372 ECOLOGY 



surface by a leathery cuticle (Fig. 241). Plants like these, 

 which bear different kinds of leaves, are said to be hetero- 

 phyllous (Gr. heteros = different). Heterophylly is common 

 in water-plants and frequently occurs in land-plants. 



The Frog-bit has floating leaves and much-branched 

 roots hanging in the water, but often not rooted in the soil 

 (Fig. 239, e). The Duckweeds, with their curious leaf -like 

 stems (phylloclades or cladodes) and roots hanging in the 

 water, are entirely floating. In the more open water many 

 plants occur which are rooted and entirely submerged, 

 e. g. Water Milfoil, Canadian Water-weed, Stoneworts, and 

 many Pond-weeds; while plants like the Bladderworts 

 have neither root nor floating leaves, but are suspended 

 unattached in the water. 



Growing among these is a rich flora of minute plants, 

 the Algae, some in the form of a tangle of delicate green 

 threads and others, though consisting each of a single 

 microscopic cell, exhibiting, as in the case of desmids and 

 diatoms, structural details which are both elaborate and 

 beautiful. 



Structural peculiarities of water-plants. Specimens of 

 the different types of water-plants should be obtained, 

 and their varied forms studied with especial reference to 

 their aquatic surroundings, on account of which water- 

 plants form a distinct type of vegetation. 



Cut sections of stems and leaves of the larger species and 

 examine them with a lens. Note that the bulky cortex 

 contains very large air-spaces (Fig. 243), and these occur 

 not only in stems and leaves, but also in the roots. The 

 roots growing amongst decaying organic matter, and in 

 badly-aerated mud, are thus able to obtain a supply of 

 air by diffusion from the shoots above. The vascular 

 bundles of the stem, as in roots (Fig. 15), are often 

 concentrated in the centre, which is a good position for 

 resisting the longitudinal strain of running water. The 



