18 ^'ül XLIir., Art. 1 — K. Yendo : 



The medullary layer in the sense as taken in the present 

 monograph is a very thin layer extending nearly the whole width 

 of the midrib. It is surrounded by the periraedullary tissue in 

 the manner related to for the complanated part of the stipe. The 

 term medulla used by former writers for the central tissue of the 

 Laminariaceous midrib comprises the medulla and the porimedullary 

 tissue together. In most species of Alar la, the layer is consider- 

 ably thickened at a point near each margin of the midrib. For 

 convenience' sake, I will call the point " marginal swelling of 

 the medulla" (Plate XVIK, fig. 14, h). 



The medullary layer in the midrib is built up of the same 

 elements as that in the stipe and the blade, i.e., trumpet hyphae 

 and fibrous cells. The proportions of the components differ some- 

 what according to the species, but in most cases there are more 

 trumpet hyphae in the midrib tlian in the blade. The marginal 

 swellings of the medulla show great variation in form and tissue 

 elements characterizing the species to a certain degree. In some 

 species, in the cross sections of the midrib, they are narrow lanceolate 

 in shape, in others, ovate or nearly roundish ; in some species, 

 they are composed of more trumpet hyphae and less fibrous cells ; 

 in others, a narrow middle portion is composed of fibrous cells 

 only, a mixture of both surrounding it. Cfr. Plate XVIII, fig. 

 15, IG. 



The part just overlying the perimedullary tissue, i.e., the in- 

 nermost part of the cortical tissue, is composed of cells which re- 

 semble the cortical cells in shape but are much smaller. The 

 differentiation of this layer is indicated in the complanated part 

 of the stipe, as has been alluded to above. The thickness of the 

 medullary sheath, as it may be provisionally called, varies in 

 different parts of the medulla, being, in most species, much thinner 



