ANATOMICAL FEATURES OF METAPHLOEM IN STEMS OF SABAL, COCOS AND TWO OTHER PALMS

Sieve tubes in metaphloem of palm stems function throughout the life of the plant and merit close investigation. A stem of Sabal palmetto estimated to be 50 years old was sampled extensively. Variation in length of' sieve-tube elements throughout this stem was measured and is discussed. In the metaphloem of individual vascular bundles companion cells are not sharply differentiated from other phloem parenchyma cells. Definitive callose deposits and slime are normally absent from mature sieve tubes, even in fixed material. Otherwise no conspicuous structural features which might account for the longevity of sieve tubes can be discerned. Occlusion of phloem strands after leaf fall is initially by callose deposition on sieve plates followed immediately by tylosoid formation. Similar sampling of Cocos nucifera, Washingtonia robusta and to a lesser extent Archontophoenix alexandrae confirmed these results except for quantitative differences. THE LONGEVITY of sieve tubes in palms and other perennial woody monocotyledons which do not renew vascular tissues by secondary growth has been commented upon by several writers (e.g., Tomlinson, 1964a) but we know of no extensive study on long-lived phloem. Russow (1882) concluded that in some perennial Liliflorae with secondary growth (by an "etagen" cambium) sieve tubes function for 15-20 years because there was no significant change in the amount of callose from outer (younger) to inner (older) bundles. Among lower vascular plants which perennate without secondary vascular tissue, it is to be expected that sieve cells are long-lived. Hume (1912) has reported a functional life of more than 5 years for sieve cells in the phloem of Pteridium aquilinum. In woody dicotyledons in which phloem is renewed seasonally, Holdheide's (1951) report of sieve tubes in Tilia that function for more than a decade seems to be the only authenticated record of long-lived phloem. Evert (1962) has confirmed that they function 5 years in Tilia americana. Studies of the metaphloem of monocotyledons by Cheadle and his I Received for publication 8 December 1966. 2 Cabot Foundation, Harvard University, Cambridge, Mass. 02138. This research was supported by N.S.F. grant GB 2486 with Dr. H. E. Moore, Jr., Bailey Hortorium, Cornell University, as Principal Investigator, together with N.S.F. grants GB 506 and 2991 and grant 488 from the Johnson Fund of the American Philosophical Society to the junior author. The writers express their appreciation for help in obtaining material used in this investigation to Mr. Richard Archbold, Dr. L. J. Brass, Mr. Alfred Clark, Dr. M. Gaskin and Mr. Albert de Vane. Technical assistance by Miss Lesley Jackson, Miss Anne Bellenger, R. V. Osgood and G. A. Vargo is gratefully acknowledged. associate (Cheadle and Whitford, 1941; Cheadle 1948; Cheadle and Uhl, 1948) were largely concerned with trends of structural specialization and do not emphasize functional aspects. In general, however, the angiosperm sieve tube, because it has been investigated largely in dicotyledons which renew vascular tissues by secondary growth, is commonly regarded as having a short conducting life. The structure of mature, enucleate sieve-tube elements supports this notion, and it is thus evident that the cytoplasm of these cells is in a highly specialized state. In palms the ability of sieve tubes to maintain this state for very long periods is worthy of investigation. Any features in which the sieve tubes of palms can be shown to differ from those of other angiosperms is likely to throw light on this longevity and in turn on sieve-tube physiology generally. The present study is an extensive examination of mature metaphloem elements in stems of Sabal palmetto (Walter) Lodd. ex J. A. et J. H. Schult. (subfamily Coryphoideae) and Cocos nucifera L. (subfamily Cocoideae) supplemented by less-detailed observations on Washingtonia robusta Wendl. (subfamily Coryphoideae) and Archontophoenix alexandrae (f. Muell.) Wendl. et Drude (subfamily Arecoideae). This survev provided information about the structural range of sieve tubes and associated elements throughout a single palm trunk. In addition some distinctive features of palm sieve tubes were revealed. The extent to which phloem was rendered nonconducting as a result of fall or abscission of older leaves was also examined. No developmental studies were made. The analysis of the vascular system in the stem