EISENIA ARBOREA. 157 



point allow us to consider, for the present at any rate, that 

 at this point the departure o£ the Eisenia-iy^Q from the 

 EcJdonia-type begins to be indicated. As the growth proceeds, 

 the involutions become more and more pronounced and are 

 accompanied by a thickening of the margins along the in- 

 rolled edges. Consequently, one surface of the blade becomes 

 somewhat rounded and convex at this point, while the other 

 becomes indented with a broad and rather shallow longitu- 

 dinal furrow. This gives to the blade a certain dorsi-ventral 

 character, i. e., gives to the two faces different characteristics. 

 A similar modification occurs in species of Agarum, and also 

 in Arthrothamnus-species, where it is produced by two scroll- 

 like involutions at the base. Another marked and important 

 change is introduced at this time and that is connected with 

 the place of origin of the pinnules. 



The pinnules appear in the young plants at the transition- 

 place and continue to be produced there until the basal mar- 

 gins of the blade begin to become involute. Then the forma- 

 tion of the pinnules is carried up, /. e. the new outgrowths 

 occur at the tip of each scroll-like fold formed in each involu- 

 tion and as this increases in length (as it continues to do) 

 the place where the new pinnules appear is removed farther 

 and farther from the transition-place. 



In Alaria, Pterygopliora and Ecklonia, the outgrowth of 

 sporophylls takes place at the transition-place where also the 

 principal increase in length both of the stipe and of the blade 

 takes place. But by the process just mentioned, the meriste- 

 matic region in Eisenia becomes elongated and soon sepa- 

 rated into two definite regions, an upper and a lower one. 



The lower meristematic region seems certainly to persist, 

 for the stipe continues to increase in length as well as in thick- 

 ness, even to a very marked degree. 



The upper portion of the meristematic region divides 

 again, but this time longitudinally and soon becomes con- 

 fined to two small regions, one at the apex of each thickened 

 involute portion. 



