278 BULLETIN OF THE BUREAU OF FISHERIES. 



of Starch within the tissues. Starch is so abundant that the air cavities are considerably 

 reduced by the distension of the cells (fig. 71). In portions of the stem where the tissues 

 are not obscured by the deposition of starch, it is seen (figs. 69, 70) that mechanical tissue 

 is scattered through the stem in greater abundance than is common in the other Pota- 

 mogetons, serving to support the heavy sprays of foliage and to give the rigidity of stem 

 which is characteristic of this species. 



In P. amplijolius the tip ends of the branches function as propagative structures in a 

 manner similar to P. Rohbinsii (fig. 58). These structures appear in the autumn devel- 

 oping only at the tips of the branches. The internodes are short and thick and densely 

 packed with starch. At the end there are a few partially unfolded leaves which con- 

 tinue to grow slowly or, at least, remain green all winter. These rapidly expand when 

 the roots develop in the spring and the entire structure forms an effective and rapid 

 means of propagation. 



PROPAG.'KTION BY SEEDS. 



While the main purpose of this paper is a consideration of the vegetative means of 

 propagation, yet it is important by way of comparison to present such data as are 

 available on the propagation of these plants by seeds. In reviewing the literature on the 

 seed germination of Potamogetons, it appears that Irmisch (1858) and Sauvageau (1894) 

 have made the only contributions of importance."^ Irmisch figures the germinating seeds 

 and two small seedlings of P. natans but otherwise gives no data concerning them. 

 Sauvageau found that P. crispiis, pcrjoliatus, and pcctinatus germinate in less than a 

 year and that P. natans remains dormant three or more years. No figures accompany 

 his account of their behavior. 



In the course of the present investigation additional observations have been made on 

 P. pcctinatus and P. americanus. The seeds of both species were gathered in October 

 and kept in cold storage through the winter. On January 24 seeds of each kind were 

 placed in aquaria and kept at ordinary room temperatures. On February 14, the seeds 

 of pcctinatus began to germinate, but this process was very irregular, extending over a 

 period of three or more weeks. These seedlings lacked vigor and nothing came of them. 

 On March 15 other seeds of the same species were taken from cold storage and placed 

 in aquaria as before. In this later planting germination was more uniform, the majority 

 of seeds sprouting within a few days of each other. Subsequent growth was rapid and 

 vigorous. It appears from the behavior of the seeds in the two experiments that the later 

 planting is advantageous. Figures 46 and 47 represent seedlings of the second planting 



3 and 5 days old, respectivel)'. Figure 48 represents a seedling of the same species 

 about ID days old, and figure 49, one about 3 weeks old. 



The seeds of P. americanus planted on January 24, showed no signs of life till May 5. 

 Those of the second planting germinated between June 13 and 15. In this species also 

 the later planting proved to be more successful. Figures 2 and 3 represent seedlings, 

 respectively, 5 and 14 days old. When the seedlings were about 3 weeks old they 

 were transplanted and kept in outdoor aquaria with running water till October. Figure 



4 shows one of these seedlings which produced winter buds during the latter part of the 

 growing season. These winter buds described in a preceding chapter are the vegetative 



a In a recent publication by Escnbcck the seedlings of P. fo/ora/wiaredcscribed. (Usenbeck. Ernst: Beitrage zur biologie 

 der gattungen Potamogeton und Scirpus. Flora, bd. 7. June, 1914, p. 151-212, fig. 59.) 



