4 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 



growth, modified characters which he beheves to be atavistic occur in 

 definite parts of the organism, particularly in the region where resump- 

 tion of growth takes place. These local modifications he calls " local- 

 ized stages " and says that the equivalents of these stages " are to be 

 sought in the adults of ancestral groups." In three recent papers in the* 

 American Naturalist, Cushman (1902, 1903, 1904) gives the result of 

 studies similar to those of Jackson, and, besides presenting evidence that 

 at the resumption of growth in perennial herbs in the spring these ata- 

 vistic modifications occur, lays stress upon senescent stages as showing 

 even more primitive conditions than are to'be found in the seedling. It 

 may be said, in passing, that this view of the significance of senescent 

 stages does not fully accord with that of Hyatt (1890, pp. 78, 79; 1897, 

 p. 221), who looked upon senescent stages as indicative of the course 

 any species in question will pursue in the process of degenerative evo- 

 lution. In Hyatt's view the senescent stages have a prophetic instead 

 of an historic significance, though he recognized, of course, that there 

 are many resemblances between the senescent and the juvenile series. 



If the propositions of Hyatt and Cope, of Jackson, and of Cushman 

 are all true, there should be at least three regions in any perennial plant 

 which will agree in presenting ancestral characters — the juvenile leaves 

 following the cotyledons, the earliest formed parts at each renewal of 

 vegetative activity, and the senescent stages approaching and accom- 

 panying the inflorescence. In numerous cases there is a general agree- 

 ment in the forms passed through at these three regions, and no incon- 

 sistency arises when they are all looked upon as atavistic, though the 

 mere fact of agreement in the several regions can not be taken as in 

 itself convincing evidence that these characters agree with the adult 

 characters of some ancestral group. On the other hand, certain plants 

 do not show the same modification of leaf-form in the inflorescence that 

 is found in the " nepionic " leaves of the seedling, and it becomes at once 

 evident that no reliance can be placed upon the forms of leaf occurring 

 in any of these regions as having more than the most general signifi- 

 cance as indications of ancestral characters. 



A plant which most strikingly illustrates this fact is the hemlock 

 water-parsnip {Sium cicutaefolium Gmel.), which presents a great 

 range of leaf-form and passes rather rapidly, sometimes suddenly, from 

 one form to another without repetition, so that each of the regions sup- 

 posed to tell of ancestral conditions tells a different story. This fact is 

 illustrated in a general way by Plate I. 



The seedling of Sium cicutaefolium is so different from the adult 

 plant that, except when the two are associated together, its identity 



