LAKES AND TARNS 139 



hood by capturing small animals, and how when it 

 dies away in autumn it does not wholly die. 



In some parts of the tarn the surface is quite hidden 

 with the glossy floating leaves of one of the Pond- 

 weeds (P o tamo get on natans), which has also thread- 

 like submerged leaves. There are other kinds (e.g., 

 Potamogeton crispus), whose leaves are all under 

 water and only the flowers at the surfaces. All of 

 these are rooted in the mud, and this fact, along with 

 their surface flowers, may serve to remind us that all 

 or almost all the flowering plants that live in water 

 have had a terrestrial origin. Most of them have 

 near relatives on shore. It might be thought that the 

 rootlessness of the Bladderwort was against the idea 

 of terrestrial origin, yet there is a terrestrial species 

 of Bladderwort, and the suppression of roots actually 

 occurs in a few land plants, such as C ' oralliorhiza 

 innata, an orchid that lives among rotting vegetable 

 matter and has a running stem that takes on the 

 root's duties. The rootlessness of many water plants 

 is to be associated with the absorption of water and 

 dissolved salts by the general surface of the plant or 

 by a large part of it. There are no roots in the 

 submerged Hornwort (Ceratophyllum), a relative of 

 the Water Lilies which are rooted and have floating 

 leaves. 



On other parts of the tarn the surface is covered 

 with the Duckweed Lemna. What look like leaves 

 are really flattened shoots, very buoyant and difficult 

 to wet. On the under side there are delicate, white 

 roots which are unattached. As Lemna is a leafless 

 plant, and as its shoot is not submerged, it requires 

 roots for absorption; it thus makes a good contrast 



