VEGETATION OF STILL WATER 27 



summer. Patches of the Common Reed border the banks, which 

 may be almost covered with the glossy yellow blossoms of the 

 Celandine and the pale lilac of the Cuckoo-flower. Even as early 

 as the middle of April, after a severe winter, the Water Crowfoot 

 may be in bloom, its white flowers above the water making it 

 conspicuous. This plant has two sets of leaves : those in the 

 water are very much cut up, presenting a large area to the water ; 

 those above the water are not cut up, but float flat and dry on 

 the surface, enabling the plant to hold its flower-stems well up 

 out of the water. After flowering, the flower-stalk curves down- 

 wards, and the fruit ripens beneath the water. There are several 

 varieties of this species : some are found in running streams, 

 others in deep still waters ; it 

 varies very much with the situa- 

 tion in which it is growing. 



In summer most ponds have 

 some species of Potamogeton or 

 Pondweed. There are a large 

 number of these Pondweeds, which 

 mostly resemble each other in 

 having a root-stock that creeps in 

 the mud, very narrow leaves in 

 the water, and usually much FIG. 8.~Stem of Potamog^ (transveisc 



broader leaves that float On the section). *, Epidermis ; c, cortical 



surface. The chief variation is in ^^ with air-spaces ; /, conducting 

 the leaves : the majority of species 



have both submerged and floating leaves ; in others the broad 

 leaves are borne under the water, and occasionally the floating 

 leaves are not broad, but narrow. A comparison of the 

 plants found in still water undoubtedly shows that their habit 

 is to float ; not only the leaves, but the stems can float. 

 A transverse section of a Potamogeton (Fig. 8) shows the 

 structure of a stem adapted for floating. The cortex has very 

 large intercellular spaces, filled with air ; these make the 

 plant very light, enabling it to float, and they also serve to 

 convey the air to the lower parts of the stem growing in the 

 deeper water, or creeping in the mud. The woody tissues 

 which support land plants are in these water plants very much 



