PHYLOGENY 



animals may have arisen, as well as their relatives which have given rise 

 to no further groups. The kingdom Plantae is thus restricted to green 

 algae, vascular plants, and bryophytes, while the kingdom Animalia is 

 similarlv restricted to the Metazoa. While there is much to justify this 

 idea, it presents some serious difficulties. First, it is subject to the same 

 criticism as is the Protista, that the extreme members of the Protoctista 

 are just as clear-cut plants and animals as are the vascular plants and 

 Metazoa, respectively. This system also substitutes three areas of con- 

 fusion for one in the conventional system. For it would, under Copeland's 

 system, be necessary to decide whether an organism were a mychotan or 

 a protoctistan; a protoctistan or a plant; or a protoctistan or an animal. 



It seems altogether probable that this confusion exists because the 

 organisms concerned are closely related to the actual common ancestors 

 of the higher plants and animals, ancestors which need not be thought of 

 as either plant or animal, though they are usually assigned to the Plant 

 Kingdom. The living algae and Protozoa then represent various stages in 

 the evolution of the characteristic differences bet\\'een the kingdoms, to- 

 gether with many specific adaptations to the unicellular ( or better, acellu- 

 lar) grade of construction. There is, theoretically, no reason why there 

 should be a sharp line of separation between those developing along 

 plant-like lines and those developing along animal-like lines. Indeed, this 

 would be in conflict with the idea of origin by evolution. This mixture of 

 plant and animal characters among microorganisms has led Dillon to 

 recommend that the entire world of life be included in one kingdom, 

 Plantae. Perhaps the most important feature of Eiiglcna and its allies is 

 this intermediacy between the kingdoms, suggesting as it does the prob- 

 ability that the euglenoids may be primitive organisms, fairly close to the 

 stem group from which both plants and animals have come. 



THE CHLOROPHYTA 



The Chlorophyta or green algae are an extraordinarily varied group, the 

 simplest members of which are imicellular, but tliere is a definite separa- 

 tion of nucleus and cytoplasm, and the chlorophyll is contained in a single . 

 plastid. They may show considerable speciali/ation of difteront cells in 

 the multicellular species, and the higher green algae may attain large size. 

 While the more primitive species reproduce by simple fission, sexual re- 

 production and alternation of generations are well developed in the 

 phylum. The green algae appear to be on or near the main line of evolu- 

 tion leading to th(» higher plants, and hence the great interest which 

 attaches to this ]ili\'luin. 



Chlamydomonas and the Origin of Sex. Sex probably originated in 

 an unknown green alga, wliicli ma)' liave resembled llic living ChhiDuj- 

 dunionas (Figure 33). Fach plant consists of a single cc>ll. It has a well- 

 defined nucleus and a single large chloroplast. it swims bv means of two 

 flagella which ar(> l()cat(>d at the anterior end ol the ct^ll. The cell is pro- 

 tected by a heavy cellulose wall. The plant may reproduce by simply 

 dividing to form two, four, or eight zoospores (so called because they are 



104 



