322 Aquatic Societies 



feet at most) to the limits of effective light. Within 

 such a range of depth conditions of movement, pressure, 

 warmth and light find also a considerable range; hence, 

 the forms differ at the inner and outer margins of the 

 zone. Its forefront is usually formed by Chara as 

 stated above, and pondweeds follow Chara, with a 

 number of other forms usually commingled, in the 

 shallower part. 



These groups are not free from intergradation since 

 some forms like the spatterdock (fig. 195 on p. 335) are 

 in part emergent, and some of the pondweeds have a 

 few floating leaves. But they are nevertheless con- 

 venient, and they represent real ecological differences. 



Distribution of these plants in depth results in their 

 zonal arrangement about the shore line. When all 

 are present they are arranged in the order indicated. 

 It is an inviolable order ; for the emergent forms cut off 

 the light from those that cannot rise above the surface, 

 and the latter overshadow those that are submerged. 

 The zones may vary in width and in their component 

 species, but when all are present and crowded for room 

 they can occur only in this order. The two accompany- 

 ing figures illustrate zonal arrangement ; figure 189C, on 

 a low and marshy shore; figure 190, on a more elevated 

 shore, backed by a terrestrial flora. 



The algce of littoral societies are those of the plancton 

 (practically all of which drift into the shoals) plus 

 numberless additional non-limnetic forms, many of 

 which are sessile. As with the vascular plants, algas 

 that are fragile (see fig. 198 on p. 338) and the larger 

 that float free (Spirogyra, etc.) develop mainly in pools 

 and quiet waters, w T hile those having great pliancy of 

 body (Cladophora, see fig. 46 on p. 125) and protective 

 covering (slime-coat diatoms, etc.) are more exposed to 

 moving waters. 



