The Simplest Leaf Organization Found in Surveys of Several Hundred Species of Dicotyledons

The number of cell-layers in leaves of dicotyledons varies considerably but within fairly definite limits. Extreme thickness of epidermis or mesophyll presently retards photosynthetic activity and is usually restricted to succulents and desert types. In connection with studies of leaves representing a large number of species of dicotyledons, no mature leaf was noted having fewer than five cell-layers in the blade. The list included, in addition to many species from Iowa, a liberal collection from southern Florida, samplings from the Pacific coast states, a number from Fiji and a fairly large collection from North Island, N. Z. with scattered numbers from various southern and eastern states. The series therefore included plants which were widely diversified in their geographical, taxonomic and ecological relations. Among these, numbering about five hundred species, only ten were noted having as few as five cell-layers in the blade. The names and general structure of nine of these are given in Table 1; the tenth, Begonia rex, is omitted from the table because it is a succulent, grown usually in planthouse, and is not comparable with the others which were growing out of doors under natural conditions. These ten represent eight families, one species each except three for the Urticaceae