186 J. O. HAGSTROM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



und beide haben nur im Laufe der Zeit sich in entgegengesetzten Richtungen fortent- 

 wickelt. » He still in 1914 suggests something of a development from a hybrid 

 alpinus X lucens (yet with?) and expects a good help from the paleontologic study. 



To this I want to say. P. nodosus is a distinct species like all the other real 

 species, the origin of which we cannot give more exactly than by the statement of 

 its near affinity with the AmplifoUi and a more remote relation to the Lucenies. 

 It is a connecting link between those two. In all its properties it shows a distinct 

 fixedness whether it is met with in Europe or in another quarter of the world. 

 The above mentioned hybrids, again, vary in all their properties and in habit to 

 greater likeness now to one, now to the other of the respective parents. The fact 

 that they in most of their forms come very near to P. nodosus is hightly interesting. 

 At the same time it renders the determination difficult, it urges to cautiousness and 

 a thorough consideration of each particular case. On account of their nature the 

 coactive species must produce forms of that aspect. The anatomical conditions of 

 the plant, however, give rise to finding the right solution in most cases. 



On the anatomical structure of P. nodosus, see C. Raunki^r, 1. c, 1903, 274 

 — 275. The characteristics there remarked I have found confirmed by examining a 

 great number of specimens from four different quarters of the world. Those properties 

 are: 1) Absence of a supporting layer (pseudo-hypoderma) in the stem; 2) Absence of 

 vascular- and bast-bundles in the cortex of the stem, as well as 3) of the petioles; 

 4) a typical 0-endodermis; and 5) a central cylinder of genuine trio-type, see fig. 1, G. 

 The importance of these facts cannot be nullified by the very rare occurrence of one 

 or a few faintly developed cortical bast-bundles in the stem, or by the circumstance 

 that the 0-cells of the endodermis rarely tend to grow a little thicker at the inside, 

 as we have observed in a few cases. Besides, this unilateral incrassation is oftenmost 

 more seeming than real. It arises in this way that thickwalled cells of the central 

 bundle-sheaths stratify themselves to the endodermis, whereby the very endodermis-cells 

 also seem to be thicker there, than at the outside, where they border to the thin- 

 walled cortical cells. This fact can also be observed in several other species with 

 0-endodermis. As to the Potamogeto7i-s])eoies there is no reason to depreciate the 

 systematic value of the anatomic facts or to give the anatomical characters a slighter 

 importance than the morphologic conditions. 



I, therefore, cannot agree with Prof. P. Graebner, who has arrived at the 

 conclusion that almost everyone of the commoner species can be divided into more 

 j> species » on account of their anatomic facts (Lebensgescli. 1906, 436). The case is 

 just the very contrary. The anatomic stem-diagram is so fixed that it is very iittle, 

 if at all, influenced by the morphologic variations within the same species; nay, 

 even morphologically well separated species being nearly related can have the same 

 anatomic diagram. But, of course, just as the leaves, for instance, vary as to the 

 width etc., so the cortical bast-bundles, f. i., can deviate a little as to number and 

 thickness, and the thickness of the endodermis-cells can vary a trifle, mostly de- 

 pending on age and alimental conditions. In the cases when a species is able to 

 grow both in stagnant and running water and the stem is lacking cortical bundles, 



