PLANTS OF FRESH WATER, SWAMPS, AND BOGS. 107 



The water-milfoils (M//n'opJi;/llu)n) differ from the pond-weeds in 

 that they have no floating leaves, but boldly raise their upper portions 

 above the water-surface. They agree, however, in the fact that the 

 aerial leaves differ from the submerged ones. This is the more in- 

 teresting because no line of demarcation on the erect stem separates 

 the two except the water-surface that is to say, the same tissue can 

 change its leaf-form according to change of outer circumstances. 

 The water-milfoils are graceful, feathery - looking plants, with the 

 leaves frequently given off, four or more, from the same height round 

 the stem. The submerged leaves are cut into fine segments, a very 

 common occurrence in many water-plants, whereas the aerial leaves 

 are broader and much less cut. Some of the water-milfoils are bog- 

 plants rather than aquatics e.g., M. Votschii* 



It is highly probable that all seed-bearing aquatic plants are 

 descended from land plants, which took to the water through competi- 

 tion with rivals better suited than they to their original stations. 

 This is no place to discuss this question, but it may be pointed out 

 that some plants can live equally well on land and in water, and even 

 do not mind being submerged. The water-starworts (Callitriche) are 

 examples. 



In many streams the native aquatic vegetation has been ousted 

 by the introduced watercress or the American water-weed (Elodea 

 canadensis). It is interesting to note how much the former varies 

 in both leaf and flower when growing on dry ground by the sides of 

 ditches. Both plants are noxious weeds, and it costs the country 

 much money annually to keep open the watercourses where they 

 flourish. 



FRESH-WATER ALGAE. 



Rivers, ponds, lakes, stagnant pools, moist soil, and many other 

 stations are the homes of the fresh-water algae, or the pond-scums, as 

 one section may be popularly called. They very frequently form 

 green, slimy masses on the surface of the water. Common forms 

 consist of what look like very fine, long, green hairs. Under a fairly 

 strong power of the miscroscope these are seen to consist of long 

 tubes, divided by thin walls into compartments, which contain plant- 

 green, sometimes in the form of bands. 



The fresh-water algae are a very large family, and. although 



* M. pedunculatum (in part) of Cheeseman's Flora, p. 152. 



