624 



ECOLOGY 



tective layer. Usually the reservoir does not discharge to the ex- 

 terior, but in Eucalyptus and in various Rutaceae there are cover cells, 

 which after a time rupture at a definite spot or along the walls, allowing 

 the secretions to pass off. In some cases, as in Peperomia and in 

 various liverworts, there are single cells that secrete mucilage (figs. 745, 

 928). The substances secreted by internal glands resemble those se- 

 creted by glandular hairs, and likewise are often odoriferous. 



The role of oils, resins, and mucilage. But little is known concern- 

 ing the role of oils, resins, and mucilage, though speculation along this 

 line has been abundant. Wax coats previously have been seen to rep- 

 resent waste products that are 

 of incidental value in protect- 

 ing against excessive transpira- 

 tion (p. 570). A similar theory 

 has been proposed concerning 

 excreted volatile oils, on the 

 ground that they absorb heat 

 in large amount, but it is most 

 unlikely that these oils are 

 present in sufficient abundance 

 to check transpiration. Simi- 

 larly improbable is the common 

 theory that sticky glandular 

 hairs, which are especially abun- 

 dant on floral stems, are of value in keeping crawling insects away 

 from the flowers, or the theory that various secretions which are 

 unpalatable or even poisonous (as in Primula) may lead to the better 

 preservation of plant species from grazing animals. Somewhat more 

 probable but scarcely authenticated are the roles commonly ascribed to 

 slime in water plants, such as protection from snails and other water 

 herbivores and from water currents. In gelatinous lichens and in am- 

 phibious algae, mucilaginous secretions may protect from desiccation. 

 In the economy of aquatic life as a whole, slime plays an important 

 part, since it is a perfect culture medium for many algae, bacteria, and 

 small animals. Probably there is no adequate reason for believing that 

 such secretions as oils, resins, and mucilage are of any particular value in 

 the economy of plants. Doubtless, for the most part, they represent waste 

 products, whose removal is of greater value than their retention. Any 

 incidental gain that these secretions may have probably is small. 



FIG. 917. An internal oil gland (a) in the 

 orange (Citrus A urantium); o, oil drops; highly 

 magnified. From TSCHIRCH. 



