POTAMOGETONS IN RELATION TO POND CULTURE. 275 



directly (fig. 31), though this occurs but seldom. On the maturity of the bur detach- 

 ment from the parent stem is an easy and natural process. The tissue just below 

 the pointed base of the bur becomes softened and the burs fall away, either by their 

 own weight or by accidental contact with other objects. On reaching the bottom, 

 anchorage in the substratum is facilitated by the peculiar shape of the bur, a sharp- 

 pointed, spindle-shaped structure that is heavier than water. A rest period occurs 

 before germination takes place. This rest period is apparently a varied one, depending 

 on the season when the bur is matured. Those which matured early in the season, in 

 so far as it could be determined, germinated in the fall, and in October bore shoots 

 from 6 to 10 inches long (fig. 59). Those maturing late passed the winter in the 

 quiescent state and germinated early the following spring. 



The slender, spicular burs (fig. 21) described by Irmisch (1858) and by Sauvageau 

 (1894) were found more or less commonly in the axils at the base of the erect stem, 

 and always few in number compared with the stouter form. It is interesting to note 

 in this connection that these spicular burs appeared more abundantly on the so-called 

 "state" of P. crispiis, a plant with flat, not undulate leaves, said to be a young state of 

 crispHs (Fryer, 1900). In one of the spring pools from which collections were made 

 the spicular buds predominated on what appeared to be matured plants of this flat- 

 leaved form. The plants were never so vigorous looking as those in the other situations, 

 and the appearance of the spicular burs upon them may be explained by differences 

 in habitat. Generally they appear to be poorly conditioned plants, and from observa- 

 tions on their development it would seem that they are a starved state of crispus rather 

 than a young state. 



The development of the large bur (fig. 22), which Sauvageau (1894) described in 

 part, has been observed in the field and in aquaria throughout the various stages, from 

 its beginning as a small branch to its completion as a mature bur. Since the steps in 

 the formation are essentially the same under natural or artificial conditions, observa- 

 tions will be presented on the material under control in the laboratory. 



Vigorous looking plants were collected in the latter part of March and anchored 

 in a soil substratum, in aquaria with running water. Cuttings also were used, some of 

 which were anchored in the soil and others allowed to float on the surface of the water. 

 Three weeks later, short, stunted-looking branches appeared in the axils (fig. 26, A). 

 They exhibited at once a noticeable thickness of the axis and later the peculiar 

 denticulate appearance at the base of the leaves (fig. 22, a). When the diameter of 

 the branch had become considerably augmented and the denticulate margin conspicu- 

 ous, disorganization of the leaves commenced from the distal end and proceeded toward 

 the base. Disorganization ceased at the tip of the denticulate base (fig. 22, a, i). By 

 this time the basal portion of the leaf was hardened, thickened, and homy like the 

 axis, and the entire structure presented the characteristic burlike appearance. Figure 

 60 shows several small-sized denticulate burs in various stages of development. 



Essentially the two kinds of burs are similar, differing only in certain minor details. 

 In the bur shown in figure 22 the leaf bases are large and always denticulate, the buds 

 in the axils are relatively small, and the internodes are short. In the spicular bur 

 (fig. 2 1 ) the opposite is true. The leaf bases are small and spinous with a smooth margin, 

 the buds are well developed, and the internodes are comparatively long. A difference 



