450 THE ECOLOGY OF PLANTS. 



421. Zones of Vegetation in Ponds. In studying the 

 vegetation in ponds and streams, note how the different 

 species tend to be arranged in zones. The arrangement 

 differs considerably in different ponds and streams, each 

 having peculiar features of its own in many cases, so that 

 here (as with other plant societies) I can only indicate the 

 kind of thing to be looked for by the student. 



On the margins we get the Reed Swamp Association, 

 consisting of plants rooted in the mud (usually with creep- 

 ing rhizomes), with their leaves and flowers above the water. 

 The most characteristic and successful "social "or gregarious 

 species are those with long erect leaves, e.g. Iris, Typha, 

 Sparganium, Triglochin, Glyceria aquatica, Digr aphis, Phrag- 

 mites, Rushes (Juncus), and Sedges. The Water Plantain 

 and the Arrowhead have broad leaves, but these are longer 

 than broad and assume a more or less erect position ; how- 

 ever, these two plants are not very successful social species. 

 In this association are also included such plants as the marsh 

 Horsetails, Marestail, Water Dropwort, Water Docks, Purple 

 Loosestrife, etc. 



The Floating-Leaf Association, further from the bank, 

 includes free-floating plants like Duckweed and Frog's-bit, 

 also various rooted plants like Pondweed, Water Crowfoot, 

 Water-lilies, etc. The Submerged-Leaf Association is 

 best developed in fairly deep water, especially when the 

 erect-leaved reed- swamp plants are not so abundant as to 

 prevent enough light from entering the water. The Marsh 

 Association has already been dealt with (Art. 402). 



The following is a description by Dr. Smith of a series of 

 ponds at Piley, Yorkshire : 



" This pond (Fig. 183) was filled with vegetation. At the southern 

 end an inflow is shown, which was bringing in water when we saw it. 

 From the appearance of the channel, it does not always contain water, 

 and only acts as an overflow to some other pond in the same trough. 

 At the northern end there is an outflow depression, which acts as an 

 overflow channel to this pond. The water is probably derived from 

 surface drainage, except in rainy seasons, when there will be some 

 inflow at the southern end, and the pond will attain its maximum 

 depth when the water begins to pass through the overflow 



The plants are arranged in four well-marked zones : (a) a marginal 

 zone of taller species of Rushes (Juncus) ; (6) a zone of Water Dropwort 

 (Oeanthe Jistulosa), mixed with Water Plantain (Alisma Planlago] at 



