OF ARTS AND SCIENCES. 199 



cortex, inner cortex, and medulla occupy about one, five, and two 

 eigbtiis, respectively, of the radius of the midrib. That is, the in- 

 crease in proportion of the inner cortex, observed in passing upward 

 from the base of the stipe, continues rapidly into the midrib, until the 

 outer cortex is reduced to a very thin tissue. The outer cortex con- 

 sists of a few layers of thin-walled cells arranged in rows perpendicular 

 to the surface, the exposed outer layer being modified to form aji epi- 

 dermis. The inner cortex forms an abrupt change from tlie last, con- 

 sisting of rounded cells with walls not so thick as in the corresponding 

 cells of the stipe, and without pits, but much thicker than those of the 

 outer cortex. The outer cells of this tissue are somewhat smaller 

 than the rest. The medulla is identical in character with that of the 

 stipe, and its filaments penetrate the inner cortex. 



The passage from midrib to lamina is rather gradual, and the latter 

 becomes thinner towards its edge. In the lamina, the distinction 

 between outer and inner cortex is lost, and the vvhole cortical tissue 

 consists of thin-walled cells. Tlie iiuier layers are rather large-celled, 

 but the outer cells are smaller, while those of the epidermal layer 

 are quite small and have their outer walls thickened. (Fig- 7.) 

 At the margin, where the frond is quite thin, the cortex is reduced 

 to a very few cell-layers. The medulla becomes gradually thinner 

 toward the margin of the lamina, where it consists of a few fine fila- 

 ments fused into an indistinct mass. The rhizoids originate from the 

 end and lower portion of the stipe, branching freely and irregularly, 

 and often are considei'ably expanded at the ends which come in con- 

 tact with and attacli the plant to the substratum. Their tissue is a 

 direct continuation of the outer cortex of the stipe, and is similar to it 

 except in being rather smaller-celled. The outer cell-layer forms an 

 epidermis, and the layer which comes into immediate contact with 

 the substratum has its exposed walls thickened very strongly, and 

 often irregularly, so that it not infrequently happens that the cav- 

 ity of a cell is nearly or even wholly obliterated by the thickening 

 of its wall. 



2. Growth. 



The cambium-like layer described by Reinke for Laminaria has 

 also been mentioned as occurring in Agarum, but there appears to be 

 another region of growth in the latter genus, which includes the epi- 

 dermis and cell-layers immediately underlying it to the number of 

 two or three. I believe that growth in thickness takes place in tliese 

 two regions, each meristem adding, by its activity, to the tissue lying 

 within it. 



