12 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 



or lance-ovate, sharply serrated leaflets characteristic of the typical leaf 

 of the stem, is very sudden (see Plates I and VII), and would again 

 indicate a remarkable mutation if the course or ontogenetic develop- 

 ment agrees with the phylogenetic. 



In fairness to Cushman it should be said that while this discussion of 

 the early spring growth is based on a literal interpretation of his papers, 

 it evidently misrepresents his intention. The condition in Sium is analo- 

 gous to that in the rosette plants, in which the writer had to look care- 

 fully beneath the rosette to find the " primitive " leaves. If he had 

 examined the rosettes at some time during the preceding summer or 

 autumn, when they were just beginning to develop, he would have 

 found less difficulty in securing the leaves for which he was looking. 

 The dissected leaves in old specimens of Sium result from the rejuvena- 

 tion of axillary buds at the base of the flowering stem, and it will be 

 seen later that the first leaves of these buds were of a more juvenile 

 type, but that they belong to the autumn growth instead of the period 

 of renewed vegetative activity in the spring. 



When we examine the senescent stages approaching the inflorescence 

 a series of forms is found entirely different from either of tho'se already 

 considered, and yet according to Cushman it is in the region of the in- 

 florescence that we should be able to trace phylogeny to its earliest 

 stages. During senescence the leaves of Sium cicutaefolium present a 

 marked contrast in every particular to the juvenile stages. The pro- 

 gressive changes (fig. 3) present a simple linear series instead of the 

 complex series seen in the seedlings, and there is an increasing con- 

 stancy of form as senescence progresses instead of the increasing vari- 

 ability which should be expected if the senescent stages repeat the same 

 story as that told by the juvenile stages, but in inverse order. Thus in 

 34 adult plants examined with reference to this point the second leaf 

 below the principal inflorescence (n 4, figs. 3 and 6) had three pairs of 

 leaflets in 10 per cent, two pairs in 56 per cent, and one pair in 34 per 

 cent ; the leaf subtending the primary umbel (n 3) had one pair of leaf- 

 lets in more than 70 per cent ; and the peduncle arising axillary to this 

 highest leaf of the principal axis bore a bract (n 2) having a single 

 lance-linear blade in over 80 per cent. In vigorous plants peduncles of 

 tertiary or still higher orders occur, and the bracts (n I and n) on 

 these show a regular reduction from the form just described as pre- 

 dominant on the secondary peduncles to small awl-shaped structures 

 which apparently correspond to the base of the petiole and leaf-sheath 

 of the earlier stem-leaves, the blade having completely disappeared. 



