402 BOTANICAL GAZETTE [November 



addition of darkly staining bodies; there is no trace left of nuclear 

 structure. 



As active growth continues on each side of the initial row, 

 rapid division occurs in the external layers of the thallus, whereas 

 the inner cells, lying on the border of the central cavity, are pulled 

 apart, not dividing to keep pace with the active outer tissue. 

 This separation leaves the filiform basal process swinging loose, as 

 it were, in the central cavity (figs. 17-20). The entire structure 

 lies at the base of a slight depression, the result of arrested growth 

 of the initial row followed by activity of abutting tissue. 



Such, then, is the history of the initial row. 77 is the result of 

 more or less disintegration of an entire linear row of the thallus, with 

 perhaps the addition of portions of a terminal hair. The exact 

 amount of tissue originally involved, whether including a terminal 

 hair in part or in entirety, determines the length and size of the 

 mature structure. For a long time the writer was puzzled by the 

 great variation in length displayed by these bodies, and the explana- 

 tion was apparent only when their origin was completely worked 

 out. 



The structure described in the foregoing is found in connection 

 with any activity of the plant, as growth at the main apex, or 

 production of lateral branches, or inception of conceptacles. 



The conceptacle 



In the formation of the conceptacle, the cavity in which the 

 initial row lies becomes deepened as more and more of the abutting 

 border tissue is involved. Rapid radial division of these border cells 

 contributes new tissue to the conceptacle at the same rate and in the same 

 way as ordinary thallus tissue is developed. In the early stages 

 of the conceptacle, except for the initial row, there is no essential 

 difference between adjacent tissue of the thallus and that of the 

 actual cavity of the conceptacle (fig. 19). Presently, however, the 

 lining cells of the conceptacle become elongated, and subsequently, 

 by transverse division at the basal end, develop into septate hairs 

 (figs. 20, 21). This behavior is exactly as noted by the writer in a 

 recent investigation of Fucus (14) as the stage in the history of the 

 conceptacle known as the "hair pit." 



