452 THE ECOLOGY OP PLANTS. 



extending across the pond. The Dropwort, however, is always nearer 

 the shore than the Bur-reed. It may, therefore, be inferred that the 

 Bur-reed prefers deeper water than the Dropwort. 



The Water Plantain does not form masses like the other plants, 

 but occurs singly amongst the Dropwort and the Bur-reed ; it is not a 

 social species, whereas the other plants shown are social. It is a 

 feature of social species of plants that they can grow closely together 

 as a mass, and hold their own more or less completely against all 

 comers. One can appreciate this in the case of the Pondweed, with 

 its broad floating leaves lying edge to edge, or overlapping, so as to 

 occupy the whole surface and shade the bottom, thus checking the 

 growth of other plants which may try to grow below them. Amongst 

 trees, the Beech is one of the best examples of this kind of social plant. 



The Dropwort, Bur-reed, and Rushes are also social plants, and it 

 is noteworthy that their long, slender, erect leaves are similar in form 

 to the Grasses which on land are amongst the most successful of our 

 social plants. Just as the Grasses, with their closely interwoven 

 roots and stems underground, and their close, erect, leafy shoots 

 above ground, can keep in check the growth of other plants and 

 maintain a close sward, so in our pond the Dropwort, Bur-reed, and 

 Rushes maintain themselves in distinct zones with little mixture. 

 The Dropwort is not quite so successful as the other two, and amongst 

 it plants of Water Plantain have found a place and have also succeeded 

 in gaining ground in the closer Bur-reed zone. 



Turning now to the Rushes on the margin. The Rushes form a 

 close well-marked zone, which at the time of our visit was standing in 

 water and therefore formed part of the pond-vegetation. Out of the 

 water there was, however, no scarcity of Rushes, and from general 

 experience most of us would regard the Rushes as plants of the marsh 

 rather than water-plants. The Rush zone of this pond therefore 

 indicates that part of the margin which is only occasionally under 

 water, and is generally marsh. 



From this one may infer that this pond, as a rule, contains less 

 water than when we saw it. It has a maximum depth which is 

 determined by the sill of the outflow channel. The ebb extends at 

 least to the lower margin of the Rush zone, but the Dropwort, Water 

 Plantain, and Bur-reed are all plants which may grow in a wet marsh, 

 and it is probable that this is the drought condition of that part of the 

 pond occupied by these plants. From the appearance of the Pond- 

 weed we should say that its area is never quite free from standing water. 



Fig. 184 represents what has been a pond, but is now so dry that 

 it might be described as a dry marsh. The shape of the hollow is like 

 that of the true ponds, and there is a distinct outflow channel. There 

 is also an inflow channel of the kind seen in Fig. 183, namely, an 

 occasional channel which drains the overflow from some adjacent hollow. 



Near the centre are patches of two water-plants Pondweed and 

 Bur-reed restricted to what are now the wettest spots. The Pond- 

 weed has small reddish leaves with short stalks, the Bur-reed has 

 short narrow leaves ; both are land-adaptations and are probably 

 remains of the original aquatic vegetation which has been displaced. 



