200 PROCEEDINGS OF THE AMERICAN ACADEMY 



As in Larainaria, the epidermis follows growth in thickness by the 

 increase of its cells through the formation of radially perpendicular 

 walls. 



The lamina of Agarum has never been observed to present any 

 evidences of periodic renewal so plainly shown among the species of 

 Laminaria, especially the digitate forms, during the spring months, 

 although it has been collected in great quantities at all seasons on the 

 coast of Massachusetts. This fact and the presence at all seasons of 

 abundantly forming perforations, and only these in the basal part of 

 the lamina, make it quite certain that the growth is continuous. 



Even in very youug fronds, the exact tip is not known. Many 

 fronds, when very young, are markedly attenuated at the upper end, 

 but none has been found with the actual tip still present. In any 

 but a very early stage, the attenuate character of the upper end is lost 

 and the lamina is ragged and water-worn at its apex. 



3. Anatomy of Young Frond. 



The specimens of the young frond of Ar/aniin Tarneri which have 

 been at my disposal are as small as any known, but none is suihciently 

 young to show the one-layered condition of the blade known in Lami- 

 naria, if it exists in this species. A young frond of 3 cm. long has a 

 stipe 4 or 5 mm. in length and not over .5 mm. in diameter, with 

 several branched rhizoids at its base. The lamina is oblong in slmpe, 

 about a centimeter broad and of the thickness of very tliin writing- 

 paper, with the margin slightly crisped or wholly flat. Through the 

 middle of the lamina the midrib appears as a rather broad Hat band, 

 of about twice the thickness of the lamina and of a firmer texture. 

 (Fig. 1.) Such a frond, while much simpler than in the adult condi- 

 tion, includes all the tissues of the latter except the medulla. 



The stipe has an outer cortex composed of radial rows of rather 

 thin-walled cells, the whole of which is deeply colored by the olive- 

 brown pigment. This tissue is covered on its surface by a homo- 

 geneous cuticula, in consequence of which the exposed ends of the 

 outer cells, while markedly rounded, are not thickened. The rest 

 of the stipe, composing about two thirds of its diameter, consists 

 of elongated cells closely resembling those of the inner cortex of 

 the adult, but with somewhat less thickeucd walls, (Fig. 2.) No 

 medullary threads are present at this stage, but a little later, when 

 the perforations of the lamina begin to appear, they may be seen 

 arising as outgrowths from the axial cells of the stipe. The stiuc- 



