DEVELOPMENT OF SIUM CICUTAEFOLIUM. 



23 



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. U.— The same as flg. 10, but 

 the distal leaflets are assumed 

 to be homologous. 



