. Lectures on the evolution of plants. Botany; Plants. 200 EVOLUTION OF PLANTS groups of Angiosperms, although at present no such forms are certainly known. The Dicotyledons exhibit great variety in the form of the stem and leaves, and this is correlated with a much more perfect development of the tissues than is found elsewhere in the vegetable kingdom. This is shown especially in the highly de- veloped vascular bundles, which in the stems of the woody forms show a second- ary thickening like that in the coniferous stem, but the tissues of the bundle are much more specialized than in the latt
. Lectures on the evolution of plants. Botany; Plants. 200 EVOLUTION OF PLANTS groups of Angiosperms, although at present no such forms are certainly known. The Dicotyledons exhibit great variety in the form of the stem and leaves, and this is correlated with a much more perfect development of the tissues than is found elsewhere in the vegetable kingdom. This is shown especially in the highly de- veloped vascular bundles, which in the stems of the woody forms show a second- ary thickening like that in the coniferous stem, but the tissues of the bundle are much more specialized than in the latter. From the continued growth of the cambium or active tissue in the bundles of the stem, an- nual growth-rings result, and soon the greater part of the stem is made up of the sec- ondary wood derived from the activity of the cambium. This results in the develop- ment of the massive woody stems characteristic of dico- tyledonous shrubs and trees. In this secondary thickening of the stems and roots, the Dicotyledons differ from the Monocotyledons and ap-. FiQ. 48. — A, a seedling of the castor-bean (Rieinus), showing the difference in appearance between the two cotyledons, cot, and the second leaves, I; r, the main or tap-root, a contin- uation of the stem; B, cross- section of the stem, showing the arrangement of the tissues; vb, the vascular bundles; C, section of the seed of the shepberd'spurse (Capsella),the embryo occupying the whole seed-cavity; cot, cotyledons; St, stem; D, section of the seed of blood-root (Sanguinaria), showing the small embryo, 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 Campbell, Douglas Houghton, 1859-1953. New York, London, The Macmillan company
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Keywords: ., bookcentury1900, bookdecade, booksubjectbotany, booksubjectplants
