DAVIS ON FLORA OF LAKES. 27 



The true aquatics or hydrophytes, are those phiiits which grow wholly 

 submerged or with but a small portion of the growing apex of the stem 

 together with the inflorescence, emersed. These plants are usually but 

 lightly rooted, their stems and leaves are filled with large air sjcucs and 

 in the exogens, the leaves are frequently much dissected into long filamen- 

 tous lobes. In the endogens. on the other hand, the submerged leaves are 

 commonly entire and frequently have broad blades. The line separating 

 these two divisions is not a very sharp one as many of the species which 

 are commonly pure aquatics will frequently survive for long periods grow- 

 ing on muddy or sandy banks from which the water has receded, and the 

 amount of adaptation to the changed conditions which some species will 

 show in these circumstances is remarkable and suggestive. The aquatic 

 plants also have the ability to survive for a considerable period floating 

 freely in the water and undoubtedly this power is of material aid to 

 them in assisting in their distribution in a given body of water. 



In discussing the flora of Lake St. Clair in Bulletin Xo. 2 of the Michi- 

 gan Fish Commission. Mr. Pieters has adopted the terminology of Magnin, 

 whose work on the lakes of the Jura demonstrated the existence of a 

 ■series of zones in the littoral and aquatic plants of that region. These 

 zones, which Mr. Pieters found more or less well marked in Lake St. Clair, 

 are liiuited by the depth of the water in which they lie and there are 

 i-ertain dominating genera of plants characterizing each zone. The 

 zones of Magnin are four in number; 1st. A littoral zone subdivided into 

 I'lii'agmitetum and Scirpetum, the former extending to a depth of 2-2| 

 meters, the latter to 8 meters. 2d. The Nupharetum, from 3 to 5 meters. 

 3d. The Potamogetonetum usually extending to (i or 7 meters ; and below 

 8 meters, -Ith., the Characetum. In Lake St. Clair, the prominent 

 plants of the first zone are Phragmites communis Trin., Typha latifolia, 

 L., Acorns Calamus L., and several others. Two species were character- 

 istic of the Scirpetum. Scirpus pungens ^'ahl., and aS'. Lacustris L., the 

 latter growing in the deeper water. The 2d. zone, the Xupharetum was 

 wanting, Nupliar advena belonging to the Phragmitetum, but the third, 

 the Potamogetonetum, characterized by the true aquatics, was well de- 

 fined, extending into water from 3 to 7 meters deep. The chief plants were 

 various species of Potamogeton, the most common being P. perfoliatus L., 

 which, together with ValUsneria spiralis L. was abundant. Beyond this 

 zone and covering the whole bottom of the lakes, so far as studied, the 

 Characetum was found in which various species of Characeae formed the 

 prevailing vegetation. These plants were found most abundant on clay 

 and alluvial bottoms, much less so on sands. Mr. I'ieters also points out 

 that these zones were not well defined in shallow parts of the lake and 

 where the bottom sloped very gradually. 



Professor Reighard in Bulletin No. 4 of the Michigan Fish Commission 

 mentions three factors which may influence the abundance of plant life 

 in a lake: 1st. The nmount of plant food which the water contains: 

 2. The amount of shallow water in the lake; 3d. The transparency of the 

 water. I would add a fourth, as decidedly influencing the number of 

 macroscopic plants, namely, the character of the bottom near the shore, 

 sand being very nearly if not quite barren, while alluvial and clay deposits 

 are usually richly inhabited. The plants of all of Magnin's zones are 

 more or less influenced by these conditions, especially, by the 2d and 



