xxi] MUCILAGE 271 



plants, which is the very antithesis of the problem of keeping 

 up the water supply namely, the danger that the osmotic 

 attraction of the cell-sap may draw an excess of water into the 

 young tissues. A certain feature, occurring widely among water 

 plants belonging to unrelated families, may possibly play some 

 part in obviating this risk; this is the development of an outer 

 layer of mucilage, clothing the young organs, whose epidermis 

 has not yet matured to a resistant coat^. This slime is secreted 

 by hairs or scale-like bodies, such as the "squamulae intra- 

 vaginales^ " occurring so frequently in the leaf axils of aquatic 

 Monocotyledons. A similar secretion exists in many land plants: 

 the young leaves of the Dock, for instance, are completely 

 invested by it. Here, again, its power of delaying the passage 

 of water, may be of some value to the plant, but, in acting as 

 a protection against excessive transpiration, it has exactly the 

 opposite influence to that exerted by the slimy coating of water 

 plants. It has also been suggested that in the aquatics the 

 mucilage may serve to prevent the soluble products of assimi- 

 lation diffusing into the water, or that it may form a protection 

 against animals and discourage parasitic and epiphytic growths. 

 These theories, regarding the possible function of the slimy 

 coating, are not easy to prove or to disprove, but there seems 

 to be some experimental evidence that, in the case of submerged 

 plants, the mucilage actually hinders the entry of water, while 

 the distribution and mode of occurrence of the slime in different 

 hydrophytes, furnish certain indications indirectly confirming 

 this view. In some cases the development of mucilage begins 

 very early, and it is thus present on the surface of the delicate 

 organs of the seedling: the hypocotyl of Callitriche stagnalis^ 

 for instance, has scarcely emerged from the fruit before the 

 epidermis shows the first rudiments of the secretory trichomes^. 

 It has also been observed* that plants of tender structure, such 

 as Limnanthemum nymphoides and Polygonum amphibium^ retain 

 their slimy coat to a much later stage than plants of tougher 



iGoebel, K. (1891-1893). 2 irmisch, T. (18582). 



3 Fauth, A. (1903). * Schilling, A. J. (1894). 



y 



