l82 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 3D Ser. 



central part of the base having shrunk away, exposing the 

 rhizoids. P'rom this it would appear as if the rhizoids 

 entered, to a slight extent only, the cuticula of the host- 

 plant. 



The young fronds arise from the cells of the external 

 layer of the prothallium. Evidently any cell may give rise 

 to a frond by a division in one plane, in advance of the 

 surrounding cells which form part of the external layer of 

 the cushion-shaped base. When in this manner a filament 

 of some five or six cells has been formed, the cells of the 

 filament begin to divide in two planes, thus giving rise to a 

 membranous frond, the length of which when fully grown 

 seldom exceeds six centimeters. 



The number of fronds a prothallium may give rise to 

 appears to be indefinite, every cell of the outer layer of 

 the base seemingly being capable of producing a frond. A 

 frond may be formed during the first stage of the existence 

 of the prothallium, cases having been observed where frond 

 formation evidently took place when the prothallium was 

 but two cells in thickness. The formation of a frond by a 

 cell of the outer layer does not mean cessation of growth 

 for the other cells of the outer layer, since frequently a 

 frond may be found, the base of which lies in a depression 

 of the prothallium several cells deep. 



An attempt was made to ascertain, if possible, if when 

 the frond has reached a certain size, the cells of the frond, 

 in the neighborhood of the base, produced rhizoid-like pro- 

 jections such as are found in the corresponding cells of 

 other species of Porfhyi'a. But an examination of the 

 bases of a large number of mature fronds of P. naiaduni 

 failed to reveal these structures. 



When Po7'phyra naiadum occurs on Zostera, it produces 

 the same wart-like growths, but only on the extreme mar- 

 gins, not on any part of the surface of the blade, and they 

 appear to be smaller. 



Other species of Porphyra probably occur but seldom on 

 eel-grass. The only species found by the writer to occur 

 occasionally on the same host-plant were P. perforata, 

 P. laciniata, and P. abyssicola. 



