Types of Foliar Dichotomy in Living Ferns

LEAVES OF CERTAIN ferns are commonly cited as "primitive" on the general theory that they have retained dichotomy of the major vascular axes of the leaf from ancient psilophyte ancestors (Zimmermann, in Verdoorn, 1938; Lam, 1948). The present study suggests a re-interpretation to be based on comparisons with ordinary fern leaves, "pinnate" in the arrangement of their main veins. The term "foliar dichotomy" will be used here to refer not only to dichotomously lobed or divided leaves, but also to those with blades that are simple or nearly so, in which dichotomous organization is indicated by over-all vein pattern. The term is thus used for the basic patterns of major veins or vascular strands, without particular reference to degrees of laminar dissection, which is known to be highly variable among closely related ferns. Fern leaves with dichotomous vascular architecture fall into four distinctive categories which have not previously been clearly distinguished. Each category has been named for the feature of the leaf which accounts for its peculiar organization. These categories are named as follows: (A) leaves with midribless blades-"Midribless-blade Type," (B) leaves in which the morphological blade midrib has become displaced-"Displaced-midrib Type," (C) leaves with crested blades-"Crested-blade Type," and (D) leaves with non-dichotomous blades, in which the petioles are once or repeatedly branched -"Forked-petiole Type." Each type will be discussed and the known examples of each listed. A. MIDRIBLESS-BLADE TYPE.-The foliar dichotomy of midribless leaves is the commonest type, with the following peculiarities: (1) forking of the main veins of the blade is regular and symmetrical, (2) the basal pinnae or segments, or veins representing them, are formed, but the central portion and the midrib of the blade are absent or fail to Iorm," and (3) blades of this type are horizontally