166 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



Order CHLOROMONADALES Engler (1898, p. 8, as "Reihe") 

 Syn.: Raphidomonadales Chadefaud (1950a, p. 789) 

 Family Vacuolariaceae Luther (1899, p. 19) 



Syn.: Chloromonadaceae Engler (1898, p. 8, nomen nudum) ex Prescott 

 (1951, p. 421); Gonyostomaceae Lemmermann (1907-1910, p. 478); 

 Tliaumatomastigaceae Skuja (1939b, p. 99); Thaumatonematidae 

 Poche (1913, p. 155) 



Phylum Phaeophycophyta 



Characterization: The algae belonging to this phylum owe their characteristic olive- 

 green to dark brown color to the presence in their plastids of certain xanthophylls, espe- 

 cially fucoxanthin (which is peculiar to them and to diatoms), that mask the other pig- 

 ments: chlorophyll a, chlorophyll c, and beta-carotene. 



Depending upon the genus or species, the cells possess one to many plastids of varying 

 form and size. Pyrenoids have been recorded for a number of species but these structures 

 may not be true pyrenoids. Ordinarily the cells are uninucleate. The cell wall is differen- 

 tiated into an inner cellulosic and an outer pectic portion consisting usually of a gumlike 

 substance, algin, which has many economic uses. Calcification of the wall occurs in the 

 genus Padina. The known food reserves are the polysaccharide laminarin, the alcohol 

 mannitol, and fats. 



The simplest Phaeophycophyta, as exemplified by certain members of the order 

 Ectocarpales, have a branched, uniseriate, filamentous, and frequently microscopic thallus. 

 The orders Laminariales and Fucales are comprised of morphologically elaborate forms 

 that in size, degree of external differentiation, and complexity of structure surpass all 

 other algae. 



Growth in length of the thallus is apical or marginal as the result of a single initial 

 or a row of initials. Some show diffuse growth. Many possess an intercalary meristem. 

 In the Laminariales it is situated between the stipe and blade and contributes cells to 

 both; in other groups (e. g., Desmarestiales) it is located at the base of a terminal hair. 

 Growth in width, thickness, or girth of the thallus is effected by repeated longitudinal 

 division of the first-formed segments, or by the formation of radially directed filaments. 

 In many forms the surface layer of cells remains meristematic and through periclinal 

 division contributes to the growth in girth or thickness. 



The majority of brown algae show an alternation of generations. The two genera- 

 tions may be morphologically identical (isomorphic) or dissimilar (heteromorphic). The 

 diploid asexual generation forms either unilocvilar sporangia, plurilocular sporangia, or 

 both. The unilocular sporangium develops from a single cell, which is not partitioned by 

 walls. It is the seat of meiosis. The haploid zoospores (aplanospores in the Dictyotales) 

 that are produced in it give rise to sexual plants. The plurilocular sporangia are formed 

 by a linear series of cells (or rarely a single cell) that are divided into compartments, 

 in each of which is formed a single zoospore. No reduction of chromosome number occurs 

 in these sporangia. Their zooids give rise to other diploid, sporophytic plants. 



The sexual plants are monoecious or dioecious and they are either isogamous, anisog- 

 amous, or oogamous. In isogamous and anisogamous forms the gametangia are pluriloc- 

 ular organs that in structure agree with the plurilocular sporangia of the sporophytic 

 generation. In oogamous species one or more eggs are produced in each oogonium and 

 one or many sperms in each antheridium. 



The motile reproductive cells are pear-shaped, usually possess an eye-spot, and are 

 laterally biflagellate." The anteriorly directed flagellum is longer than the posteriorly 

 directed one, except in the Fucales, whose sperms have a long posterior and a short 

 anterior flagellum. Longest (1946) has shown that in Ectocarpus the long anterior 

 flagellum is of the tinsel type, an observation that has been confirmed by Manton and 



9. This is true even of the sperms of Dictyota, which were described by Williams 

 (1904b) as possessing only an anterior fiagellum. Shortly afterwards, however, he dis- 

 covered the presence also of a short posterior flagellum, a fact recently mentioned in his 

 obituary notice by Knight (1947). In a paper that appeared after the manuscript of this 

 chapter had gone to press, Manton, Clarke, and Greenwood (1953), on the contrary 

 describe and illustrate the sperms as being uniflagellate. 



