320 



var. trichophijüa Klz. was growiii;,', 3— 10 cm liigli, willi fruit, 

 Seplbr. 30. 1901, Angma^'salik, 65°37'N. L. (Krcüse 1906 p. 224). 



The habitus ol the (iilVerent species, if livirii: in water, is 

 very similar, small plants with slender branching stems and 

 opposite leaves, the uppermost of these, except in C. autum- 

 nalis, forming a rosette on the emerged part of the stem. 

 When living in dry places C. venia is short and branches 

 abundantly, looking tuftlike, often it is prostrate (cfr. f. minima 

 Hoppe and f. caespitosa Schultz). The forms, however, vary 

 very much and do not always correspond to the description. 



Most likely the species of CallitricJie in arctic as well as in 

 temperate regions are as a rule perennial without making 

 special winter-buds (Irmisch 1859 p. 354; Schenck 1886 a p. 84; 

 Warming 1884 p. 90; Stlvén 1906 p. 187; Birger 1908 p. 57). 

 H. Jonsson (1895 p. 290| observed lots of C. hamulata in a 

 slowly streaming channel, Vallanes, Iceland, Jan. 10; the water 

 had been frozen from the beginning of Novbr. until Decbr. 28. 

 That Callitriche is able to live below the ice during the winter 

 is often seen, e. g. C. venia in the Botanical Gardens of Copenhagen 

 and in ditches near Furesø, but how it hibernates if the water is 

 frozen to the bottom, seems not to have been investigated; per- 

 haps at least parts of the stem will survive even then (cf. p. 332). 



The observation that species of Callitriche are annual 

 (e. g. Hartma.nn 1879 p. 383; Neumann 1901 p. 307— 308) probably 

 applies only to the terrestrial forms. Such is the case according 

 to Irmisch (1. c.) and Stlvén (1. c.|. Ostenfeld (1908 p. 938) writes 

 that occasionally C. hamulata is annual on tlie Faeroes; i. e. 

 when the plant grows on land, according to verbal information 

 of the author. 



The primary root is succeeded by adventitious roots. 



The ability of the nodes to form roots is very great. They 

 are short and rather stout in many of the land-plants, long and 

 slender in aquatic ones. They may be found, just as sometimes 

 in Myriophyllum^ even at the nodes bearing flowers. 



