368 ANNTJAL REPORT SMITHSONIAN INSTITUTION, 1962 



The stony material laid down by the living cells of the plants is 

 largely calcium carbonate, although some magnesium carbonate is 

 present. This takes the form of calcite in the Corallinaceae and of 

 aragonite in the other red algae. While the encrustation first appears 

 in pectin-bearing layers of the cell walls and then extends into the 

 cellulose layers, the living protoplast of the cells always remains 

 enveloped by an uncalcified membrane. 



In the segmented corallines, some groups of cells remain completely 

 uncalcified, thus forming flexible joints between rigid segments. Such 

 a delicate, erect, bushy plant as Corallina is admirably suited to surf 

 shock and agitation — hardly less so than is the rigid, rock-encrusting 

 Litho'phyllum with which it may grow. 



The calcareous red algae, like most of the members of this large 

 plant phylum, fulfill their life history by means of an alternation of 

 generations (see pis. 4 and 5). That is to say, a given plant does not 

 reproduce one of the same kind as itself. Instead, it gives rise to a 

 different generation of plants, and these, in their turn, repeat the first 

 generation. These generations are of two kinds: one is sexual, and 

 consists of separate male and female plants. The other is asexual, 

 producing spores that germinate to grow into sexual plants. 



At maturity, the asexual plants form small, domed cavities on 

 their exterior surfaces. These asexual conceptacles produce small, 

 red structures consisting of four spores in a row (tetraspores). The 

 spores are released into the water by means of one or more exit pores 

 from the cavity. The released spores then settle down, germinate, 

 and grow into new plants. Some grow into male and some into 

 female plants which, except for reproductive details, look very much 

 alike. 



The male plants produce quite small conceptacles in which are 

 formed exceedingly minute, non-motile, male cells (spermatia). 

 These, usually only %ooo of a millimeter in diameter, are released 

 into the water through a pore in the conceptacle roof. 



Meanwhile, the female plants have produced much larger concep- 

 tacles, on the floors of which stand minute receptive organs called 

 carpogonia. Each carpogonium has a sensitive, tubular, hairlike ex- 

 tension, called a trichogyne. This organ probes the empty space 

 beneath the conceptacle pore, waiting for the stimulus of contact with 

 one of the many male spermatia that are then drifting freely about in 

 the surrounding water. 



By what mechanism the drifting, but nonmotile male cell enters the 

 tiny pore of the female conceptacle to accomplish the sexual union 

 we do not know. Wlien a spermatium does make contact with a 

 trichogyne, however, its nucleus enters the carpogonium, effects sexual 

 fusion, and so begins the development of a new generation. 

 Strangely, this new generation remains as a distinct parasitic plant 



