124 The Leaf 



where there is a good deal of salt, all the water they 

 suck up contains salt dissolved in it. When the water 

 vapour passes out through the leaves the salt remains 

 behind, hence such plants must be able to get on with 

 as little water as possible. That is why most grasses 

 and reeds found by the sea have their long leaves 

 closely rolled up. When the air is moist so that there 

 is no danger of evaporation going on too quickly, some 

 of them unroll themselves, and curl up again in dry 

 weather. 



Many desert plants live in a climate where, not only 

 is the air dry, but during a portion of the year there is 

 no moisture at all to be got from the ground. Such 

 a plant must do one of two things : it can die down to 

 the root during the drought, doing all its rapid growing 

 in the rainy season ; or it can live through the dry 

 weather on a store of water which it put on one side 

 when there was plenty to be had. The leaves or some- 

 times the stems of such plants become thick and fleshy 

 and act as storehouses. This is seen in members of the 

 cactus family, and to a smaller extent in some of our 

 stone-crops which are accustomed to live on rocky 

 ground where the water supply is not to be depended 

 upon. 



In our country there is plenty of water to be had as 

 a rule, but the length and coldness of our winters makes 

 it impossible for plants to go on actively growing 

 through it, hence most of our British trees cast their 

 leaves before the winter comes, and do not begin to 

 sprout again until the warmer weather. Those leaves 

 that are evergreen are furnished with tough leathery 

 coats which prevent evaporation from going on quickly 

 and protect the inside of the leaf from cold winds. 



