270 BULLETIN OF THE BUREAU OF FISHERIES. 



the tuber would seem to be advantageous, especially if growth occurred under untoward 

 conditions. 



The tubers, hardened by the great quantity of starch that is packed into the tissues, 

 normally pass through the winter in a dormant state. This, however, is quite easily 

 disturbed, and by supplying continuously ordinary room temperatures the tubers may 

 send forth shoots as early as October. Figure 45, noted above, illustrates such a response 

 to growth conditions, the plant having been developed between the dates of October 22 

 and December 20. 



The propagation of tubers in aquaria has shown that when tubers occur in twos, for 

 example, figure 40, the larger one develops the shoot. The smaller one has never been 

 seen to sprout unless by chance it became detached. In that case it developed an 

 individual plant. It has been frequently observed that plants of this species when 

 propagated in aquaria never attain their full size or vigor when deprived of a soil sub- 

 stratum, an observation that is in accord with the results of Pond's (1903) experiments 

 on rooted aquatic plants. 



The remarkable versatility of P. pectinatus as regards the origin of tuber-bearing 

 runners has been clearly shown by Irmisch (1858). There is, moreover, in each of these 

 situations, on rootstock, stem, and spray, a considerable variation in size and number of 

 tubers. For example, an underground stem or rootstock may develop them at the ends 

 of slender, stolon-like branches which arise from the axils of fertile nodes as shown in 

 figure 43. These have been found singly or in pairs, large or small, depending upon 

 the richness of the substratum and the size of the plant. Again, the rootstock itself may 

 be terminated by tubers which occur singly, in pairs (fig. 42), or in threes (fig. 39). 

 Plants bearing rootstocks of this character have been collected at various times during 

 the growing season, and from each collection the specimens have shown comparatively 

 short underground stems without other tuber-bearing structures. Some rootstocks 

 have shown no tendency to produce tuber-bearing runners or tubers at the end of the 

 horizontal axis, but send up a succession of leafy shoots from the fertile nodes. It is 

 suspected, however, that had such plants been undisturbed tubers might have devel- 

 oped, especially since at the base of these upright shoots there was always a bud, either 

 latent or showing a tardy development. 



In autumn pectinatus develops tubers on the leafy spray. They are generally 

 smaller than those which occur on the rootstock, but quite conspicuous because of their 

 pale, yellow color. They are borne singly or in pairs at the ends of runners that are 

 bright green and stouter than the stems from which they arise (fig. 44, B, C). These 

 structures are readily distinguishable about the time the plant begins to show signs of 

 decay. They may occur on attached or detached parts of the plant. The remarkable 

 proHficity of these sprays is a characteristic of this species. Repeatedly detached 

 parts of the leafy spray have been placed in aquaria and tubers have been developed in 

 abundance until the spray became completely disorganized. It is interesting to note 

 that when this species grows in the currents of the stream the tendency to form pro- 

 liferations on the leafy spray is conspicuously lessened, although portions of these plants 

 when caught in the drift and carried to quiet water readily produce them in the new 

 environment. 



For the most part tubers are more numerous on sprays devoid of fruiting spikes, 

 although exceptions are frequent. In examples of this kind, figure 44, B, C, shows the 



