Comparative anatomical study of three species of Microseris Public Deposited

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  • Three growth forms of the genus Microseris, of the tribe Cichorieae, family Compositae, were compared with respect to their anatomical structure. The purpose of this study was to clarify lines of evolutionary specialization in the genus. Two of the three species studied, M. laciniata and M. borealis, are perennials, while the third, M. lindleyi is an annual. Seedling development and primary growth of the root is essentially similar in all three species. There are two groups of initials in the root apex; the upper group giving rise to the stele and the lower group differentiating into the rootcap, epidermis and cortex. Differences occur in secondary growth of the primary root. The cortex is persistent in all three species. Microseris lindleyi produces a relatively large amount of secondary xylem in relation to secondary phloem. The cambium consists largely of fusiform initials and some non-typical fusiform initials, Definite ray initials were not observed. Outwardly, the cambium derivatives differentiate into storage parenchyma in which phloem strands are embedded. A phloem strand consists of sieve tubes, companion cells, phloem parenchyma and laticifers. The secondary xylem consists of both tracheids and vessels, both with scalariform to pitted lateral walls and the latter with simple perforated end-walls, thick-walled fibers, appearing only in the later part of secondary growth, and xylem parenchyma. The two perennials, M. laciniata and M. borealis have fleshy roots which consist largely of storage parenchyma, in which the phloem strands are arranged in concentric rings. Here also there are connections between most of the phloem strands of each ring by means of anastomosing laticifers but not between those of different rings. The secondary xylem is scanty, and consists of tracheids, vessels and xylem parenchyma. The vessels are arranged more or less in radiating rows which correspond with the location of phloem strands in the storage parenchyma. Both perennials produce adventitious roots that replace the primary root. Their anatomical structure differs from that of the primary root in that there are more than two protoxylem ridges, and there is a pith present. The adventitious roots show transverse wrinkling in the upper regions as a result of contraction. In the primary root, contraction is indicated primarily by folding of the Casparian strips, and is not obvious externally. All three species possess a one-layered tunica in the shoot apex. Aerial stem structure was described only for M. lindleyi and M. laciniata. The rosettes of the perennials form a primary thickening meristem which accounts for much of the increase in diameter of the stem of the rosette stage. In all three species, the stem is a eustele. Vascular bundles are always bordered at their outer margins by laticifers which are the first elements to mature in a procambium strand. In M. laciniata, a ring of phloem strands or medullary bundles develops at the inner periphery of the vascular cylinder. These phloem strands consist of sieve tubes, companion cells, phloem parenchyma and laticifers. Xylem elements may be differentiated at a later stage, and secondary growth may occur also. This work suggests M. lindleyi as the most advanced species because of its woodiness, alternate leaf position, absence of medullary bundles in addition to morphological and chromosomal features described by previous workers. Among the two perennials, M. borealis is more advanced over M. laciniata because of the horizontal rootstock and the absence of medullary bundles. M. laciniata is considered to be closest to the putative common ancestor than any of the two other species.
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