Right-Handed and Left-Handed Corn Embryos

Abstract

The vertical stem of a corn plant, with its two rows of leaves arranged in a plane, has the appearance of a bilaterally symmetrical structure, but a close examination shows that this appearance is deceptive. The sheath of each leaf of such a plant is wrapped around the internode with its edges overlapping. If the observer imagines himself as standing in the position of the internode of the stem, with the leaf sheath around him like a coat, he will find that sometimes the right half of the sheath overlaps the left, and sometimes the left overlaps the right. For convenience we shall designate the one as dextral, or right-handed, and the other as sinistral, or left-handed. Further examination of the plant reveals that the dextral and sinistral leaves are arranged in alternate sequence along the stem. Thus it follows that the overlapped sides of the sheaths all fall on one side, and, if we should split the plant lengthwise in the plane of the midribs of its leaves, one half would have only the overlapping sides of the sheaths and the other would have only the overlapped sides (pl. 34). This asymmetry of the shoot, which is characteristic of almost all grasses, is further shown in various species by other characteristics. Usually there is some difference between the two sides of the collar at the top of the sheath; the auricles, when present, ordinarily show some differences and alternately reverse their direction of overlapping in successive leaves; the leaf blade is frequently asymmetrical; and the insertion of the base of the sheath marks a spiral line on the stem. This structural pattern may be regarded as a sort of morphological dorsiventrality, but it is apparently not always correlated with orientation in rhizomes, stolons, or other horizontal stems. Since the overlapping edge of the sheath is inserted on the stem a little lower than the overlapped side, it follows that the line of insertion of a right-handed leaf describes a left-handed spiral, and that of a left-handed leaf a right-handed spiral. This reversal of direction of the spiral at each node is an aspect of the phenomenon known as antidromy, which received much attention, especially from German morphologists, during the latter half of the nineteenth century. It occurs in one way or another in many kinds of plants and has been cited in objection to the theory that the distichous arrangement of the leaves of a grass can be interpreted as a form of spiral phyllotaxy (Elias, 1942, p. 29). The significance of this reversal of pattern in consecutive phytomers has never been fully explained. As far as the grasses are concerned, it is only a part of a more comprehensive pattern in which various other structural differences appear in successive internodes. In some species, for example, the stem has alternately long and short internodes, the short ones sometimes being so reduced that the leaves appear to be opposite (pl. 34).