KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55- N:0 5- 11 



The anatomical conditions play such an important part in the Amplifolii as to 

 divide them in a sclerenchymatous and a non-sclerenchymatous series. 



Hybrids are not rare in this genus, but the successfull study of them is com- 

 paratively new. 



All hitherto exactly known Pota?nogeton-hyhrids have proved to be very ste- 

 rile. But, stränge to say, the sterility also occurs, although rarely, in genuine 

 species. No one can suspect P. Robbinsii to be a hybrid of any known species, 

 nevertheless it seldom is to be found fruiting. To this is to add that species com- 

 monly fruiting freely, occasionally, by adverse conditions appear fruitless. In her- 

 baria such forms are often met with and have sometimes been considered as hybrids. 

 This is the case with some broad-leaved forms of P. gramineus, viz. P. gram. var. 

 fluvialis, f. Wolfgangii et f. septentrionalis, that in Herb. Stockholm have been taken 

 for and in Hartman, Handbok i Skandinaviens Flora, ed. 12, 1889, described as 

 »xP. alpina X graminea var. graminifolia{' > .)». The genuine P. alpinus X gramineus 

 is somewhat låter detected and described (see below). 



According to my experience, in most cases an exact judgment is nearly quite 

 impossible except by means of the anatomy. This makes it necessary to have each 

 species even anatomically examined and described. 



Concerning the very possibility of hybridization I will remark that it seems 

 not to occur between species with different stigtnatic papillae. Thus no hybrids are 

 observed between Coleogeton- and Eupotamogeton-species. On the contrary it is not 

 rendered impossible by the differences in form and size of the pollengrains. An evid- 

 ent proof of it is the hybrid P. nitens. One of its parents, P. gramineus, has small 

 oval pollengrains, the other again, P. perfoliatus, has spheric and conspicuously larger 

 grains. In this case it is surprising, that when the hybrid eventuahy produces 

 fertile pollen, we see part of it evoluted to the gramineus-type, another part to the 

 perfoliatus-lype. But sometimes one or the other species is exclusively prevailing. 



As I already in 1901 and 1908 have given a partly new interpretation of P. 

 Zizii and P. nitens, as hybrids and not genuine species, and established some new 

 hybrids, and on the following pages am going to present several others, it may be 

 necessary to give an account of the principles I have followed in establishing all 

 those forms. 



The hybrid forms mostly present a pollen to the greatest part unfit and ste- 

 rile and deformed stigmata. Accordingly no fruits can generally be produced, or the 

 fruiting faculty is at least considerably reduced. In very rare instances, for exam- 

 ple in P. gramineus X lucens (P. Zizii M. & K.) the development of fruit seems to be 

 almost normal. But it is not yet made out, if such individuals through several ge- 

 nerations are able to keep their existence and multiply themselves by seeding. This 

 is probably not the case since those forms appear only locally. In many cases, be- 

 sides, an abundant evolution of fruit in this hybrid and in the hybrid P. nitens is 

 only seeming as most fruits are obviously lacking adult and fertile embryo. 



The hybrids further lack a distribution area of their own. They accompany 

 two species and appear sporadically in their distribution area, less often pretty com- 



