Variations in anatomy and stomatal characteristics of cotyledons were studied with light microscopy and scanning electron microscopy during postgermination development. Species investigated wereAcer negundoL.,Ailanthus altissima(Mill.) Swingle,Fraxinus pennsylvanicaMarsh., andRobinia pseudoacaciaL. During their development, cotyledons of all species progressed through storage, transition, photosynthetic, and senescent stages. When germination was initiated, most cells of embryonic cotyledons were filled with reserve materials (lipids, proteins, and carbohydrates). Embryonic cotyledons lacked functional stomata, and those of very few species had developing guard-cell mother cells. In embryonic cotyledons, the mesophyll was differentiated into palisade and spongy parenchyma. Amounts of cotyledon expansion varied greatly among species, and cotyledon expansion was due primarily to increased cell size. Emergence of cotyledons from seed coats occurred over a range of seedling ages, depending on species. After cotyledons emerged, chlorophyll was rapidly synthesized and stomata became functional. Anatomical characteristics of cotyledons were much different from those of foliage leaves. Leaves were much thinner and had smaller parenchyma cells than did cotyledons. Cotyledons ofAcer negundoandRobinia pseudoacaciawere relatively short-lived (less than 30days), while those of the other species were more persistent.
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