82 



GROWTH AND WORK OF PLANTS 



less danger of injury. The work which leaves perform in con- 

 junction with light is very important and the leaves must have a 

 good light exposure. Divided leaves permit the light to shine 

 through to leaves below which otherwise would be too greatly 

 shaded were the large leaves continuous. 



3. DECIDUOUS AND EVERGREEN LEAVES. 



129. Fall of the leaf. Leaves are not permanent outgrowths 

 of the stem as most branches are. Their origin is superficial as 

 compared with the origin of a branch, and they sooner or later 

 fall away from the stem. In many trees and shrubs the leaves 

 formed during the growing season fall at its close. These trees 



and shrubs are said to be 

 deciduous. The stems re- 

 main bare during the resting 

 season which in our climate 

 is the winter season. In the 

 spring new leaves are again 

 formed on the new shoots. 

 Other trees and shrubs hold 

 each season's crop of leaves 

 for several (two to four or 

 more) years, and usually one 

 crop, the oldest, falls away 

 each year. These trees and 

 shrubs are said to be ever- 

 green, because they are holding several crops of green leaves 

 during summer and winter, as in the pines, spruces, firs, 

 balsams, rhododendrons, etc. When the time has come for the 

 leaf to fall, a separation layer is formed at the junction of the 

 petiole with the stem, and the leaf falls away leaving a scar (the 

 leaf scar) on the stem with a smooth surface (Chapter IX). 

 The scars, therefore, enable us to determine the position and 

 arrangement of the leaves of deciduous shrubs and trees during 

 the winter. 



Fig. 72. 

 Shoot of white pine with "needle" leaves. 



