FLAGELLATES 329 



The tetrasporic plant in character of plant body is very similar 

 to the sex plants. On it no sex organs occur. It bears spores 

 known as tetraspores, so named because the number occurring in 

 a sporangium is four. Why the plant is called a tetrasporic plant 

 is now clear. The sporangia, which have a one-celled stalk, arise 

 laterally from the axial siphon and push their way through the 

 peripheral cells. The tetraspores escape and upon germination 

 give rise to plants that bear sex organs, either antheridia or 

 procarps. 



This type of life history in which sexual plants alternate with 

 asexual plants is a feature of considerable significance because 

 it is a feature characteristic of plants above Thallophytes. It 

 is known as "alternation of generations" and its significance will 

 be explained in the groups where it is a well-established feature. 

 The alternation of generations, the cystocarp, and more complex 

 female sex organs are the chief features introduced by the Red 

 Algae. 



When Polysiphonia is compared with some of the simplest 

 forms of Algae, as some of the one-celled Blue-green Algae or even 

 Pleurococcus, it is obvious that the Algae made notable advance- 

 ments. The plant body, a single cell in the simplest forms, 

 becomes a multicellular plant body showing considerable differ- 

 entiation of parts as to form and function in the higher forms. 

 Reproduction, accomplished entirely by cell division in the 

 simplest Algae, gradually becomes more complex through the 

 groups, involving zoospores, gametes, the differentiation of 

 gametes, and the development of sex organs. 



There is very little reliable data as to how each group of Algae 

 arose. It is not probable that they arose from each other, but 

 probably all have developed independently from some unknown 

 ancestor. Regardless of how they arose, the groups mark in a 

 general way some of the steps in the evolution of the higher 

 plants. 



Some Alga-like Thallophytes not Definitely Classified 



There are three groups of alga-like plants, the Flagellates, Di- 

 atoms, and Stoneworts, which have not been definitely classified. 



Flagellates. — These are free-swimming unicellular organisms 

 of fresh water. They have both plant and animal features, on 

 which account they are regarded as intermediate between the 



