THE LARGER AQUATIC VEGETATION 187 



Since the escape of oxygen and withdrawal of carbon dioxid are 

 simultaneously in progress during the time the plant is making 

 starch, both processes may operate to precipitate the neutral 

 carbonate. If the plants secrete an alkaline carbonate this would 

 immediately upon its escape from the plant decompose the soluble 

 bicarbonate in the water with the formation of the neutral insol- 

 uble carbonate. It is uncertain, however, that such an alkaline 

 carbonate is secreted by the plant and not much emphasis can be 

 placed upon this hypothesis. The most recent explanation rests 

 upon the discovery that a soluble calcium salt of succinic acid is 

 present in the cell sap of Chara. The occurrence of this salt in 

 the sap of other plants has not been determined, but as succinic 

 acid is a very probable by-product in the ordinary processes of 

 plant physiology, its wide distribution may reasonably be expected. 

 As the calcium salt escapes from Chara by osmosis it is most likely 

 decomposed with the formation of the insoluble carbonate. 



Possibly the incrustation offers protection to the plant in some 

 way, but this seems hardly probable, and at present one can only 

 say that its formation is a consequence of processes in the plant 

 and that its presence is of little benefit or of harm to the plant. 



Various plant organs are often found to be covered with a gelati- 

 nous coating. This may occur on the lower surface of floating 

 leaves as in species of Nymphaea. Young leaves and growing tips 

 are often encased with it. In the axils of leaves arising in a 

 rosette around a shortened stem it is likely to occur. Seed coats 

 are often slimy and in some fruits the seeds at maturity are em- 

 bedded in a mass of gelatinous substance which on sweUing rup- 

 tures the ovary walls and allows the seeds an exit. Some plants, as 

 Brasenia peltata, have special glands to furnish the slime, but often, 

 as in leaf axils, there are no distinct structures for furnishing this 

 substance. Many of the algae are embedded in a mass of slime 

 just as the eggs of frogs are. Amphibious plants and those sub- 

 ject to temporary exposure, as in the case of plants which grow in 

 tide-water, are doubtless protected from too rapid loss of water by 

 such covering. It may also serve as a protection for young buds 

 and leaves against devouring animals. It is quite possible tliat 

 the gelatinous masses in which seeds are found embedded arc of 



