POTAMOGETONS IN RELATION TO POND CULTURE. 277 



itself first by the establishment of an erect axis, from which very soon a subterranean 

 system arises in the manner shown in figure 27. By further extensions of these axes 

 the number of branches is greatly augmented and the capacity for multiplication 

 greatly increased. 



P. crispus, like most of the Potamogetons, propagates readily by detached stems. 

 Many of these have been picked up in the drift along the lake shore where under favor- 

 able circumstances some, doubtless, find lodgment and establish new centers of growth. 

 Besides, in the spring there have been found leafy axes which, while still remaining 

 attached to the parent stem, lie prone upon the muddy or sandy substratum and, be- 

 coming rooted at the nodes, send up a long series of erect stems (fig. 20). In this manner 

 P. crispus combines the rapid growth from stolons with the normal spread of the subter- 

 ranean system and forms an effective means of possessing the soil. 



The large number of burs which are developed indicate that they are the chief source 

 of distribution in this species. Some plants doubtless develop from seed, though they 

 can not represent any great number of the whole since comparatively few seeds mature. 

 To obtain some data on this point a large number of young plants were pulled up and to 

 the most of them a bur was attached, an observation which shows that, for the region 

 at least, this structure was the chief agent of propagation. From the standpoint of 

 prolificity, P. crispus represents a desirable species for cultivation. It remains to be 

 shown that this abundant herbage is of importance in the economy of aquatic life. Data 

 relative to this are recorded under the heading "Economic aspects of Potamogetons." 



PROPAGATION I!Y FRAGMENTS OF STEMS. 



In P. Robbinsii the propagation occurs exclusively by vegetative means, depending 

 upon a more or less complete dismemberment of the plant. This breaking of the plant 

 into propagative structures does not take place at random, but occurs at very definite 

 points throughout the leaf-bearing part of the plant. At intervals along the axes of the 

 stems, a few intemodes develop which are very short, and in them starch is stored 

 so abundantly that they become hardened and stiff and noticeably thickened in diameter. 

 At the limits of these indurated regions where the stems appear constricted, the tissues 

 soften when the structures are mature, and dismemberment becomes a natural operation. 

 The process of separation is similar to that which is met with in P. crispus and which 

 causes the detachment of the bud from its parent stem. Besides the main axes of the 

 plant which break up into many potential units, there are also numerous short, axillary 

 branches which possess the characteristic feature of the propagative structure. The 

 intemodes are likewise short and stiff and conspicuously augmented by the deposition 

 of starch. Moreover, they are always provided with a growing terminal bud, a feature 

 which facilitates rapid propagation. When an axillary shoot becomes 6 or more inches 

 long it behaves like the main axis of the stem eventually breaking up into several propa- 

 gative structures. In figure 67 is represented a single branch showing the constricted 

 appearance which distinguishes a stem bearing more than one propagative structure. 



In the spring, often before a general dismemberment of the plant occurs, very long, 

 white rootlets are developed at the nodes (fig. 57). These rootlets ser\-e to anchor the 

 new growth, whether it be an attached part of the plant or a scattered fragment of the 

 stem. The provision for the initial growth in these fragments of stems lies in the storage 



