POTAMOGETONS IN RELATION TO POND CULTURE. 257 



tive period in the fall, tlie shoots of recent formation have a singular appearance, the last two thin 

 intemodes bearing tubers at the end. At first the tuberous end resembles a conical terminal shoot or bud 

 surrounded by scales. Intemodes make their appearance and soon become thickened; eventually the 

 scales split and disclose a tuber of two swollen intemodes. Simultaneously a slender bud forms at the 

 distal end of the tuber, and axial outgrowths develop from tlie sides that bespeak the shoots of ordinary 

 branches. These axial shoots in turn develop swollen intemodes which follow two thinner ones as in the 

 preceding case and produce a series of tubers dichasial in form. The excellent series of drawings by 

 means of which the author depicts the transition from intemode to tuber leaves nothing to be desired 

 in the morphological interpretation of them. They are clearly two modified intemodes. 



Tuber-bearing shoots grow out of the upper leaf axils also, and follow the usual development of genera- 

 tions of intemodes with leafy shoots, besides the tuber-bearing ones in two or three series. The anatom- 

 ical structure of the tuber resembles that of the stem excepting that all tissue not fibro-vascular is filled 

 with starch. The observations on P. ohiusifolius are incomplete, but the presence of winter buds is 

 noted. For P. naians and P. lucens the morphology of the rootstock, stem, and shoot is completely 

 determined, and the details are clearly shown in the drawings. The method of branching is fundamen- 

 tally the same in the two species. In brief, the growing tips of the rootstocks branch dichotomously, 

 giving an erect a.xis and a horizontal one. Each generation of the developing rootstock brings forth two 

 horizontal intemodes and a bud which is the incipient erect axis. The terminal bud at the end of the 

 horizontal axis reproduces this condition as long as the plant lives. In the development of the erect 

 shoot, the scales, usually three in number, grade into stipular sheath, phyllodes, and foliage leaves. A 

 two-fifths arrangement of leaves is noted and the shoots follow the same order. The winter condition 

 of P. lucetis consists of rootstocks by means of which the plant propagates itself rapidly in the spring. 

 The intemodes of these rootstocks are shorter and thicker than the ordinary ones and are borne in a suc- 

 cession of three or more with terminal and axillary buds containing the incipient axes of the horizontal 

 and erect shoots. 



Irmisch made observations also on P. crispus, investigating especially the "burs" or propagative 

 shoots, although this work was anticipated in part by D. Clos in his Mode de Propagation particulier au 

 Potamogeton crispus L. Irmisch, however, found two forms of the bur of cm/>u.y, the slender spicular bur 

 as well as the stout, homy, denticulate one observed by Clos. The former bur he observed growing in 

 the axils of detached shoots in late autumn and afterwards breaking away from the axils and settling in 

 the mud. The origin of the latter form he did not observe, but he found it in the muddy bottoms of 

 ponds in great abundance. These "burs "or modified twigs, as Irmisch sometimes called them, he con- 

 sidered important examples of propagative structures. 



In connection with these plants, Irmisch first pointed out the "Scheiden-Schiippchen, squamulse 

 Intravaginales, " scale-like structures developed at the leaf bases, having as a possible function the pro- 

 duction of slime or mucilage for the protection of young and slender shoots. 



This monograph is of special importance in presenting the morphological data of a few species of 

 Potamogeton. From time to time further contributions have been made to the subject by other investi- 

 gators in the field, but this still remains the greatest work of its kind. 



As a result of these studies on the tubers of P. pectinaiiis, the rootstocks of P. lucens, and the burs of 

 P. crispus, Irmisch came to appreciate the advantage of artificial propagation in this group and remarked 

 in conclusion, in an observation that is prophetic of present day interest, "That many of the Potamoge- 

 tons, as well as other aquatic plants, possess in a singular way that possibility of domestication which 

 has given us the tame animals from the wild ones." 



ROBBINS, 1867. 



Thus far the work of the Potamogetons was confined principally to European species. In 1867, 

 however, the American species were reduced to something like a complete intelligible systematic shape 

 by Dr. G. W. Robbins, whose descriptions, as far as they came within the range, were incorporated in 

 Gray's Manual, edition 5. Later descriptions of the western species were published as they became 

 known. 



MORON'G, 1893. 



The greatest contribution to the literature on the North American species of Potamogeton is by 

 Thomas Morong in his Naiadaceae of North America, a monograph which includes 37 North American 

 species, 14 of which are confined to this country. Many of these species were studied through succes- 



