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



P. amplifoliiis Tuck, x illinoensis Mor. (P. scoliopliyllns Hagstr. n. hybr.). 



Folia submersa ± recurvata modo P. ampUfolii, natantia in omnibus partibus 

 prsesertim ad apicem, nervaturam, magnitudinem, petiolosque medium inter P. ajnpli- 

 folium et illinoensem tenentia. 



The plant here concerned manifests its connection with P. ampUfolius by the 

 densely running longitudinal nerves of the floating leaves. P. illinoensis can be 

 traced in the obtuse apexes and dense cross-veins of the amphibial leaves, the shorter 

 ligules, the rudimentary denticulation of the submersed leaves, the short inter- 

 nodes of the branches of the stem-prolongation, and in the anatomical conditions of 

 the stem. In the respect last mentioned it corresponds nearly quite with P. illino- 

 ensis. Only in the complete absence of subepidermal bast we detect an influence 

 from P. amplifoliiis. The petioles of the submersed leaves correspond with those of 

 the two parent species. The petioles of the proper floating leaves are not quite so 

 long as the laminse and consequently intermediate. Such floating leaves are found 

 first in the second rank branch of the stem-prolongation and are small, 35 — 50 X 20 

 mm, an obvious influence from P. illinoensis which often lacks proper floating leaves. 

 The peduncles are slightly incrassate. The plant is quite barren and the pollen is 

 also sterile. 



The collector of the plant has labelled it as P. lucens but added in a paren- 

 thesis »n?i P. Zizii M. & K.?» and, in fact, it has a great habitual resemblance of 

 it. On closer examination, however, it appears, that the plant has nothing to do 

 either with P. lucens or gramineus. In the Berlin Museum there are some specimens 

 from the same locality labelled » Pota7nogeton lucens X gramineus den 6 febr. 1899. 

 P. Graebner det. » I know this beautiful hybrid from only two other stations, viz. 

 » Cedar Lake, Ontario » and » Buckingham, Quebec ». Several North-American »P. 

 Zizii -i, nevertheless, might be this hybrid. Since it is probable that the American 

 botanists will find forms of this hybrid with more amplifoli'us-like aspect in the 

 floating leaf region the now distinguished form with late developed and small 

 floating leaves may conveniently be named : 



forma barensis n. f. — Forma P. scoliophylli foliis coriaceis parvis et sero 

 evolutis. 



Distribution: »In lacu Bar ad Manistee, Michigan, U. S. A.», 1882, »Morong 

 (lib. Stockholm., Berolin.). Canada, see above! 



Subsectio 18. Muricati Hagstr. 



Caulis ut in prsecedente. Folia natantia saepe breviter petiolata obtusa; sub- 

 mersa ± petiolata vel sessilia. Ligulae ± persistentes. Pistillum stylo magis elongato 

 vel saltem conspicuo. Fructus carinatus rostratus ± muricatus. Anatomia caulis: 

 0-endodermis ; fasciculi libriformes subepidermales et intraloculares semper adsunt. 

 Fasciculi centrales liberi, mediani semper duo. 



