Leaves. 97 



they become torn away by their own weight, or are easily 

 blown off by the wind. The separating layer of tissue is 

 often of the nature of cork and then serves also to heal 

 the wound. In evergreens, however, the leaves may re- 

 main on the branches for several years. Figure 41 shows 

 a branch of pine bearing 

 leaves some of which are ' 

 three years old. 



The fall of the leaf is a 

 wise provision for the con- i 

 ditions of winter. When 

 the ground is very cold or 

 frozen, the roots are no ' 



longer able to absorb 



FIG. 41. 

 water from the soil, and 



...... . . Branch of Pine tree bearing leaves three years 



it the broad transpiring old . There are gaps between the leaves of 



surfaces of the leaves re- each y ear>s s rowth where the bud scales 



were 



mained, the plant would 



suffer from too great loss of water ; the weight of the snow 

 also which would accumulate on the leaves would break 

 the branches, as may sometimes be observed when early 

 snows overtake the trees with their leaves still on. 



76. Size and Form of Leaves. Leaves show great varia- 

 tion in size and form. The leaves of mosses, for instance, 

 are only a few millimeters in length and breadth, while 

 those of the palm Raphia tcedigera, growing in Brazil, 

 have petioles from four to five meters long and leaf-blades 

 from nineteen to twenty-two meters long and twelve meters 

 broad. The student can at any time during the growing 

 season find endless materials for the study of variations in 

 leaf forms. We should not, however, look at the differ- 

 ences in leaf forms as expressions merely of the power to 

 vary, for we may find that the form of the leaf is nicely 



