POTAMOGETONS IN RELATION TO POND CULTURE. 269 



PROPAGATION BY TUBERS. 



A conspicuous method of vegetative propagation is seen in the development of 

 plants from tubers in P. pectinatus and P. filiformis. The tuber-forming habit of 

 pectinatus has been described by Irmisch (1858), who carefully worked out the morpho- 

 logical details of the tubers in terms of the ordinary stem structure. His figures illus- 

 trate the development of tubers on detached parts of leafy stems, on the erect axis, and 

 on the underground stems. It is not clear from which forms of pectinatus these drawings 

 were made. In general, however, they bear a close resemblance to our most common 

 representative of pectinatus, though no hint of the variability in this species is given 

 beyond the fact that some plants were collected in deep water, and that the tubers 

 were varied in shape, some being more cylindrical than others. 



The work of Sauvageau (1894) confirms the observations of Irmisch as regards the 

 tuber-forming habits of pectinatus, but this investigator also makes no allusion to the 

 remarkable forms that exist in this species. His drawings, moreover, are, as he states, 

 modifications of those by Irmisch. Both of these workers in this field recorded the time 

 of tuber formation to be in the autumn. Jepson (1905) suggests an earlier development 

 for those on the rootstock and the erect stem. He says: "The slender threads which 

 develop one, two, and even three tubers at the end, are not only borne on the horizontal 

 rootstocks and on the soil at the bottom of the ponds, but are also produced on the 

 upright stems, and at the end of the season on the uppermost leafy portion." 



Regarding the presence of tubers on rootstock, stem, and spray, the present investi- 

 gation is confirmatory. Tubers have often been observed on all these parts of the plants. 

 Additional figures and observations relate more especially to the season in which they 

 occur and to their artificial propagation. Collections of plants made on the 15th of May, 

 1913, and thereafter throughout the growing season, show the presence of tubers in great 

 numbers on the proliferating shoots of the rootstocks. Many of these tubers are well 

 grown in May, though others subsequently arise on the extensions of the subterranean 

 system which develop after this time. 



Figure 41 represents the basal part of a small immature plant of P. pectinatus col- 

 lected in shallow water June 20, 1913. Many plants at this time were more nearly 

 mature and bore larger tubers, but it seemed desirable for illustration to select a small 

 plant because in such all parts may be preserved intact during the collection of material, 

 a task that is attended with considerable difficulty when the plant has attained a large 

 size and great complexity of parts, especially in the subterranean region, where the 

 underground stems are exceedingly brittle and tender. This figure (fig. 41) illustrates 

 the general sequence of growth in what may be termed the typical vegetative life cycle 

 of the plant. The order of development is as follows: The production of a leafy, erect 

 shoot (C) from the tuber of the preceding season (A) ; the growth of the horizontal axis 

 or rootstock (D) ; and the production of the stolon-like branch or runner which in turn 

 bears a tuber or tubers at the end (B). 



As the season progresses the tubers become solidly packed with starch in sufficient 

 amount, apparently, to bring the plants developed from them to a very advanced stage 

 of growth, at least to render them quite independent of the soil for a considerable length 

 of time. Figure 45, B illustrates the typical condition in this respect when tubers sus- 

 pended in aquaria without contact with the substratum produce the future propaga- 

 tive structure. Thus the continued dependence of the plant upon the stored starch in 

 19371°— vol 33—15 18 



