Microphyll

Abstract: The independent origin and evolution of leaves as small, simple microphylls or larger, more complex megaphylls in plants has shaped and influenced the natural composition of the environment. Significant contributions have come from megaphyllous leaves, characterized usually as flat, thin lamina entrenched with photosynthetic organelles and stomata, which serve as the basis of primary productivity. During the course of evolution, the megaphylls have attained complexity not only in size or venation patterns but also in shape. This has fascinated scientists worldwide, and research has progressed tremendously in understanding the concept of leaf shape determination. Here, we review these studies and discuss the various factors that contributed towards shaping the leaf; initiated as a small bulge on the periphery of the shoot apical meristem (SAM) followed by asymmetric outgrowth, expansion and maturation until final shape is achieved. We found that the underlying factors governing these processes are inherently genetic: PIN1 and KNOX1 are indicators of leaf initiation, HD-ZIPIII, KANADI, and YABBY specify leaf outgrowth while ANGUSTIFOLIA3 and GROWTH-REGULATING FACTOR5 control leaf expansion and maturation; besides, recent research has identified new players such as APUM23, known to specify leaf polarity. In addition to genetic control, environmental factors also play an important role during the final adjustment of leaf shape. This immense amount of information available will serve as the basis for studying and understanding innovative leaf morphologies viz. the pitchers of the carnivorous plant Nepenthes which have evolved to provide additional support to the plant survival in its nutrient-deficient habitat. In hindsight, formation of the pitcher tube in Nepenthes might involve the recruitment of similar genetic mechanisms that occur during sympetaly in Petunia.

Abstract: BECK, C. B. (U. Michigan, Ann Arbor.) Reconstructions of Archaeopteris, and further consideration of its phylogenetic position. Amer. Jour. Bot. 49(4): 373-382. Illus. 1962.-Two reconstructions, one of a branch bearing the basal part of 3 leaves and the other of the habit, are presented with the evidence upon which they are based. An analysis is made of some features which characterize the ferns. These, and characters of Archaeopteris, are considered in the light of present knowledge of evolutionary trends, and the conclusion is reached that Archaeopteris was neither a primitive fern nor a fern ancestor. Additional evidence is discussed which supports the view that the plant was a progymnosperm. THE CORRELATION of Archaeopteris and Callixyton (Beck, 1960a,b) makes possible the preparation of relatively accurate reconstructions of the Upper Devonian and Lower Mississippian plant represented by these organ genera. Two reconstructions and presentation of the evidence on which they are based form the first objective of this paper. The second is to discuss further the phylogenetic position of this plant, including additional evidence which supports the view that Archaeopteris is a progymnosperm (Beck, 1960b). RECONSTRUCTIONS-No single species of Archaeopteris is known in sufficient detail to provide an accurate picture of a large part of the plant, but such a picture can be obtained by pooling information from several species. Figure 1 shows a branch segment bearing the basal parts of 3 leaves, one of which is fertile. This reconstruction, including the size and spatial relationships of various structures portrayed, is based largely oIn the illustrations of Archaeopteris fimbriata in Nathorst (1902) and on the specimen which I have recently described and identified as Archaeopteris cf. macilenta (Beck, 1960b). Archaeopteris fimbriata Nathorst (1902) is considered a synonym of A. macilenta Lesquereux (1884) by Kriiusel and Weyland (1941). The spiral arrangement of the leaves was determined by Arnold (1930) on the basis of the sequence of leaf-trace departure in Callixylon zalesskyi. This is confirmed by the specimen of A. cf. macilenta in which several leaf fragments are borne spirally on a branch identified as C. zalesskyi. Arnold reported the origin of 4 leaf traces within a distance of 1.5 cm in his specimen which suggests that the leaves were closer together than I Received for publication August 31, 1961. Supported by National Science Foundation Grant 13223. 2I am indebted to Professors Arthur J. Eames and Irving W. Bailey for their critical reading of the manuscript. For assistance in the preparation of Fig. 1, I extend thanks to G. Kadambari Kumari. The skill of Elizabeth Anthony is apparent in Fig. 2. Her patience and cooperation are acknowledged with appreciation. those illustrated in Fig. 1. This apparent discrepancy may be explained by assuming that the leaf traces diverged in their passage from the stele to the leaf bases. Or, I may have incorrectly identified the wood of A. cf. macilenta as Callixylon zalesskyi, since the identification was based on only a few slides of pyritized wood. The distance between leaves may be related to leaf size; the larger the leaves, the more widely spaced. And considerable variation in leaf size among the several species of Archaeopteris is to be expected. In this regard, a rather intriguing suggestion has been made by W. S. Lacey (personal communication, 1961) who wonders if Archaeopteris might not have borne both simple and compound leaves. There is no evidence of simple leaves on the specimen of A. cf. macilenta, unless the stipules fall into this category. The stipules do appear to be attached to the stem, immediately subtending the leaves, rather than to the petiole (Beck, 1960b, pl. 25, Fig. 2, 3). Lacey's suggestion is especially interesting in view of the proposal of the occurrence of both microphylls and megaphvlls in some members of the Zygopteridaceae by Eggert (1961). The presence of a stipule at the base of the Archaeopteris leaf has been illustrated by Nathorst (1902) in both A. fimbriata and A. roemeriana (halliana), Johnson (1911) in A. hibernica, Beck (1960b) in A. cf. macilenta, and possibly others. The forked nature of this structure is well illustrated by Nathorst. The basal enlargement of the rachis (petiole) is illustrated in Nathorst (1902) and Johnson (1911). The adaxial concavity near the base and the convex abaxial surface of the rachis are entirely hypothetical, since only compressions have been uncovered in the fossil record. The rachial pinnules near the base of the leaves have been often observed and may be documented by reference to Lesquereux (1884), Nathorst (1902) and Johnson (1911). Opposite to sub-opposite, rachial pinnules between both fertile and vegetative pinnae are illustrated in the 2 latter papers and by Arnold (1939) in a vegetative specimen. I have recently