LEAVES, THEIR FORM AND MOVEMENT 



75 



3. FALL OF THE LEAF. 



124. 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 remain 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 of cells is formed at the 

 junction of the petiole with 

 the stem, and the leaf falls 

 away leaving a scar (the leaf Flg> 69 ' . 



snoot of white pine with needle leaves. 



scar) on the stem with a 



smooth surface (Chapter VIII). The scars, therefore, enable us 

 to determine the position and arrangement of the leaves of decid- 

 uous shrubs and trees during the winter. 



4. ARRANGEMENT OF LEAVES. 



125. The arrangement of leaves on the stem seems to follow 

 certain definite laws, and, barring accidents, is always the same 

 for a given species. 



126. Opposite leaves. Leaves are opposite on the stem 

 when two arise at the same level, or node, but on opposite sides. 

 The milkweed (Asdepias) is a good example, but there are many 



