86 



PLANT MORPHOLOGY 



Porphyra. Poiyhyra is a typical member of the Baii^iales, the most 

 primitive order of Rhodophyceae. It is widely distributed along rocky 

 seashores, occurring on both coasts of North America. It grows in the 

 intertidal zone on rocks and other algae. The thallus is plate-like and 

 attached by means of a small basal holdfast (Fig. 73A). It is only one or 





S^'as). 



E 



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Fig. 73. Porphyra perforata. A, thallus, one-half natural size; B, vertical section of 

 vegetative portion of thallus; C and D, vertical sections of thalli with carpogonia and 

 developing carpospores; E, surface view, showing liberation of carpospores; F, amoeboid 

 carpospores; G, vertical section through portion of a thallus liberating spermatia. {From 

 Gilbert M. Smith.) 



two layers of cells in thickness and, in most species, is less than 50 cm. 

 long. The cells lie in a tough gelatinous matrix derived from their walls. 

 They are without apparent cytoplasmic connections (Fig. 73B). Each 

 cell has a nucleus that divides by a primitive type of mitosis. It also 

 has a single large plastid with a central pyrenoid. 



Some species of Porphyra are monoecious but most of them are dioe- 

 cious. The antheridia develop directly from the vegetative cells. A cell 

 undergoes repeated divisions in three planes until 64 or sometimes 128 

 small cells are formed. The walls gelatinize and free the protoplasts, 

 which function as male gametes (Fig. 73G). Such naked, nonmotile male 



