DEVELOPMENT OF SIUM CICUTAEFOLIUM. 



15 



tinues the cycle on to the close of the senescent series, beginning at or 

 slightly in advance of the stage attained by the axis at the level from 

 which the bud springs. But when it fails to develop at once it can not 



resume activity except 

 through a process of reju- 

 venescence, which throws 

 its development back to 

 some earlier point in the 

 cycle, and always at least 

 as far down 

 the juvenile 

 side of the 

 cycle as the origin of the 

 bud is down the senescent 

 side, a relation which is 

 clearly indicated by the 

 slant of the " rejuvenes- 

 cence " lines in the dia- 

 gram (fig. 4). 



It is this less differen- 

 tiated, juvenile condition 

 which usually occurs when 

 a resting bud is reawak- 

 ened into active develop- 

 ment which suggested to 

 Jackson (1899) his con- 

 ception of localized stages 

 having phylogenetic sig- 

 nificance. 



Some cases of rejuve- 

 nescence were observed in 

 a specimen of Sium cicii- 

 taefolimn which had been 

 torn up by the roots and 

 thrown down in such a 

 way as to submerge the 

 upper portion of the stem 

 and the immature inflores- 

 cence. The axillary buds 

 of the upper part of the stem, which might otherwise have produced 

 secondary branches of the inflorescence, and which would then have 



Yio. 6.— Part of the stem of a specimen of Sium 

 cicutae/oliiim with three rejuvenated buds. 

 Drawn by J. Marlon Shull from a photograph. 



