OF ARTS AND SCIENCES. 197 



present. At their points of contact with the substratum, the exposed 

 cell-walls become very much, and often irregularly, thickened. 



Each of the papers above quoted contributes something in regard to 

 the growth or development of the species treated, and reference may 

 be had to them for full details. It seems well established that the 

 seat of growth in length is the point of union of stipe and lamina, 

 where growth takes place either periodically or constantly. How the 

 stipe grows in thickness seems as yet hardly clear. Reinke describes 

 a " meristem cylinder " composed of the inner cells of the outer cor- 

 tex in Laminaria. to which he ascribes a function similar to that of the 

 cambium of Dicotyledons. And Will describes a peculiar layer im- 

 mediately outside of the medullary tissue in tlie young stipe of Macro- 

 cystis, which serves as the source of increase of the medulla ; this 

 ring disappears in the adult stipe. The cells of the epidermis in- 

 crease by the formation of walls at riglit angles to the surface, thus 

 keeping pace with growth in thickness, as well as in length. 



The differentiation of the frond into stipe and lamina is evident 

 from a very early stage. When quite young, the stipe consists of a 

 cortex and a few thick-walled axial cells, from which the medul- 

 lary filaments develop later, as outgrowths. At a corresponding 

 stage the lamina consists of two large-celled layers and epidermis, 

 althouirh an earlier condition is known in which it is but a single cell 

 thick. The lamina is annually renewed from the region of growth, 

 and Reinke considers it probable that not much change occurs in 

 the structure of a given lamina, but that the successive steps in the 

 development of the complex adult frond from the comparatively 

 simple young condition are accomplished by, an increase in complexity 

 of structure in successively formed fronds, until the adult type is 

 reached. 



Agarum Turneri. 



Ar/arum Turneri, the so-called Sea Colander, is one of the Lami- 

 nariaceai, having a cylindrical stipe 10 to 30 cm. in length, which 

 becomes flat toward its upper end and finally expands into a broad 

 lamina of an ovate or oblong shape, with a strongly cordate base and 

 crisped margin. Through the middle of the lamina passes a broad, 

 flat continuation of the stipe, the midrib. The lamina is abundantly 

 perforated by holes of considerable size, which will be treated in detail 

 later. The stipe splits up, at its base, into numerous branching rlii- 

 zoids, by which the plant is attached to the substratum. The whole 



