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The Development of the Shoot Apex and of the Primary Thickening Meristem in Phoenix canariensis Chaub., with Comparisons to Washingtonia filifera Wats. And Trachycarpus excelsa Wendl.
American Journal of Botany
Vol. 28, No. 9 (Nov., 1941), pp. 820-832
Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2436668
Page Count: 13
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The shoot apices of Phoenix canariensis, P. dactylifera, Washingtonia filifera and Trachycarpus excelsa are small in size and their structure is highly evolved. All cell walls of these apices are anisotropic. Numerous simple pit pairs occur in these youngest cell walls of the plant. They differ only in their lesser degree of development from those of mature parenchyma cells. Conspicuous thickenings, which do not persist in mature tissues, occur in these cell walls. During development of P. canariensis from embryo to maturity there is a constant increase in the volume of the shoot apex until a maximum is attained, before the plant begins to grow in height. In the mature palm with a tall trunk the shoot apex has a slightly smaller volume. The average cell volume in the shoot apex increases slightly during developmental stages of the palm, but at maturity it is only slightly greater than it is in the embryo. There is little variation in cell volume in the shoot apex during any stage of development. The nuclei appear to be of three types in apices showing the acid fixation image; large, lightly-staining; large, densely-staining; and small, densely-staining ones. Cells with nuclei of intermediate size and staining reaction are frequent. In apices killed in fluids that give the basic fixation image, the size variations are also evident, but all nuclei stain with equal intensity in haemotoxylin. Only after young plants of Phoenix canariensis have attained a shoot diameter of 5 cm. do their shoot apices exhibit the zonation of tissues that is characteristic of the terminal meristems of mature plants. The shoot apices of smaller plants have a more delicate aspect. Coincident with this change in zonation of tissues of the shoot apex there is a pronounced increase in its volume. The shoot apex and the primary thickening meristem have a certain independence of each other in the processes of growth of the shoot. The primary thickening meristem goes through a definite developmental history. In the embryo it is a flat zone of tissue beneath the leaf and sheath primordia. In the seedling stages it is a steep cone. During succeeding growth stages it is again a flat zone, and finally a concave region that produces the bulk of the tissues of the stem. In the growth of the palm to maturity, the primary thickening meristem contributes at first largely to the diameter of the shoot, and later to the height of the stem. Beneath the bowl-shaped meristematic zone of a mature palm long files of cells may be observed that add to stem length. The primary thickening meristem owes its origin to periclinal divisions that occur beneath the attachment surface of very young leaf and sheath primordia. It does not have an initial cell layer like that of true cambia, but is "tiered" like the young cambia of certain woody members of the Liliaceae. Provascular strands originate in two fashions in these palms. Their minor source is the tissue produced by the shoot apex, and their major source is the tissue produced by the large area of the primary thickening meristem. From a phylogenetic point of view, these palms are considered to have achieved a massive plant body by the development of a specialized meristem apart from the shoot apex itself. The cycads, on the other hand, may represent the opposite extreme since they have the largest shoot apices of any seed plant and have achieved the massive plant body largely by the activity of the shoot apex.
American Journal of Botany © 1941 Botanical Society of America, Inc.