LAKES AND THEIR FORMATION 



599 



from the banks towards deeper water. 

 The ad\-ance guard consists of such forms 

 as MynophylhiDi (the Water ^Milfoil). 

 Potamogetons (Ponchveeds), Hippiins 

 (Mare's Tail), and Chara, whose parts are 

 \\-h()lly or in large measure submerged 

 beneath the surface of the water. Then 

 come the plants with floating leaves such 

 as Nymphcca (Water Lily), and the broad 

 Potamos.eton. Next we find the Reed 

 zone, whose roots, and portions of their 

 stems, are submerged, while the main 

 portion of their vegetative parts is above 

 the surface. They flourish where the 

 depth of the water does not exceed four 

 feet, and include such forms as Phragmites 

 (the Common Reed). Scirpits lacustris (Bul- 

 rush), and Typhas (Reed Maces). Behind 

 these, in still shallower water, is a dense 

 growth of Iris, Juncus, Gallium (Bedstraw), 

 Orchids, Water Forget-me-not, Spiraea and 

 Meadow-sweet. On the verge of the 

 dry land we meet with a grassy zone 



habitat nearer and nearer the middle. 

 Eventually the whole basin may be filled, 

 and its former site is marked only by a 

 peaty soil. 



Thus we come to see that lakes are only 

 evanescent features, and none can claim 

 an}^ great antiquity. Their presence in 

 the landscape is a sure sign that the 

 drainage system of which they form a 

 part is young and immature. They are 

 but incidents in the hfe of a river. Similar 

 conclusions as to their age are reached 

 when we consider the distribution of lakes 

 of this class. In Great Britain they are 

 only found in areas which were over- 

 ridden by glaciers in the great Ice Age. 

 The same associations hold good in North 

 Germany, Scandinavia, Russia, and North 

 America. We know that during the ice 

 period the ordinary drainage of the coun- 

 tries affected was interrupted and modi- 

 fied. When the ice melted away many 

 rivers found their old beds filled up, and 



\\-ith Carex and Scirpiis. the King Cup and they were forced to chscharge their waters 

 Cuckoo flower, and the rearguard consists along new channels. We cannot count 

 of Alders, Willows, and other trees, whose in years the time which has elapsed since 

 roots strike deep into the waterlogged the Ice Age, but we know that it is one 

 soil. Floating plants such as Duck- of the most recent events in the Earth's 

 weed. Bladder- wort, and the Water history, and the rivers in developing new 

 Violet may ex- 

 tend into the 

 interior beyond 

 even the sub- 

 merged zones. It 

 does not follow 

 that in aU waters 

 the same assem- 

 blage of plants 

 wiU be found, 

 but the same 

 physi ological 

 zones are met 

 with, the species 

 occurring at any 

 one place being 

 dependent o n 

 local conditions. 

 Year by year 

 dead leaves and 

 other vegetable 

 products sink to 

 the bottom, and 

 gradually raising 



me nOOr, tne r'toto-tafll l'yVo:iitii:son,J.tve:/^\u. 



marginal forms "one of these fans has crept far into the lake." 



find their proper Tai-y-Liyn. 



