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Foliar Venation in Angiosperms from an Ontogenetic Standpoint
Adriance S. Foster
American Journal of Botany
Vol. 39, No. 10 (Dec., 1952), pp. 752-766
Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2438624
Page Count: 15
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A critical review of the literature concerning the ontogenetic aspects of foliar venation is presented first, with particular reference to the observations and theories of Prantl, Deinega, Goebel, Schuster and Troll. A common feature of all the literature surveyed is the absence of information regarding the histogenesis of the procambial network in the leaves of angiosperms. Most of the speculations and assertions are concerned with an attempt to correlate the course of the main veins with the "distribution of growth" in the young lamina. The system of finer veins and veinlets which usually imparts the most distinctive features to the venation has received much less attention; Schuster interprets the meshes of the network as "areas of minimal perimeter," while Goebel contends that the smallest areoles represent the partitioning of the free intervening spaces into approximately equal parts. The second portion of the paper describes in detail the origin and development of the procambial network and interveinal parenchyma in the lamina of the pinnatifid juvenile leaves of Quiina pteridophylla. The procambium originates and develops from the third layer (from the adaxial surface) of the embryonic lamina. A series of paradermal sections reveal the salient features in the histogenesis of this procambium-forming layer. At the earliest stage, the layer appears cytologically "undifferentiated" and consists of groups of 2-5 cells, each of which has arisen by the subdivision of a mother cell. The walls of these cell groups are oriented prevailingly at right angles to the course of the adjacent secondary veins. In intercostal areas, 200-250μ in width, continuous strands of cells become demarcated by the greater density and deeper staining of the cytoplasm and prove to be the precursors of the tertiary and intermediate veins. From the surrounding groups of cells, by means of appropriate but highly variable planes of division, branches of these veins and anastomoses between them and other veinlets are produced. A distinctive feature of Q. pteridophylla is the relatively early, sharp demarcation between the procambial strands and the interveinal ground meristem. The latter consists at first of narrow strips of dividing cells which closely conform in growth to the longitudinal extension of the adjacent procambial strands. Later, as a result of further division, enlargement and the production of chloroplasts, the interveinal meristem develops into areas of parenchyma tissue. Concommitantly there is a continued increase in the thickness of the procambial strands, the appearance of tracheary elements and the final development of external strands of fibers. The paper concludes with a brief contrast between the organization of the procambial net in Liriodendron tulipifera and Q. pteridophylla. The differences observed indicate the existence of widely divergent patterns of histogenesis and emphasize the need for a combined intensive and extensive attack upon the problem of foliar venation.
American Journal of Botany © 1952 Botanical Society of America, Inc.