276 BULLETIN OF THE BUREAU OF FISHERIES. 



between them is also apparent in the time of occurrence and in position on the stem. 

 Irmisch (1858) recognized a disparity between them and suggested a difference in origin, 

 though he was not able to determine this for both forms. The spicular burs he found 

 originating from the axillary buds of decaying, floating stems in autumn. The den- 

 ticulate ones he found always mature and detached from the parent stem in muddy 

 bottoms of pools. Sauvegeau (1894) describes and figures both forms of burs, giving 

 their origin as well. My observations, however, are not in full agreement with their 

 representation on the stem as expressed in Sauvageau's figures. According to his 

 illustrations, both forms are abundant on the same branch and at the same season of 

 the year. This has not been found to be the established order in vigorous and healthy- 

 looking plants. Numerous collections of P. crispus indicate that when the denticulate 

 burs are abundant — that is, in the early part of the growing season — the spicular burs 

 are scarce, and if present on the same stem they are sparsely represented at the base of 

 the axis. In every case the large denticulate bur seems to be the product of strong 

 and vigorous-looking plants, and the spicular bur a result of poorly conditioned ones. 

 That the spicular bur is a weakling would appear to be borne out by observations on 

 their development. When grown in aquaria they have been found on sickly-looking 

 plants and when germinating burs have been deprived of their vigorously growing 

 shoots, small shoots bearing spicular burs have replaced them. In this instance a dis- 

 turbance of the natural trend of growth would be the occasion of their formation. 

 When the spicular burs germinate they produce shoots bearing leaves not crisped, 

 but narrow and flat (fig. 25). 



The internal structure of the bur is fundamentally like that of the ordinary stem. 

 No new features appear in the tissues of any part of the bur, but starch grains are 

 present in such great quantities that the cells become distended with them. In the 

 fully developed bur (fig. 71) the cells become so greatly expanded that the air cavities 

 are practically obliterated. It is to these distended cells so compactly stored with 

 starch that the hardened, indurated character is due. 



The accumulation of starch in the bur furnishes an abundant storage supply for 

 rapid growth, after a rest period of greater or less prolongation, depending upon the 

 time of formation. Burs formed early in the summer may germinate early in the fall, 

 or, like those of later development, pass the winter in a quiescent state. Figure 23 

 shows a stage of germination which is usual in the early spring. It is obvious from the 

 general appearance of the shoots that burs of this character passed the early part of the 

 winter in the resting stage. At the same time burs much more advanced in stage of 

 growth (fig. 32) are frequent, and it is assumed that these are comparable to burs that 

 germinated in the fall (fig. 59) and grew but little during the winter. In aquaria a varia- 

 ble rest period is common. Under these conditions burs have been germinated after 

 periods of six weeks and of three months. 



In the germination of a bur there are as many possibilities for the production of 

 stems as there are axillary buds on it, although usually not all of the buds germinate. 

 The greater number of burs bear but one shoot eventually, but several may begin 

 growth and produce short shoots (fig. 23). By experiment it has been found that when 

 a bur is broken into bits with one bud per node, each bud will produce a shoot. In 

 the development of a plant from the bur, progress in the growth of a shoot manifests 



