaves or stems, where they are called 

 extra-floral nectaries (Fig. 461). The epidermis of these gland areas 

 has no cuticle and consists of a layer of narrow, highly protoplasmic 

 cells called epithem. 



A peculiar type of gland, associated with the excretion of water, is that 

 called a hydathode. These are usually modified stomata permanently open 

 and supplied beneath with a strand of vascular cells ending in a group of 

 epithem cells (Fig. 462). They are often to be found at leaf margins, where, 

 in moist weather, the drops of excreted water may be seen hanging. 



Internal glands may be simply cavities in the tissues. They originate in 

 two ways. Firstly, by the dissolution of cells, leaving a space in which the 

 secretion accumulates. These are known as lysigenous, and they have 

 usually ragged remains of cells lining the cavity (Fig. 463). Secondly, by 



