596 - Heredity and Evolution 



The Euglenophytes (Euglenoids). Euglena 

 (Fig. 10-4) may be chosen to represent this 

 relatively small group of unicellular organ- 

 isms, which are partly plantlike and partly 

 animallike in their characteristics. 



Most of the 250 or so species, like Euglena, 

 possess definite chloroplasts. The Eugleno- 

 phyta, therefore, can carry on photosynthesis 

 and grow in the manner of green plants 

 generally. But unlike typical plants, the 

 euglenoids do not possess rigid cellulose 

 walls. In fact, the surface covering is very 

 flexible and many forms display active eugle- 

 noid movement (p. 196) by which the cells 

 may change shape quite drastically. More- 

 over, euglenophytes have other animallike 

 features. Active locomotion is achieved by 

 means of flagella, which usually number one 

 or two; and there is always a gullet, which 

 serves for the ingestion of solid nutrients in 

 some species, but not in others. The main 

 storage product of reserve organic food is 

 not starch, as in most plants, or glycogen, as 

 in most animals, but an intermediate type 

 of polysaccharide, namely paramylum. A 

 definite nucleus, delimited by a membrane, 

 is typical of the group; but no sexual repro- 

 duction is found, except in one genus. The 

 usual habitat is fresh water, especially in 

 shallow ponds and lakes containing rich ac- 

 cumulations of organic debris. Many of the 

 species, like Euglena, can carry on a sapro- 

 phytic kind of nutrition, which supplements 

 their growth (p. 178). 



Obviously the classification of the Eugleno- 

 phyta presents a difficult problem; and vir- 

 tually no fossil remnants are available to 

 help in the solution of this problem. Perhaps 

 the most logical viewpoint is to think of 

 them as an offshoot from a very early stock — 

 the stock that later gave rise to the sepa- 

 rately evolving plants and animals. The 

 cellular structure, on the other hand, is dis- 

 tinctly less primitive than that of the Cyano- 

 phyta. In any event, the Euglenophyta are 

 often classified as a phylum in the Plant 

 Kingdom, even though the basis for this 

 classification is rather arbitrary. 



The Chlorophytes (Green Algae). This 



fairly large group consists of some 6000 

 species, which vary widely as to form and 

 habit. Many, like Chlamydomonas (Fig. 31- 

 2), are unicellular and motile; others, like 

 the Desmids (Fig. 31-2), are unicellular, but 

 nonmotile; others, like Ulothrix (Fig. 31-2), 

 Spirogyra (Fig. 3-17), and Volvox (Fig. 3-19), 

 form a variety of colonial aggregates; while 

 a few, like the sea lettuce, Ulva (Fig. 31-2), 

 take the form of very simple multicellular or- 

 ganisms. Most of the species live in shallow 

 fresh or salt waters, although a few grow on 

 damp soil, or even on snow. 



Among the green algae the reproductive 

 processes are of great interest because they 

 show how sexual reproduction probably orig- 

 inated. Except for a few forms in which no 

 sexual processes have been found, all the 

 chlorophytes reproduce both sexually and 

 asexually. Usually, however, there is not a 

 very regular alternation of sexual and asex- 

 ual generations, as is the rule among higher 

 plants. 



In the sexual phase of the life cycle, the 

 green algae show a series of gradations 

 which culminates in the production of true 

 eggs — which are large nonmotile female 

 gametes — and true sperm — which are small 

 highly motile male gametes. In Ulothrix 

 (Fig. 31-2), for example, the fusing gametes 

 are isogametes, both of the same size and 

 form, and both motile; in Pandorina and 

 Eudorina (Fig. 3-19), the heterogamies, 

 while still motile, show a progressive differ- 

 ence in size; and finally in Volvox (Fig. 3- 

 19), the female gamete is a typical large non- 

 motile egg cell and the male gamete is a 

 small motile sperm cell, as in most higher 

 plant (and animal) groups. 



Although the fossil evidence is scant, the 

 modern chlorophytes are presumed to rep- 

 resent an offshoot from the ancient stock 

 that gave rise to the higher plants (see Fig. 

 29-11). The cell structure is generally typical. 

 Each cell is covered by a rigid cellulose wall 

 and typically there are definite nuclei and 

 well-defined chloroplasts. Also the pigments 



