POTAMOGETONS IN RELATION TO POND CULTURE. 273 



appears at the beginning of the winter, is seen in figure 1 1 , although variations in the 

 length and thickness of internodes are not uncommon. 



Tuberous rootstocks have been transferred to aquaria, where the growth has cor- 

 responded exactly with that exhibited in the natural habitat except in one respect, the 

 development of aerial tufts of leaves. But the explanation of this omission in the 

 cycle of development is clear, since the plants remained submerged in the aquaria. 

 The period of desiccation not having been interpolated, it is assumed that the tuber 

 formation progressed in a natural manner for the species. Figures 16 and 17, drawn 

 from aquarium specimens, show how in the purely aquatic phase of its existence the 

 natural habit of growth and reproduction in this Potamogeton is reproduced under 

 artificial cultivation. 



PROPAGATION BY SUBTERRANEAN STEMS NOT TUBEROUS. 



Among the species studied, P. perjoliatus, P. amplifolius, and P. americanus are 

 propagated in this manner. The plants are carried over the winter by means of the 

 terminal portions of underground stems, which are generally stouter than the ordinary 

 ones and which bear conspicuous scaly buds. These buds are the incipient shoots from 

 which the elaborate plant structures of the following season are developed. Sauvageau 

 (1894) has figured this propagative structure for P. perjoliatus as he found it at the 

 approach of winter. He states that the entire plant dies in autumn, except a few inter- 

 nodes which bear the buds for the continuation of growth in the spring. In figure 

 18 is represented a portion of an underground stem that survived the winter and pro- 

 duced the first few internodes of growth. The scales on the part that lasted through 

 the year are distinctive in appearance. They are larger and looser than the ordinary 

 ,ones, black in color, and leathery in texture (fig. 18, A.) 



In P. amplijolius perennial parts are also found in the underground stem. Figure 

 7 represents the characteristic features of such a structure at the beginning of the winter. 

 The young, erect shoots A, A, A, wit partially unfolded leaves at the tips, pass 

 the winter unchanged and serve to promote rapid growth in the spring. The buds 

 terminating the horizontal stems remain latent through the winter and on unfolding in 

 the spring push out in all directions through the substratum. In these ramifications a 

 subterranean system of interlocking stems and roots is developed that fixes the plant with 

 exceeding firmness in the soil. 



In P. americanus vegetative propagation is accomplished by subterranean scaly 

 buds which generally grow in pairs at the end of the rootstock (fig. 4 and 5). The general 

 structure of the bud resembles that of P. perjoliatus. It is an incipient shoot, possessing 

 a succession of very short internodes and young leaves, with scales surrounding* the whole 

 axis. A small portion of the rootstock generally remains attached to the buds and 

 persists through the winter. 



PROP.'iGATION BY WINTER I!UDS. 



The winter buds afford the only means of vegetative propagation which have been 

 observed for P. zosterijolius and P. obtusijolius. These structures develop at the ends 

 of the shoots. The terminal internodes remain short and, becoming completely cov- 

 ered by closely overlapping leaves and stipules, form a hard, compact, cone-like bud. 



