THALLOPHYTA. 667 



and are termed tetraspores. In the simpler forms they arise in little projecting 

 sporangia, in other cases they are on specialized branches or embedded in the sub- 

 stance of the frond. 



The sexual organs, however, are very remarkable, and differ from those of other 

 Thallophytes. The male cells arise from clusters of cells (antheridia) at the tips of 

 branches or in groups upon the surface of the frond. Each antheridium liberates 

 a single, non-ciliated male cell termed a spermatium. The female organs or pro- 

 carps consist of a filamentous receptive organ, the trichogyne, and a basal part, 

 the carpogonium (cf. figs. 204 7 and 204 8 , p. 53). Besides the carpogonium there are 

 auxiliary cells, to be described directly. The process of fertilization has been 

 followed in detail in Nemalion since the earlier portions of this volume passed 

 through the press, and we now know that the suggestion of an osmotic fertilization 

 in the Floridese (cf. p. 60) is erroneous. The spermatium becomes attached to the 

 trichogyne, and at this point the wall of the trichogyne is absorbed. The nucleus 

 of the spermatium enters the trichogyne and travels down it to the carpogonium at 

 the base, where it fuses with the carpogonial nucleus. The carpogonium now divides, 

 giving rise to a mass of spores (the carpospores). In other cases the process is less 

 simple. As before there is a trichogyne and carpogonial cell, but associated with 

 the latter a number of auxiliary cells. The carpogonium when it is fertilized does 

 not give rise at once to a mass of spores, but enters into a process of conjugation 

 with these auxiliary cells, and from them the carpospores arise. This conjugation 

 may be an immediate fusion, or may be brought about by the instrumentality of 

 special tubes. Thus we may suppose the fertilizing influence to be transmitted. 

 In some cases (as in Dudresnaya) this may take place over considerable distances, 

 from branch to branch. In the simpler forms the procarps are modified branches 

 projecting freely into the water; in the more fleshy and ribbon-like forms they are 

 sunk in hollows on the thallus and often associated with a large number of auxiliary 

 cells. The trichogyne projects into the water through a small pore in the portion 

 of the surface which roofs over the procarp and auxiliary cells. As a result of 

 fertilization of the carpogonium and its conjugation with the auxiliary cells, a large 

 mass of carpospores arises, which raises up the surface like a blister. 



Fossil remains of red sea- weeds occur under the name of Nullipores. These are 

 the calcareous incrustations of the Corallinas, Melobesias, Lithothamnions, &c., men- 

 tioned above. They occur in both the secondary and tertiary rocks. The Leitha 

 limestone, largely used for building purposes in Vienna, comes from extensive nulli- 

 pore banks in the Leitha Mountains, south-west of Vienna on the Hungarian 

 frontier, and, just as in Paris many of the finest buildings are constructed of the 

 consolidated calcareous remains of Foraminiferae, so in Vienna are the incrustations 

 of certain red sea- weeds put to this purpose. 



