THE LYCOPODS (CLUBMOSSES) 



in Britain, and secondly, that L. Selago, if it is a hybrid, is Hkely to be the most ancient 

 impure species that cytology has so far detected. 



Leaving Lycopodium we come to the two heterosporous * genera Isoetes and Selaginella. 

 The aquatic Quillworts, Isoetes lacustris L. (Fig. 252) and /. echinospora Durieu, are not 

 unfamiliar inhabitants of the pure waters of our glacial lakes and mountain tarns, and 

 their conditions of culture are fortunately of the simplest. It is only necessary to put a 

 httle garden soil at the bottom of an inverted bell-jar, which is then filled up with water, 



/soeAes n= 54 



Fig. 253. Explanatory diagram to Fig. 256a. x 2000. 



and Isoetes will grow indefinitely in a laboratory if kept near a cool north window. In 

 addition to the two species recognized taxonomically, there are undoubtedly many true- 

 breeding strains characteristic of different lakes, which form local inbreeding com- 

 munities. Maximum stature appears to be partly under genetical control and ranges 

 from the fairly small plants with leaves 4 in. or so long, characteristic of Windermere, to 

 immense plants with leaves over a foot in length such as the var. Morei Moore found 

 in Loch Bray in County Wicklow in Ireland. These size diflferences are constantly 

 maintained in culture and do not appear to be environmentally induced growth forms. 



* By heterosporous is meant the production of spores of two sizes, the large, or megaspores, being few 

 in number and heavily stored with food for the production of exclusively female prothalli; the small, or 

 microspores, being formed in greater numbers destined to produce only diminutive male prothalli. The 

 microspores oi Isoetes and Selaginella are the equivalent of the pollen grains in the higher plants. 



