198 PROCEEDINGS OF THE AMERICAN ACADEMY 



the top, which is the oldest portion, the simplest structure of a single 

 layer of cells. It then becomes two-layered, three-layered, four- 

 layered, five-layered, and finally still more complex by the growth of 

 the hyphal branches. 



That the stipe develops somewhat differently is evidenced, not only 

 by its different structure, but also by its different shape. In the ab- 

 sence of a series of earlier stages to indicate the way in which the 

 complexity came about, we have almost nothing to help us ; for in a 

 specimen only 6 mm. long the stipe is 12 or 13 cells in diameter. We 

 get a slight clue, however, from a young specimen of Laminaria 

 Phyllitis collected at Nahaut. In this the stipe consists of a central 

 vertical row of cells and a cylinder of cells about it only one layer 

 thick, so that a transverse section is only three cells across. This 

 seems to indicate that changes to produce the stipe have followed 

 something of the same course that has been followed in the blade, 

 but have been modified in such a way as to produce a cylindrical 

 rather than a flattened structure. 



Kiitzing * figures some details of the development of Laminaria 

 saccharina during this second period. Reinket also mentions the 

 partially one-layered stage of the same species, and gives a rough 

 sketch of it. 



Third Period. — The most important feature of the third period, as 

 marked out above, is the development of the rhizogen and the two 

 whorls of hapteres. At the same time, the blade and stipe increase 

 greatly both in size and complexity. The early stages are almost 

 exactly like the later stages of the previous period, except for the 

 development of the swelling which marks the appearance of the 

 rhizogen. It will be best, therefore, to describe first the changes in 

 structure which accompany this. 



A longitudinal section through a very young rhizogen shows that 

 the central portion consists wholly of elongated cells arranged in ver- 

 tical rows. There are no long tubular cells present in tins portion. 

 The changes which result in the swelling begin at the centre and pro- 

 ceed outwards. The very central cells increase greatly in diameter, 

 and are followed in this by the outer rows. They tend also to in- 

 crease in length, but, being hemmed in on all sides, soon have the 

 appearance of being cramped. This process, thus taking place in a 

 certain definite region of the stipe, causes the sides to bulge outwards, 



* Phyc. Gen., p 345, Taf 24, I, Figs. 1 to 6, 



t Pringsh. Jahrb.,Bd. X. p. 375, Taf. XXVII. Fig. 18, 1876. 



