12 J. O. HAGSTRÖM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



monly. The latter is the case with P. gramineus X perfoliatus (P. nitens Web.) fre- 

 quently occurring at the stations where its parents grow together at least in the 

 Scandinavian countries. That we here, in general, have to do with as many crosses 

 as we have stations is clear from the slight fertility mostly much reduced or quite 

 failing, as well as from the fact that one form is not like the other, i. e. the spe- 

 cimens of each station are isolated and independent. In Sweden P. Zizii follows 

 P. gramineus and P. lucens to the north, but does not appear beyond the 63 d pa- 

 rallell, where P. lucens has its northern limit. P. nitens, again, proceeds northward 

 as far as its two parent-species spread and accompany one another. The same is the 

 case as to the hybrid P. gramineus X natans. 



The correct determination of the hybrids of this genus has proved very diffi- 

 cult. It belongs indeed to the most intricate problems of the Potamogeton-study and 

 presupposes a thorough knowledge of each genuine species and a careful attention to 

 each detail. Instances can nevertheless occur when one of the parents is recognizable 

 at the first glance, but the more the other partner in such a case uses to conceal it- 

 self owing to its appearance in the fructification. And the difficulties increase when 

 the parent-species have similar styles and stigmata or the malformation of these organs 

 is so great that their true shape is not distinguishable. Then we try to get notice of 

 among which species the form was found growing. Too great a weight, however, must 

 not be set on this circumstance. It has been proved that the hybrid Potamog el on -forms 

 can abide at the place where they have arisen for long spaces of time, while at last both 

 the parents or either has become extinct and other species have occupied their place. 

 Spikes, buds and turios can also be conveyed by waves and current far distances 

 from their native place. It has happened that in a small river or on a spöt very 

 much retrenched four or five different hybrids partly entangled in one another have 

 been met with. For instance in Gudenaa at Kongensbro in Jutland. From there I 

 have, in my own herbarium, P. gramineus X natans, crispus X prcelongus, lucens X per- 

 foliatus, lucens X prcelongus and lucens X nodosus. P. nodosus itself is at present gone 

 out in Jutland. — At all events the distribution areas of the hybrids must now 

 coincide or must once have been, at least partly, coinciding with those of their 

 parents. 



The hybrids must be studied according as Nature produces them. Cultivation 

 and experiments in hybridization may not lead to great results as to the solution 

 of this intricate question. A thorough study of the specimens already exactly known 

 as to their origin is much more profitable for the purpose of a correct identifying 

 of others. Rather soon you will learn in which way the one or the other species 

 makes itself prevalent to its compartner, and to recognize the so-called intermediate 

 properties. 



Properties not occurring in the supposcd parents oannot appear in their cross- 

 breed. Thns, for instance, P. Billupsii Fryer cannot be the hybrid tcoloratuå 

 Zizii- on account of the long petioles of the hybrid. The two parent-species, namolv, 

 have short petioles. P. lanccohitus Sm. cannot be as it has been supposcd (Bknnhtt, 

 FRYER, AsOHERSON, GraebNER), either »gramineus >: mucroualiis. or »hcUrophyllns 



