POTAMOGETONS IN RELATION TO POND CULTURE. 28 1 



predominates. Propagation by tubers is, as we have seen, the more rapid method and 

 the one which produces a luxuriant foHage early in the growing season. 



In view of the observations and experiments, it is clear that in any project in which 

 the propagation of Potamogetons is an important feature, success will be measured by 

 adherence to the general principle that vegetative reproduction is the dominant mode 

 of increase in the genus Potamogeton. 



ECONOMIC ASPECTS OF POTAMOGETONS. 



In the study of the various phenomena attending the propagation of Potamogetons 

 opportunity was afforded to observe, more or less closely, various aquatic animals which 

 abounded on these plants. Their presence in such great numbers suggested the pos- 

 sibility that the Potamogetons might play an important role in the economy of life 

 beyond that of mere shelter and support, or other mechanical and indirect relations 

 which have been ascribed to the larger aquatic plants for many years. 



It has been stated by Pond (1905) that — 



The larger aquatic plants, as such, are, while living, little used as food by aquatic animals, yet they 

 greatly increase the surface available for the attachment of microscopic plant ''onn^ which are eaten 

 by smaller animals, and the latter in turn by the fishes. 



In the very recent publication by Shelf ord (191 3), bearing on the life relations of 

 aquatic animals, but little importance is attached to the larger aquatic plants beyond 

 the various mechanical and indirect relations that have so long been attributed to 

 them. He says: 



The smaller aquatic animals are commonly either alga-eaters or predatory. The larger aquatic 

 animals are commonly predatory or scavengers. The rooted vegetation is eaten only to a small extent. 

 Small floating or swimming plants and animals are the basis of the food supply of larger animals. We 

 could probably remove all the larger rooted plants and substitute something else of the same form and 

 texture without greatly affecting the conditions of life in the water; that is, so far as the life habits of 

 the animals are concerned. * * * Plants in water are of particular use to animals as clinging and 

 nesting places. 



Recent research bids fair to modify these generalizations by Shelford. Such a 

 relation as Pond describes has frequently been observed in P. pectinatus in the autumn 

 when myriads of midge (chironomid) cases have been found apphed to the leaves (fig. 56). 

 The leaves are not eaten but they are thickly covered with diatoms and other small 

 algae which, doubtless, afford foraging materials for the larvae. A small caddis fly 

 (hydroptilid) larva, with characteristic elliptical case, has also been obser\'ed in con- 

 siderable numbers in the same relation with pectinatus, the larvae apparently feeding 

 on the epiphytic algal growth. The larvae of both of these insects, after wintering on 

 the algal-covered leaves, have emerged as adults in the spring. Other midges and caddis 

 flies, flies (aquatic Diptera), moths (aquatic Lepidoptera), and beetles (Coleoptera) 

 have been found in great numbers on the various species of Potamogeton. The other 

 smaller invertebrate animals most frequently seen on these plants are Crustacea, snails, 

 and worms. 



Another interesting relation existing between the Potamogetons and aquatic insect 

 forms is seen in the striking resemblance between the cases of a caddis fly (Leptoceridae) 

 and the stipules of the leaf of P. amcricanus (fig. 75). The cases in which both larvae 

 and pupae dwell are attached along the stems and leaves in so characteristic a manner 

 as to become almost, if not quite, indistinguishable from the plant parts. 



