208 FRESH-WATER BIOLOGY 



still further reduces the oxygen supply and increases the carbon 

 dioxid. The green plants on the other hand during sunlight are 

 constantly using the carbon dioxid for making starch and giving 

 off oxygen as a waste product of the process. In this process the 

 volume of oxygen released equals the volume of carbon dioxid 

 used, so that an aquatic meadow, growing vigorously in a still- 

 water cove, would be very efficient in keeping the water well aerated 

 and much to the advantage of all the animal life finding food and 

 shelter there. 



As affording shelter and refuge for small animals. In these 

 aquatic meadows many kinds of young fish spend their early life 

 during the period when they require protection from enemies. 

 Here, too, many of the smaller animals pursued by enemies find 

 temporary refuge or are able to evade their pursuers. 



As a base of attachment for algae. Wherever the larger plants 

 occur may be found also many smaller and more lowly organized 

 forms which use the larger plants as supports. The simple forms, 

 which are usually algae, would be unable to live as free swimming 

 individuals, and since many of them are used as food by the animals 

 it is important that they should be allowed to develop abundantly. 

 The dead as well as the living larger plants may be useful in this 

 way and only direct observation will reveal to one how much of 

 importance the larger plants are in this one particular. 



As organizers of matter and distributors of nourishment for the 

 plankton. If, as formerly supposed, water plants take their min- 

 eral food from the surrounding water and not from the soil at all, 

 then it is evident that during the growing season these rooting 

 aquatics would be continually diminishing the food supply of the 

 free swimming plants, or the plant plankton, and this would, of 

 course, result in a decreasing food supply for the animals dependent 

 upon the plant plankton for nourishment. In view of the evidence 

 now at hand it is certain that these larger plants rooting in the 

 bottom absorb inorganic matter from the soil and organize this 

 material into plant tissue which can be used as food by the animals 

 and also by parasitic and saprophytic plants which can also be used 

 as food by the animals. This, then, is perhaps the most important 

 role of the larger aquatic plants, in that their life work results in an 



