. Carnegie Institution of Washington publication. DEVELOPMENT OF SIUM CICUTAEFOLIUM. creases regularly as the inflorescence is approached. In order to make this leaf fit his view of basal differentiation, Cushman (1903, p. 254) assumes that as each proximal leaflet disappears the next higher leaflet takes on this same inequilateral form. According to this view the proximal leaflets are the newest. If, on the other hand, it be conceived that the leaflets appear in apical succes- sion by the division of the terminal leaflet, and that they disappear through the loss of one pair of incisions after
. Carnegie Institution of Washington publication. DEVELOPMENT OF SIUM CICUTAEFOLIUM. creases regularly as the inflorescence is approached. In order to make this leaf fit his view of basal differentiation, Cushman (1903, p. 254) assumes that as each proximal leaflet disappears the next higher leaflet takes on this same inequilateral form. According to this view the proximal leaflets are the newest. If, on the other hand, it be conceived that the leaflets appear in apical succes- sion by the division of the terminal leaflet, and that they disappear through the loss of one pair of incisions after another, proceeding now basipetally, the proximal leaflets, which have in every late senescent leaf the same peculiar shape, will be the oldest instead of the newest, and the inequilateral leaflets of one leaf will be homologous with those of all of the other leaves. In the latter case it would occasion no sur- prise to find that these leaflets show a well-established form which occurs with considerable constancy in each succeeding leaf. If these proximal leaflets were in a state of perpetual nascence and evanescence we would expect the process to result in frequent imperfect or incom- plete differentiations and consequent great variation, a condition never realized. Every one will re- call frequent instances in which the terminal leaflet of a pinnate leaf showed imperfect formation of lateral leaflets by incisions of greater or less depth near its base (n—4 and y, fig. 3), and every degree of division will have been noted, from a slight notch to the complete formation of a pair of adjacent leaflets. This ap- pears to me to be the best possible evidence that in this region, just above the base of the terminal leaflet, is the place of progressive differ- entiation and reintegration in pinnate leaves, and that it is here and not in the proximal leaflets that new characters are to be looked for. FIG. 11.—The same as fig. 10, but the distal leaflets are assumed to be homologou
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