3o6 



Walker : Pond Vegetation. 



Flowering Plants. 



The principal factors which determine the distribution of the 

 flowering- plants are the character of the bottom of the pond and 

 the depth. 



The vegetation is richest in the shallower ponds with gently 

 sloping muddy bottoms. Fig. i shows the distribution of the 

 flowering plants in a pond of this type. The slope of the bottom 

 is about one inch per foot and the depth of the mud about 

 eight inches. The outermost zone of vegetation consists 

 principally of Juncns communis Meyer ; this extends inwards 

 to a depth of six inches, but the plant thrives best upon the 

 extreme edge of the pond, especially upon the steeper banks, 

 where the rigid rhizome lies just below the surface of the soil.'''' 

 A transverse section through the rhizome (Plate XIX., Fig. i) 

 shows a large cortex, consisting principally of a peripheral 

 close parenchyma ; within this is a narrow zone of aerating- 

 tissuet, containing irregular, radially-arranged air spaces. 

 The central region is occupied by a large vascular cylinder, 

 including numerous concentric vascular bundles embedded in 

 a highly sclerenchymatous ground tissue. The feeble develop- 

 ment of aerating tissue in the rhizome seems to be correlated 

 w^ith its existence near the surface where oxygen would be more 

 plentiful than in the putrefying mud at the bottom of the pond ; 

 and its riofiditv would allow of extension throus'h the stiff soil. 

 The roots are long and numerous, and, like the rhizome, 

 contain a considerable amount of sclerenchyma. The cortex 

 contains large radiating air passages (Plate XIX., Fig. 2) due 

 to the collapse of many of the cortical cells ; these allow of 

 gaseous communication between the stem and the more deeply 

 embedded parts of the plant. 



Growing between the rushes at the edge of the pond are 

 a few other moisture-loving plants. The commonest are Galium 

 palustre L., Myosotis lingulata Lehm., Ranunculus Flammula L., 

 and Senecio aquaticus Huds. Towards the middle of the rush 

 zone and at a depth of three inches, Water Dropwort [CEiiauthe 

 fistulosa L.) beg^ins. The slender stolons of this plant burrow 

 in the soft mud and stop short when the stiffer soil is 



It is here only intended to touch upon the more obvious relations 

 between the structure and environment of the flowering- plants. It is h.opetl 

 later to make dominant plants of each zone the subjects of separate papers. 



t This term is used to desig-nate the pronounced air-containing- tissue, 

 characteristic of water plants. 



Niitiiralist, 



