OP ARTS AND SCIENCES. 203 



In this way the perforation of tlie lamina is completed, aud we have 

 a small hole, not usually exceeduig .5 mm. iu diameter, formed 

 partly by the pushing aside, and partly by the death and breaking 

 away of the tissues which formerly occupied its place. 



Where the tissues have been pushed aside by the ingrowing epi- 

 dermis, they remain covered by it, but where a part of the tissue has 

 died and fallen away, as in case of the cortex of one surface, that 

 part which becomes thus exposed is not at first protected. But very 

 soon the exposed living cells of the cortex begin to thicken their outer 

 walls and to acquire the characteristic brown color of epidermal cells. 

 Finally they become completely indistinguishable from and continuous 

 with the cells of the original epidermal layers of the two surfaces of 

 the lamina. (Figs. 6, e, and 9.) In this we have distinct evidence 

 that the epidermis consists simi^ly of modified cortical cells, and that 

 any cortical cells may become epidermal in character, if exposed to 

 external influences. 



Further changes in the perforations affect their size only, which 

 increases with the growth of the lamina. 



The results of the investigations detailed above may be summarized 

 as follows : — 



1. The structure of the adult frond of Agarum Tarneri agrees 

 closely with that of other Laminariaccae, and especially with that of 

 Laminaria. 



2. The frond of Agarum grows in length at the union of stipe and 

 lamma, as in other Laminariaceas ; growth in thickness probably takes 

 place in two regions. 



3. The structure of the young frond is much simpler than that of 

 the adult, but its relations to the latter are clearly traceable. 



4. The perforations of the lamina are formed by the simultane- 

 ous occurrence of an indentation of one surface and the death of the 

 corresponding part of the opposite surface. 



These results will serve at least to call up still unanswered ques- 

 tions, many of which require for their satisfactory solution access to 

 abundant growing material in all stages. For a student who can 

 command these desiderata, the work to be done on the Laminariaccae 

 is as interesting and important as the many unanswered questions 

 connected with the study of other groups of marine alga3. 



