Comparative Anatomy and Morphology of Embryos and Seedlings of Maize, Oats, and Wheat

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Abstract
The scu-tellum in maize, oats, and wheat is the cotyledon. The "ventral scale" of the cotyledon, when present, may be interpreted as its ligule. The epiblast, when present, cannot be considered a rudimentary cotyledon; it prob-ably has little morphological significance. These inter-pretations hold also for other grasses examined, in-cluding barley, rye, and rice. The coleoptile is homol-ogous with a foliage leaf, and is the 2nd leaf of the plant, the leaf distal to it being the 3rd leaf of the plant. In maize and oat seedlings, the elongated struc-ture between the cotyledon and the coleoptile is the first internode of the axis. The term "mesocotyl" as applied to this structure is meaningless, it being in no sense the middle portion of the cotyledon. In wheat, the corresponding structure, although not elongated, is like-wise the 1st internode of the axis. This holds true for other Gramineae that develop similarly to maize, such as oats and rice, and those that develop like wheat, such as barley and rye. The principal elongating structure in the young wheat axis is the 2nd internode. It is always sheathed by the coleoptile. There is no hypoco-tyledonary region of "transition" in the grasses. The transition is confined to stem structure. It starts and takes place largely in the vascular plate at the 1st node, and in the 1st internode. The transition continues in the 2nd internode and to some degree even in the 3rd and 4th internodes. The 3 morphological types of grass seedlings distinguished by Van Tieghem are fundamen-tally 1 type, appearing differently upon development be-cause of the difference in location of the meristematic region in the 1st internode. These differences often show in the mature embryo, before germination begins. The similarity of vascular skeletons of so-called seedling types is pointed out.

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