SILICA 530 



concerning their physiological significance. As a matter of fact it is 

 not improbable that they are actually repositories for the reception of 

 any excess of silica that may be absorbed by the plant. Like so many 

 other physiologically valueless structures, however, they may become 

 secondarily adapted for ecological purposes. Warming believes that they 

 play a mechanical part in the Podostemaceae, which all grow in 

 swiftly flowing rivers and streams, and which, therefore, require to be 

 specially protected against the shearing action of the violently agitated 

 water. 200 



IV. THE SECRETORY AND EXCRETORY SYSTEMS IN THE 



THALLOPHYTA. 



The Laminariaceae contain in all parts of their thalli mucilage- 

 canals which have a very interesting structure. According to Guignard, 

 they form a network of schizogenous clefts, lined by groups of cells, 

 the glandular character of which is indicated by their abundant 

 protoplasm and conspicuous nuclei. These secretory cells are not 

 arranged as a continuous epithelium, but are disposed in separate 

 patches, each of which may be regarded as a mucilage-gland. It is 

 noteworthy that this extensive meshwork of mucilage-cavities occa- 

 sionally sends forth passages or clefts towards the surface, where they 

 end blindly immediately beneath the epidermis. Although these 

 centrifugal passages do not actually open to the exterior, it is probable 

 that they act as outlets for the mucilage. 207 



In many Fungi, resinous secretions are deposited in the interstices 

 of interwoven masses of hyphae ; but it is quite uncertain whether 

 these substances are ever secreted by special glandular hyphae. 

 Fungal hyphae are very frequently encrusted with crystals of calcium 

 oxalate, but actual reservoirs for the reception of this substance are 

 rarely found. In Phallus caninus, however, certain segments of the 

 slender mycelial hyphae develop into large spherical vesicles, each of 

 which is almost completely filled by a sphaerocrystal of calcium oxalate. 

 The various hyphae with special contents recently observed by 

 Istvanffy, Van Bambeke and others in the Thelephoraceae, and 

 certain other groups, cannot be considered here, if only because it is 

 not definitely known whether the substances that form the contents 

 have any nutritive significance or not. Very possibly some of these 

 specialised hyphae may actually represent excretory organs, while 

 others contain stores of plastic materials, and others again serve as 

 conducting tubes. 



