86 PROCEEDINGS OF THE AMERICAN ACADEMY 



enlarged, taking on at the same time a dark brown color. Basal cells 

 are cut off from these enlarged cells by the formation of walls parallel 

 to the surface of the frond. The upper cells then rapidly become very 

 much larger, and when they have attained their full size their con- 

 tents divide into four spores, which on escaping may develop into new 

 plants. There may or may not be a cuticular envelope covering the 

 tetrasporic sorus in its young stages. In Dictyota the spores are found 

 strewn irregularly over both sides of the frond. In Padina and Taonia 

 they develop on the upper side in the region of and following the 

 same general course as the trichomes. In Zonaria, again, the arrange- 

 ment is irregular, but the sori are more compact than in Dictyota. 

 While in a majority of cases the oogonia and autheridia are found on 

 separate fronds, in Padina they occur together. The oogonia develop 

 very much in the manner of the tetraspores, by an enlargement of the 

 cortical cells ; their contents, however, do not divide into four, but 

 remain as a single mass. The antheridia differ considerably from the 

 oogonia and tetraspores in their growth ; the cortical cells after becom- 

 ing somewhat elongated lose their color entirely, and then by a series 

 of divisions in three directions are cut into many cells, in which the 

 antherozoids are formed.* In Dictyota, which is one of the dioecious 

 forms, the oogonia and antheridia are found in sori scattered irregu- 

 larly over both sides of the frond. In Padina they occur in zones 

 situated on each side of the concentric lines of hairs. Of these zones 

 the oogonia constitute by far the greater part, being in large masses 

 separated at irregular intervals by narrow spaces of antheridia. Tao- 

 nia like Dictyota is dioecious, and the oogonia and antheridia are found 

 as in the latter genus. In Zonaria tetraspores alone are known. 



In a few words, then, Zonaria may be distinguished from Padina 

 and Taonia as follows. The absence of the inroUed margin is suffi- 

 cient to separate it from Padina, while the absence of any distinct 

 bands of hairs and the consequent position of the reproductive organs 

 distinguish it, though not so distinctly, from Taonia. It is true that 

 most species of Zonaria are marked with concentric lines, but the lines 

 are rather zones of growth than bands of trichomes. It is indeed con- 

 cerning these zones of growth that I propose to speak later, but it will 

 be necessary before discussing them to trace the growth and general 

 histological structure of the frond. 



Zonaria variegata, Lam'x,* was the form used in the following 

 investigation, and the material, as I have already said, was collected 



* Thuret, Ann. Sci. Nat., Ser. III. Vols. XIV., XVI. 



