AQUATIC VEGETATION. 625 



strand flora by different plants, according to the soil conditions, 

 locality, and other conditions of environment or factors of dis- 

 tribution (herbs, grasses, etc.). The second zone, mid zone, is 

 characterized for the most part by plants with floating leaves 

 and slender petioles or stems, like the water-lilies, the floating 

 potamogeton (P. natans), the water-fern (Marsilea). It is 

 interesting to note from year to year, as the water may be at 

 different depths, that the perennial plants fixed to the bottom 

 have short or long petioles according as the water is shallow or 

 deep. Some plants in this zone are entirely immersed, like 

 the quill wort. The latter sometimes in small ponds makes 

 pure formations. In fact any of the zones may be pure or 

 mixed, one or several fades making a formation, as in the case of 

 land plants. The third is a submerged zone and has plants of the 

 -true pondweed type. These occupy deeper water, and while 

 nearly always submerged, their leaves are brought near the surface 

 for light by the elongation of their stems. These should be 

 contrasted with such a plant as the quillwort, which does not 

 have this power of adaptation. Some of the pondweeds grow 

 where the water is up to 6m (20 ft.) or more deep. 



1114a. The three zones mentioned above are to be taken in 

 a broad sense, and relate to: ist, littoral zone, the semi-aquatics; 

 2d, mid zone, the forms with floating leaves; and 3d, submerged 

 zone, the completely submerged plants. In regular bowl- 

 shaped ponds or lakes there might be, and usually are, several 

 plant formations arranged zonally within each of the zones 

 mentioned above. To take, for example, the formations illus- 

 trated in fig. 534. In the littoral zone of semi-aquatics there 

 are three zonal formations: typha (i), the bulrushes (2), and 

 arrow-leaf (3). In the mid zone (adjacent to this) the yellow 

 water-lily, and potamogetons with floating leaves (4). In the 

 submerged zone are two zonal formations, pond-lilies (5), and 

 bass-weed = Chara (6). According to Magnin, in the small 

 lakes of Jura there are usually six zones, and these correspond 

 as follows to the three zones mentioned above: ist, littoral zone 

 of semi-aquatics, zone of sedges (i), zone of reed-grasses (2), 



