274 BULI^ETIN OF THE BUREAU OF FISHERIES. 



Such buds become conspicuous during the month of August. Later when they are 

 mature they easily fall away from the parent axis, which thereafter dies down com- 

 pletely. Being heavier than water, the buds sink to the bottom and by the middle of 

 October they have either disappeared or have become entangled in the accumulations 

 of Elodea, Myriophyllum, Ceratophyllum, etc., which still remain intact. In the dis- 

 organization of this mass of vegetation, a gradual settling of the entangled buds takes 

 place and they eventually find lodgment with the others in the substratum of mud, 

 where they remain in a quiescent state till early spring. Such buds may properly be 

 called hibemacula, since they pass through the unfavorable winter season in a state of 

 rest. 



The general external aspect of the winter buds is seen in figures 33, 63, and 64. In 

 size and form the two buds are quite similar but the leaves of ohtusijolius are less stiff and 

 harsh. In the internal structure of the bud (fig. 34) the typical branch-like character 

 is apparent with the young leaves closely crowded toward the tip. 



Plants of both species have been reared in aquaria by anchoring the buds in sand 

 or mud. The latter operation is not necessary, however, since mature buds sink naturally 

 to the bottom, but it was a precautionary measure against the disturbance of buds under 

 observation in aquaria. The plants of zosterijoliiis thus propagated did not bloom, but 

 produced winter buds; those of ohtusijolius bore flowers and fruit early in August. 



During the winter the loose leaves on the outside of the bud decay, but, on the whole, 

 the entire bud is well preserved. This resistant character is more especially true of 

 zosterijolius , in which many of the enveloping leaves of the bud persist long after the 

 new plant has become established. In the spring the first sign of growth is manifested 

 by a spreading of the inclosing leaves. Then follows the development of roots from 

 successive nodes (fig. 35) and the elongation of the intemodes at the tip of the bud. 

 This elongation carries the young leaves forward and upward, and in a short time the 

 general habit of the plant becomes apparent (fig. 65). The various stages in the growth 

 of the bud in the spring are, in so far as they have been observed, similar in those two 

 species of Potamogeton, except that obtusif alius lags behind zosterijolius. 



PROPAGATION BY BURS. 



P. crispus is the single example of such vegetative propagation. The first 

 evidence of propagative structures by means of which the growth of this species is 

 rapidly extended became noticeable early in May. At that time the so-called "burs" 

 (fig. 22) made their appearance. They were enormously abundant, appearing in the 

 axils of nearly all the leaves. Many of them became fully mature by the middle of 

 the month, especially those which developed in pools of standing water where the daily 

 temperature of the water was comparatively high. In the colder waters of spring pools 

 and of the open lake these propagative structures, like the flowers and fruit, were 

 retarded in development, maturing about two weeks later. As the summer advanced 

 the development of the burs decreased until by the middle of July only scattered 

 individuals were to be found. 



As a rule, the burs occur in the axils of the leaves. They may, however, terminate 

 the growth of the axis (fig. 30). In this latter position they may occur in pairs (Savau- 

 geau 1894), often with a flowering spike. They may develop from the rootstock 



