. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. 142 CALLUS covering the leaf-scar is interrupted only where the vascular bundles are broken across, and here the vessels are usually plugged by tjdoses. The detachment of flowers and the pheno- menon of self-pruning, by which small branches of certain trees [ Poplar) are regularly shed, is due to a similar development of a definite abscission-layer. When a branch is artificially re- moved, the living cells, and especially those of the exposed cambium, give rise to a large-celled, thin-walled tissue which
. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. 142 CALLUS covering the leaf-scar is interrupted only where the vascular bundles are broken across, and here the vessels are usually plugged by tjdoses. The detachment of flowers and the pheno- menon of self-pruning, by which small branches of certain trees [ Poplar) are regularly shed, is due to a similar development of a definite abscission-layer. When a branch is artificially re- moved, the living cells, and especially those of the exposed cambium, give rise to a large-celled, thin-walled tissue which at first forms a ring, but often gradually becomes broader till the whole wounded surface is completely covered, the outermost cells usually becoming suberised. The tissue thus formed is called a callus, and may subsequently produce adventitious shoots { pollarded trees) or adven- titious roots { cuttings). Regions of injury in the parenchymatous tissues { of cortex or leaf) are commonly isolated from the healthy surrounding tissue by the formation of cork from a cambium produced by tangential divisions in the uninjured cells nearest the wound. In this way the attack of a parasitic Fungus maj' often be localised. The cut surfaces of scion and stock, brought into contact in the process of grafting, become intimately associated owing to fusion of the intact cells, which is usually accompanied by some cell-division. Renewed activity in the tissues of a plant may likewise result from wounding by various organisms, either vegetable or animal. The local enlargements produced are termed galls (Fig. 70), and in their formation the, probably chemically, stimulated cells may either enlarge or undergo division. Little or no tissue differentiation exists in galls caused by. Fig. 69. — Diagrammatic longitudinal section through part of a node of the Sycamore (Acer pseitdoplatanus), show- ing the abscission layer (S.). , axillary bud ; p., petiole; I'., vascular Pl
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