516 SECRETORY AND EXCRETORY SYSTEMS 



concerned to glide smoothly, and without risk of damage, over opposing 

 obstacles. Such a lubricating arrangement would be especially useful in 

 the case of unfolding buds, where it would greatly reduce friction 

 [between the closely-packed organs]. As a matter of fact, mucilaginous 

 secretions do occur very regularly in plants that possess more or less 

 well-developed leaf-sheaths, which for a time completely envelop the 

 younger portions of the bud (e.g. Polygonaceae, Ficaria ranunculoides, 

 Hclleborus, Viola sylvestris, Valerianella spp., etc.) ; the large surfaces of 

 contact which occur in such cases, would experience great frictional 

 resistance in the absence of special lubricating arrangements. The 

 frequent occurrence of numerous mucilage hairs on the young leaves of 

 Ferns, while these are still tightly rolled up, is explained by Hunger 

 from a similar standpoint. There is certainly much to be said for this 

 mechanical interpretation of the prevalence of mucilaginous envelopes 

 on young organs. 255 



Finally, it is probable that mucilaginous secretions often help to 

 prevent or retard desiccation. 



2. Internal glands (secretory sacs or reservoirs). 256 



Axial and foliar organs generally, and foliage-leaves in particular, 

 are often furnished with glands which are situated either immediately 

 beneath the epidermis, or amid the more deeply-seated tissues. Such 

 internal glands, which are often visible to the naked eye as translucent 

 spots or patches, occur, for example, in the leaves of species of 

 Hyijericum, Lysimachia, Citrus, Amorpha, etc. ; on the whole they 

 display less variety of structure than the previously described dermal 

 glands. The differences which do occur, are very largely due to the 

 various modes of formation of the walls of the gland, and also depend 

 to some extent upon the presence or absence of special arrangements 

 for the discharge of the secretion. 



The intercellular cavity of an internal gland is generally more or less 

 spherical, less frequently sac-like or tubular. In the Myrtaceae and 

 Hypericaceae, and in the genera Lysimachia and Amorpha, it is formed 

 by the partial separation of the secretory cells from one another ; in a 

 fully developed gland of this type, the secretory cells form a continuous 

 layer enclosing the schizogenous cavity. 



In the Rutaceae, on the contrary, the glandular cavity owes its 

 origin to a precocious degeneration of the secretory elements, which 

 takes place as soon as they have produced their secretion. The walls 

 of the secretory cells disappear altogether, while the secretion, which 

 was originally present in the form of numerous small drops, 

 gradually runs together into larger masses. When such a lysigenous 

 cavity is completed, no traces of the secretory cells remain apart 



