KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55. N:0 5. 9 



by their always ha^ang a pseudo-hypoderma even in species lacking it in the stem, 

 and finally by the channels of the cortex increasing in number but decreasing in 

 width which process causes the well known incrassation of the organ. By this rigi- 

 dity of the peduncles the plant is able to lift the spikes over the surface of water for the 

 pollination. Thus we have here a biological adaption causing me to say some words 

 of the biological facts employable for distinguishing the species. 



I have alreadj^ spoken of the adaption of some leaves for a double life. Another 

 important fact is the adaption for different levels of water. In this respect some 

 species prolong the internodes next below the primary spike, seldom the peduncle 

 also, others, again, prolong the stem by branches from the topmost two leaves, 

 the first involucral leaves, and then produce, often dichotomously, branches of se- 

 veral different ranks. Instances of the first -mentioned way are P. lucens, P. pa- 

 normitanus and others. P. foliosus, pusillus, densus etc., on the contrary, use the 

 latter way. — Each species has also its own way of producing the greatest number 

 of spikes from which it rarely or not at all deviates. Some species give off long 

 branches from the basal or lower axils each terminating itself by a spike and a 

 short or two spike-bearing branches of the first rank from the base of the primary 

 spike, fig 2, A (4-type). The other way consists in developing a rich rosette of 

 spikes on short branches of 1st— 5th rank above the primary spike, with or without 

 the power of producing the above-said long spike-bearing basal branches, fig. 2, B 

 (5-type). A third way is to produce shorter branches from the axils next below the 

 primary spike together with or without the mode of branching characteristic to P. 

 vaginatus, vide p. 10!, fig. 2, C (C-type). Other species, again, like P. perfoliatus 

 etc., form the above mentioned toprosette, often dichotomously, to a bushy tuft of 

 rather long spiciferous branches of as far as the fifth or sixth rank sometimes together 

 with spike-bearing branches from the axils beneath the tops-pike, D-type, fig. 2, D. 



Beside the development of fruits most Potamogeton-sTpecies have an effective 

 vehicle for the propagation and spreading in their turios, which also serve as a rest- 

 ing-stage during the unfavourable season. Therefore these organs are mostly called 

 either propagating-buds or winter buds (hibernating-buds). They appear either as 

 subterranean organs on the rhizome or as cauline ones on the leafy shoot, in which 

 case they shed and at least sink to the bottom. They are tops of rhizomes and bran- 

 ches which have been metamorphosed and with their scales and leaves adapted to 

 depositories for starch, or they are wholly transformed short branches. Sometimes 

 a single sort of them occurs, sometimes again principally one sort, sometimes finally 

 all sorts together in a single species. In some species no such turios at all are ob- 

 served. In some groups especially in the Pusilli, they are of a prominent importance 

 for recognizing the species and hybrids often habitually similar among themselves. 



Another adaption are the arrangements in the stems and leaves for the pur- 

 pose of resisting the impetus of the waves and the current of water. The short epi- 

 dermis-cells, the mechanical tissue especially in the border and along the midnerve, 

 the wavy shape of the margins, the canaliculate shape of the tip, the linear or nar- 

 row form of the lamina and the lubricity of the surface are the chief expedients 



K. Sv. Vet. Akad. Handl. Band 55. N:o 5. 2 



