. Plant life and plant uses; an elementary textbook, a foundation for the study of agriculture, domestic science or college botany. Botany. PROTECTION 245 nection with this subject. You are familiar with trees like the pine, whose needle-like leaves stay on the year round, and you may be wondering how they avoid injury in winter. One glance at a pine needle does much to answer this question. Both by shape and structure it is evidently far better protected than wide, thin leaves are protected. It is in no such danger from loss of water or from injury by the freezing of the water in it. Instead
. Plant life and plant uses; an elementary textbook, a foundation for the study of agriculture, domestic science or college botany. Botany. PROTECTION 245 nection with this subject. You are familiar with trees like the pine, whose needle-like leaves stay on the year round, and you may be wondering how they avoid injury in winter. One glance at a pine needle does much to answer this question. Both by shape and structure it is evidently far better protected than wide, thin leaves are protected. It is in no such danger from loss of water or from injury by the freezing of the water in it. Instead of being a disadvantage, leaves of this kind are a positive advantage to plants in winter. On many days it is warm enough and bright enough for photosynthesis, and on such days the needle-like leaves are at work. (It is to be noted that the roots of the plants which bear such leaves usually penetrate below the frozen zone of the soil.) In summer, however, such leaves are a disadvantage as compared with broad, thin leaves, for the latter have much greater exposure to light and much greater chlorophyll content. Most plants which lose their leaves in the fall, as the time to shed them approaches, form a special layer of cells at the base of the petiole. By means of this layer the leaves are shed by a sort of self-amputation. This special layer is called the absciss layer. (The word absciss means cutting-of.) It loosens the leaf from the stem, and forms a place at which any gentle breeze or even its own weight is likely to detach the leaf. (See Figure 85.) After the leaf falls, the wound which might be expected is often. Fig. 85. — Diagram il- lustrating prepara- tion for leaf fall. A section through the base of the petiole of a mature leaf; a indicates the absciss layer. This layer runs through the cortex, indicated by c. The leaf is held till the last by its vascular strands, ».. Please note that these images are extracted from scanned page images that may have been digitally
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1913
