yiiii li/sl.rlli A II II Nil I Mrrfiiif/ 



IVoiii th.-il |i;i|ici', wliicli >li()\\s rii'iii'ly llic |i(i>ili()ii ;iiiil iiiiioiiiil 

 nf tli;s iiniiHirafliircd (i\vi:cii, jiiid tlic iiildilidii wliicli il iii;ik<'- 

 to tlic tliickiK'ss of thai part (if the lake that has alniiidance ol' 

 oxv.iivn. Lakes wliosc hahitahh' poilion would otherwise be only 

 Iwehc lo twenty feel in thickness may liave this depth douhlcd 



if the n)aiuiraclui-('d Dwu'en. The phiiils in this 



iindislni-hed ceoler water lind a peculiarly favorahle situation 

 for .urdwtli. They ohlaiii Ini' their food llie products of deconipo- 

 s'tion. which is taking- place rapidly in the lowei' watei-; and not 

 infriMpiently a far lai'u'ei' amount of oi-_i;anic iih' may he found 

 in these strata than in any othci- p(U'tion of the lake. 'I'his pi'o- 

 e(.'ss is necessarily limited to lake> whose uppei' warm layci' is 

 thin, and is conliinMl to the uppci' part of the cold watei', since 

 oidy thei'e can the l',uhl ha\-e snllicient intensity to caia'y on the 

 operation. lUit even as thus resii-icted. it is of y-i-eat value to some 

 lakes. 



J have said little hitherto of the cai-hon dioxide — a .i^a- 

 wlio-e importance is (piite e(|nal to that of oxyp'n — and now can 

 oidv skt'tch a part of its com])it'X story. This ,a'as iila\s many 

 roles in the vesp'ration of the lake. It is at once the waste pro- 

 duct of the tissue activity of ])laiit and animal, the jiroduct or 

 liy-])roduct of decompo-it ion. and the indispensahle food of li'reei: 

 plants. The lake may ohtain the uas from tlu' air. and to some 

 extent does so. Cai'lion dioxide exists in the atmosphei'e in \-ery 

 small amount — ahout four parts in 10.000. Minute as [hi- 

 (|uantit\- is. the land jjlants are able to secnre from it ample 

 sn|:)}ilies of carhon. The moveinent of the air is so fri'e and such 

 cnoi-mous (piantities pass oxer the surface of the plants, that 

 thev readily pii-k up the pis in lai;i;'e amounts. Uut the situa- 

 tion of the al.uae and other plants of the lake i> very dilferent. 

 as they must secure their carhon d'oxide throuiih the intei-- 

 medinm of the watei'. This readily absorbs lar_i;e (luantities of 

 the ,uas. l)ut the iiercenta.u'e existiuii' in the air is so small, the 

 absorbing:- surface of the lake is so restricted, and the means of 

 transport are so iwor that the lake is (piite unable to take from 

 the air enoiiu'li carbon d'oxide to maintain a xiii'orous ,<:-rowtli of 

 plants. The lake is forced to depend on its own resources to a 

 lai'iic de-Tee b)r this plant food. Fortnnatelx . tlics.' resources 

 are considerable, (ireat amounts of carbon dioxide are inanu- 



