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 zodspores, 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 
