206 STRUCTUKAL BOTANY 



Most Florideae are dioecious, and as the tetraspores 

 also are produced on distinct plants, we usually have 

 three forms of each species, asexual, male, and female. 

 Sometimes, however, as has occasionally been observed 

 in our type, all these organs occur on the same 

 individual. 



b. Sexual 



If we examine a male plant we find that the 

 antheridia occur in dense clusters on some of the 

 thallus-cells usually just below the point where a 

 branch, is given off (see Fig. 89). Each cluster is 

 really a little system of densely crowded and very 

 short branches, all springing from the same point* 

 Each terminal cell of all these crowded branchlets 

 becomes an antheridium, and there are so many of 

 these that they form a continuous mass, quite hiding 

 the short branches on which they are borne. It is often 

 easy to recognise the clusters of antheridia in red sea- 

 weeds with the naked eye, for they have no pigment, and 

 so appear as white patches on the red thallus. Every 

 terminal cell of the cluster, then, is an antheridium^ 

 Its contents round themselves off, becoming free from 

 the cell -wall, which splits open at the end, often 

 detaching a little lid. Then the cell-contents, which 

 have a single nucleus, escape through the opening; they 

 have only a protoplasmic membrane at first, but no 

 cell- wall. Thus each antheridium produces a single male 

 cell, which in this case is called a spermatium. It has no 

 cilia (in fact cilia are altogether unknown among red 

 seaweeds), but is borne passively along by the movement 

 of the sea. Often, after an antheridium has discharged its 

 contents, the cell next below grows up into the empty 



