PAPENFUSS: CLASSIFICATION OF THE ALGAE 119 



the Siphonales the cellulose is frequently replaced by callose. In the Dasycladales and in 

 many Siphonales the pectic layer of the wall is impregnated with calcium carbonate. (In 

 some seas the lime-incrusted fronds of Halimeda form an important constituent of coral 

 reefs.) In the desmids and a few other forms the wall consists of two overlapping pieces. 



The chloroplasts are usually well defined and ordinarily lie in the peripheral cyto- 

 plasm, but axile plastids are not uncommon and are especially characteristic of the des- 

 mids. According to the genus or species, the cells are provided with one or more plastids 

 of varying form and size. The pigment complex is essentially the same as in higher 

 plants, consisting of chlorophyll a, chlorophyll b, xanthophylls, and carotenes. The Sipho- 

 nales contain two xanthophylls (siphonein and siphonaxanthin) that are peculiar to 

 them (Strain, 1951). A few forms (e.g., Polytomella) are colorless. The customary food 

 reserve is starch. In many species the chloroplasts contain pyrenoids. Usually, the 

 pyrenoid is enclosed by a starch envelope consisting of separate plates of starch. 



In the majority of green algae the vegetative cells are uninucleate. The Sphaeropleales, 

 Cladophorales, Siphonocladales, and many Chlorococcales have multinucleate cells, and 

 the nonseptate filaments of the order Siphonales are, of course, also multinucleate. 



Except in the Polyblepharidaceae of the order Volvocales, the Oedogoniales, and 

 Derbesia (Siphonales) the motile stages are provided with two or four terminal fiagella 

 that are of equal length and devoid of cilia. In the Polyblepharidaceae the cells bear 

 two, three, four, five, or more fiagella of equal length and in the Oedogoniales and in 

 Derbesia the reproductive cells bear a subterminal collar of many equal fiagella. 



Asexual reproduction by ordinary cell division is of common occurrence in the unicel- 

 lular forms. Many species reproduce by zoospores. In most instances the cells in which 

 the zoospores are produced are not differentiated as specialized sporangia. The zoospores 

 are formed singly or in numbers l)y the cell contents. Nonmotile spores (aplanospores, 

 akinetes) are produced in a number of genera. 



Sexual reproduction has been established for a large number of genera representative 

 of all the orders, with the possible exception of the Schizogoniales (cf., however, Fuji- 

 yama, 1949). The species may be monoecious or dioecious and isogamous, anisogamous, 

 or oogamous. In isogamous and anisogamous forms the gametangia may or may not be 

 morphologically differentiated structures. Oogamous forms ordinarily produce differen- 

 tiated oogonia and antheridia — SphaerojUea is an exception to the rule. Except in Sphaer- 

 oplea, only one egg is formed in each oogonium. The egg is ordinarily fertilized in posi- 

 tion within the oogonium. In a few instances (CJilorangium oogamum, Chaetonema ir- 

 regular e) it is extruded prior to fertilization. There is clear evidence that oogamy has 

 evolved independently in several of the orders. 



In almost all freshwater species, the zygote is a thick-walled resting cell. In marine 

 species, on the contrary, it is a thin-walled cell which ordinarily germinates directly. It 

 thus Seems reasonable to assume that those freshwater species in which the zygote is 

 not a resting cell are derived from marine species or conversely that the marine species 

 in which the zygote is a resting cell are derived from freshwater species. 



A large majority of the green algae and almost all the freshwater representatives of 

 the phylum apparently are haploid, with meiosis occurring in the germinating zygote. 

 Certain representatives of the orders Volvocales, Ulotrichales, Cladophorales, and possibly 

 some Chlorococcales and Siphonales, show an alternation of generations. The Siphono- 

 cladales, the majority of the Siphonales and the Dasycladales are diploid as far as known, 

 with meiosis occurring at gametogenesis. The Volvocales constitute a basic group from 

 which the other orders apparently have evolved. 



History: Recognition of the Chlorophycophyta as an autonomous group begins 

 with Lamouroux (1813) who, largely on the basis of color, established an "ordre" 

 Ulvacees to receive certain genera of green algae, Viva, Bryopsis, Caulerpa, and 

 Asperococcus. Asperococcus, however, was later shown to belong to the brown 

 algae. Harvey (1836) erected the "division" Chlorospermeae and assigned to it 

 not only the green algae but also the bluegreen algae and a few genera that 

 were later found to be red or brown algae. The currently accepted class name 

 Chlorophyceae was proposed by Kiitzing (1845). 



