. A text-book of botany for secondary schools. Botany. LEAVES 33 in some deciduous leaves, as those of oaks, there is no special preparation for falling, in most of them a special plate of cells is formed at or near the juncture of the leaf with the stem, known as the cutting-off layer, which gradu- ally loosens the leaf from the stem, so that it falls by its own weight or is wrenched off by the wind (Fig. 29). In connection with the deciduous habit there often appears the autumn coloration of leaves, so striking a feat- ure of temperate forests. The colors that appear are shades of yellow and
. A text-book of botany for secondary schools. Botany. LEAVES 33 in some deciduous leaves, as those of oaks, there is no special preparation for falling, in most of them a special plate of cells is formed at or near the juncture of the leaf with the stem, known as the cutting-off layer, which gradu- ally loosens the leaf from the stem, so that it falls by its own weight or is wrenched off by the wind (Fig. 29). In connection with the deciduous habit there often appears the autumn coloration of leaves, so striking a feat- ure of temperate forests. The colors that appear are shades of yellow and red, either pure or vari- ously intermixed. They are the result of the wan- ing activity of the leaf, the yellow mostly being the color of the dying chloroplast, and the red coming from the pres- ence of a new substance manufactured in the en- feebled cells. The pop- ular belief that these colors are caused by frost is only partly true, for they often appear before any frost; but they may be induced by any conditions that tend to diminish the activity of the leaf, and cold is one of the con- spicuous conditions. 19. Leaves of evergreens.—In contrast with the decidu- ous shrubs and trees are the so-called evergreens, in which. Fig. 30.—The needle-leaves of a 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 Coulter, John Merle, 1851-1928. New York, D. Appleton
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectbotany, bookyear1906
