. A textbook of botany for colleges and universities ... Botany. 684 ECOLOGY. Secondary conductive tissues. — The cambium ring. — Soon after the formation of the primary vascular tissues, renewed growth takes place, especially in conifers and dicotyls; this results in the formation of secondary tissues, the active element in their formation being the cam- bium, a layer of cells that retain a capacity for active growth and hence known as meristematic (fig. loii). In addition to the fascicular cam- bium, representing the meristematic progeny of the procambium from which the primary xylem and phl
. A textbook of botany for colleges and universities ... Botany. 684 ECOLOGY. Secondary conductive tissues. — The cambium ring. — Soon after the formation of the primary vascular tissues, renewed growth takes place, especially in conifers and dicotyls; this results in the formation of secondary tissues, the active element in their formation being the cam- bium, a layer of cells that retain a capacity for active growth and hence known as meristematic (fig. loii). In addition to the fascicular cam- bium, representing the meristematic progeny of the procambium from which the primary xylem and phloem were developed, there is an interfascicular cambium which develops in the rays between the primary bundles. The most active division of the cam- bium cells is tangential, the new cells arising inward developing into secondary xylem, and those arising outward into secondary phloem. Radial division also takes place, resulting in the merging of the fascicular and interfascicular cambium into the cambium ring, whose circum- ference is subject to constant enlargement, owing to the outward growth of the secondary xylem cells, and to the tangential growth of the ring itself. This circumferential enlargement is responsible for the continued rupture and of the bark. Mature secondary tissues. — Secondary xylem remains as permanent tissue in shrubs and trees, thus bringing about an annual increase in diameter; secondary phloem is relatively ephemeral, being subject to yearly renewal within and exfoliation without. An old tree has thicker bark than a young tree, partly because each year the new secondary phloem a little more than offsets the amount exfoliated, and partly be- cause of the activity of the phellogen (p. 705). As in primary tissues, the phloem is made up of sieve tubes, companion cells, parenchyma, and mechanical elements (mostly bast fibers), while secondary xylem con- sists of tracheids or tracheae, parenchyma, and characteristic mechanical elements kn
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