154 PLANT LIFE. 



stipules^ wing-like developments at the base of the leaf, 

 or the leaf stalk. 



The life-history of a leaf is somewhat as follows : 

 When the buds swell and develop in the springtime 

 the young leaf begins to expand ; the scales separate, 

 and the stem internodes develop slightly. After grow- 

 ing for some time in this erect position, sideways to the 

 sun, it bends over, and gradually takqs its place at the 

 best available angle to the light, and -begins to manu- 

 facture sugar for the good of the tree. It is necessary 

 to remember that it is by no means always in the same 

 position. A great many leaves alter their position, or 

 go to sleep, at night. Sometimes .this "sleeping" 

 position is very different from that taken up by the 

 leaf during the day. When asleep, the leaf gener- 

 ally takes a more or less vertical position. Sometimes, 

 as in the Wood-sorrel, the leaflets droop or hang from 

 the stalk ; at others, as in the Clover, instead of falling 

 they rise so as to stand more or less vertically upright. 

 In some Solanaceae and Malvaceae the sleeping leaves 

 are brought much nearer the stem, as can be seen in 

 the tobacco plant often grown in gardens. The reason 

 usually given for these curious movements, is that heat 

 is not so rapidly radiated from the leaves in this more 

 vertical position ; but we cannot say that this explana- 

 tion has been proved to be correct. Under special 

 conditions of the weather, also, leaves take up different 

 positions. Thus, in severe frost the leaves of the 

 Portugal Laurel hang almost vertically in a very limp 

 and benumbed-looking manner. In very hot and dry 

 weather, the leaves of Rhododendrons droop in a way 

 which might lead one to suppose that the plant was 

 utterly exhausted, and nearly dead. The Wood-sorrel 

 may even be said to take a siesta, for in very hot 



