28 THE BOOK OF NATURE STUDY 



reduced, and are placed in the centre not midway, as in land 

 plants for the weight of the plant as it floats is supported by 

 the water, and the strain in still water is very slight. The structure 

 of the leaf is also adapted to the floating habit. Leaves of this 

 kind are usually round, or entire, or very slightly lobed ; they 

 bear stomates on their upper surface, which is usually very glossy 

 owing to the wax that covers it and prevents it getting wet. 

 Kerner gives 11,500,000 stomates for the average-sized leaf of the 

 Water Lily. 



The water of ponds, is, as a rule, shallow enough to be pene- 

 trated by the rays of light throughout its whole extent ; in fresh- 

 water lakes the depth may be considerable, and in these, three 

 regions of illumination are distinguished : 



1. The bright region, in which the intensity of light is sufficient 

 for the development of flowering plants. 



2. The dim region, where most flowering plants can hardly 

 exist. Below a depth of two yards the green alga Chara forms an 

 important part of the vegetation ; below seven or eight yards 

 Nitella takes the place of Chara, and a few Mosses may be found 

 with it. 



3. The dark region, where vegetation is very scanty indeed, 

 and is practically confined to plants without green leaves. 



It must be remembered that the depth to which light pene- 

 trates water depends on the clearness of the water, and that varies 

 with locality and season. In Lake Geneva, in September, a 

 photographic plate became slightly dark at a depth of 170 metres, 

 and very dark at 120 ; whilst in April, even at a depth of 250, 

 light was not altogether absent. 



The following regular succession of plants has been observed 

 off Scotch lochs : 



(a) Marsh plants, such as Sedges and the Lesser Spearwort, 

 mix with the Meadow Clovers and Grasses, where the soil is 

 water-logged. 



(b) A narrow belt of Rushes. 



(c) Iris or Sweet Flag, where the water is less than eighteen inches. 



(d) Reeds, such as Scirpus, from two to four feet. 



(e) Water Lilies, the rhizomes of which may be at a depth of 

 nine feet. 



