Dec, 1910.] Self-Dividing Laminae of Certain Kelps. 225 



the superficial and hypoclermal cells lining the gap divide chiefly 

 with periclinal walls until the newer tissue comes to have a fan- 

 like structure, similar to that seen in Nereocystis (Fig. 19). By 

 the same process described in that plant though on a much larger 

 scale, the furrow is strongly widened and deepened. This results 

 as in Nereocystis not only in attenuating the lamina locally but 

 in filling up the middle region with young cortical tissue (Fig. 

 20) which has no strength and is easily torn apart by wave action. 

 After ripping apart the cortical cells exposed from a to b 

 (Fig. 20) take on the aspect of epidemial elements with \^•hich is 

 associated a tendency of the contiguous tissues to close around the 

 injured portion. The two lamina edges resulting from the entire 

 process of division are lined with new tissue, the superficial layer 

 of which becomes epidermal in character and remains so. These 

 edges show for some time a groove running in them which is wholly 

 accounted for by the peculiar splitting process obtaining in this, 

 plant. 



GENERAL C0NSIDER.\TI0NS. 



That in all cases the split actually progresses through the blade 

 is clearly shown. The exposure of the inner tissues lying just 

 behind the apex of the advancing cleft is evidence enough to prove 

 that the fissure is cleaving the blade and is not a pseudo-cleft 

 brought about by intercalary growth of the portion divided by the 

 perforation. This latter, however, is of much importance in the 

 apparent elongation of the split. 



As to the rapidity of advance of the cleft, nothing whatever 

 was detcnnined. It is doubtful that this can be worked out even 

 in the field with any degree of accuracy. The facts of intercalary 

 growth and mechanical ripping would tend to vitiate any measure- 

 ments that might be made. 



In the study of the various genera special attention was given 

 to the advance of the distal end of the split, or the division of 

 the blade. In each genus however observations on the proximal 

 end of the cleft were made, which not only showed that the stipe 

 was dividing, but the splitting process was in each instance 

 identical with that described for the blade. Owing to the thick- 

 ness and compactness of the stipe the changes in it are much slower 

 than in the blade. The elongation of the stipes is chiefly by inter- 

 calary growth. 



An arrangement of the five genera on a basis of specialization 

 in the splitting woiild be as follows: PosteLsia, Nereocystis, Les- 

 soniopsis, IMacrocystis and Dictyoncuron. The relative arrange- 

 ment of the first three might be ciuestioned but that the process 

 in Macrocystis and particularly Dictyoncuron is a very definite, 

 and specialized one admits of no doubt. 



