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 

 e.xfoliation 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 known as wood fibers ; all gradations exist between these fibers 

 and ordinary tracheids or tracheae. Rays of parenchymatic cells, the 

 medullary rays, traverse the secondary tissues radially (fig. 1012). In 

 woody monocotyls and lianas secondary growth diverges considerably 



Fig. ion. — A cross 

 section of a vascular bundle 

 in which secondary thick- 

 ening is in progress; p, 

 phloem; c, cambium from 

 which secondary phloem 

 and xylem are forming; jc, 

 xylem, composed of prim- 

 ary xylem (^i) and second- 

 ary xylem, {pc^i ; highly 

 magni^ed. — From Barnes 

 (Part II). 



