LAKES AND TARNS 133 



dim for green plants to flourish. On the floor of the 

 lake there are incalculable numbers of very simple 

 plants, such as flinty-shelled Diatoms and curious lime- 

 secreting, blue-green Algae, which often form tor- 

 tuous markings on stones. These simple blue-green 

 Algae (Cyanophyceae) have got in addition to chloro- 

 phyll a blue-green or verdigris-coloured pigment, 

 called " phycocyanin," which perhaps helps them to 

 make the most of the scanty light. Some of them, 

 such as Anabcena flos-aquce, are interesting also be- 

 cause their living matter secretes gas which collects 

 in vacuoles and buoys the plants up to the surface, 

 where they often form large floating masses. There 

 is a kind of Anabaena that enters into a partnership 

 with a little water fern, mainly of tropical distribution, 

 called Azolla. It appears to be invariably present in 

 a cavity in the upper lobe of the bilobed floating leaves 

 of the Azolla, but the meaning of the companionship 

 remains obscure. 



The Water Lily (Nymphcea) is a good example of 

 fitnesses. It is rooted in the mud and perennial; its 

 shoot is buoyant with air spaces and very lightly 

 built; the broad leaves float on the water, presenting 

 a large surface to the light. The little openings, or 

 stomata, which allow of gaseous interchange with the 

 atmosphere and regulate the transpiration of water- 

 vapour, are confined to the upper surface, whereas in 

 ordinary leaves they are mostly or exclusively on the 

 under surface unless the leaf grows more or less 

 vertically. It is plain that stomata on the under 

 surface of a Water Lily's leaf, which is continually 

 washed with the water, would be useless ; and they are 

 absent. An oak-leaf has on each square millimetre of 



