PROTECTIVE FUNCTION OF DERMAL GLANDS 515 



the upper surface only with a glistening coat of resin, which is 

 generally secreted by dermal glands. Such " varnished leaves," as 

 they are termed by Volkens, are very effectually protected against the 

 danger of excessive transpiration by their resinous covering."'' 



Glands which secrete ethereal oils or resins, also frequently serve 

 to provide a means of protection against the assaults of animals ; but 

 it is improbable that their importance in this respect is so great as 

 Stahl is inclined to suppose. It is certain that many aromatic plants 

 are readily devoured by a number of different animals. Kaltenbach 

 states that the foliage of Thymus serpyllum is eaten by two beetles, by 

 twenty-five different kinds of caterpillar, by the larva of a fly (Trypcta 

 sapylli), by an Aphis (Aphis serpijlli), and by a Mite (Calycophthora 

 serpylli). No less than six of these animals are " specialists," which 

 live exclusively on the Thyme. 253 



The well-known physicist, Tyndall, attributed a totally different 

 ecological role to those external (and internal) glands which secrete 

 volatile ethereal oils. He showed that a layer of air which is saturated 

 with the vapour of an ethereal oil, is much less pervious to heat- 

 radiations than an equally thick layer of ordinary air. The screening 

 effect of such vapours is very considerable ; hence plants which 

 surround themselves with a layer of vapour produced by the vola- 

 tilisation of ethereal oils are, to a certain extent, protected against 

 rise of internal temperature and consequent increase of transpiration 

 in bright sunshine, and, conversely, against excessive loss of heat by 

 radiation under a clear sky at night. This theory, though attractive, 

 is open to many objections." " 4 



Floral organs are frequently provided with dermal glands secreting 

 fragrant volatile esters, which help to attract insect visitors for the 

 purpose of cross-pollination. 



A coating of mucilage, which is generally secreted by glandular 

 hairs, occurs on the vegetative organs of many water-plants. Stahl 

 regards this slimy envelope as a means of protection against snails. 

 The author himself, while accepting this interpretation, believes that 

 the mucilaginous covering is also useful in preventing colonisation 

 of the young organs by epiphytic Algae and Bacteria. Goebel assumes 

 that the mucilage prevents the young organs from coming into direct 

 contact with the surrounding water, thereby preventing the "extrac- 

 tion " (by exosmosis) of organic and inorganic materials ; this hypo- 

 thesis must be rejected in the entire absence of experimental 

 evidence in its favour. Hunger attempts to explain the ecological 

 significance of the mucilaginous coverings of young organs, in the case 

 both of terrestrial and of aquatic plants, from a mechanical point of 

 view ; he regards the mucilage as a lubricant, which enables the organs 



