Box.— Vol. II.] HUS—PORPHYRA. 151 



more or less flattened structures. Finally, they cover the 

 blades of eel-grass in such large numbers that they grow 

 next to and over each other, and lose their natural hemi- 

 spherical shape, obscuring the normal color of the eel-grass, 

 and giving a rough appearance to the blade. 



As the season advances, examination with a lens shows a 

 greater or smaller number of short, blunt protuberances 

 issuing from the wart-like growths. Under the microscope 

 they appear to be composed of a number of cells placed 

 end to end. Further observation demonstrates the fact 

 that these cells, by division in two planes, give rise to a 

 monostromatic frond. From this it is but a step to estab- 

 lish a genetic connection between the hemispherical struct- 

 ures on eel-grass and the fully grown fronds of P. naiadum 

 on the same host-plant. Evidently we have the prothalloid 

 form of P. iiaiadtcm before us. This was already suspected 

 by Dr. Setchell when he called my attention to the matter. 



The prothallium, when young, consists of but a single 

 layer of cells, placed side by side on the blade of the eel- 

 grass. For a certain length of time these cells continue to 

 divide in a single plane. After that, division in the second 

 plane begins to take place, gradually giving rise to the 

 wart-like growths referred to above. In section they appear 

 to consist of layers of large, thin-walled, parenchymatous 

 cells (PI. XXI, fig. 19). The cells of the central layers 

 possess ordinary cell-contents, but only a very small chro- 

 matophore. The two or three outer layers are made up of 

 slightly smaller cells, and possess a large chromatophore. 

 The cells of the layer adjacent to the surface of the blade 

 of the host-plant also contain a large chromatophore. Upon 

 these latter cells evidently devolves the function of attach- 

 ing the prothallium to the eel-grass. Each cell is extended 

 so as to form a short, sharp, unicellular rhizoid. The 

 writer has been unable to determine with any satisfaction 

 whether these rhizoids entered the cells of the host-plant or 

 not. Careful sectioning and staining has failed to reveal 

 anything of the kind. But in material which had been 

 shrunken by reagents, only the rhizoids at the periphery of 

 the cushion-shaped base were attached to the eel-grass, the 



