. Introduction to botany. Botany. Fig. 41. 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- ditions of winter. When the ground is very cold or frozen, the roots are no longer able to absorb water from the soil, and .J- ,1 V J 4. • • Bra
. Introduction to botany. Botany. Fig. 41. 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- ditions of winter. When the ground is very cold or frozen, the roots are no longer able to absorb water from the soil, and .J- ,1 V J 4. • • Branch of Pine tree bearing leaves three years It the broad transpirmg o]d. There are gaps between the leaves of surfaces of the leaves re- ^^'^^ y<^^^'^ growth where the bud scales 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 milhmeters 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Stevens, William Chase, 1861-. Boston, D. C. Heath & Co.
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