PILEOPIIITE.E. The Brown Alga also form a very large group, which contains diverse lines of development, the forms ranging from simple filaments to the gigantic Kelp or Devil's Apron (q.v.), and highly specialized rock weeds. This class also presents excellent illustrations of the principal stages in the evolution of sex. There are two sub-classes: the Ph:eosporea, whose reproductive cells, whether sexual or asexual, are swimming spores, and the Cyclo sporea, whose reproductive cells are large eggs fertilized by highly specialized sperms. The motile reproductive cells of the group, whether sexless spores or gametes (sexual cells), are peculiar in being bean- or kidney-shaped, with the pair of cilia inserted laterally. There are a dozen or more orders in this group. the largest being the Ectocarpales, comprising some of the simplest filamentous forms; the Laminariales or kelps, and the Fucales, which include the rock weeds and Sargassum. In vegetative complexity some of the Fucales are probably the highest of all the Alga. For illustration see PILEOPIIITE.E.
ElIODOPIIITE:E. The Red Alga are acknowl edged to be the most beautiful of all the Alga, because of the delicacy of their structure and brilliancy of color. The vegetative structure is not so highly differentiated as in some of the Brown and Green Alga, but the method of sex ual reproduction is especially complex. As the result of the fertilization of the female cell by the fusion of a sperm with the Trichogyne (a hair-like process of the female cell, q.v.), there arises a growth of filaments constituting a new generation (sporophyte), which remains at tached to the parent plant. The filaments of this sporophyte sometimes establish secondary con nections with the sexual plant (gametophyte) for purposes of nutrition. Certain cells of the sporophyte become spores (carpospores). The masses of spores constitute the fruit, called a eystocarp, which frequently includes a highly developed receptacle formed from the tissue of the parent plant. The sperms of the Red Alga are non-motile. There. is an asexual method of reproduction by tetraspores, so called because they are generally formed in the mother-cell in groups of four. For illustration see 11YDRD1'1YTES.
The Alga furnish especially good illustrations of some biological phenomena of general interest.
Perhaps the most remarkable are the physiologi cal conditions surrounding the development of the reproductive cells. The commonest form of
reproductive cell is the swimming spore, which became established very early in the develop ment of the Alga, as far back as the Proto coccales, whose members frequently pass a con siderable part of their life history in a motile condition, essentially like that of a swimming spore. Whenever a higher alga develops swim ming spores, which generally happens at a cer tain period of its life history, it may ,be said to return to one of the conditions of its early ancestors. As has been mentioned before, the simple motile sex cells (gametes) which fuse in pairs in the water are unquestionably swim ming spores endowed with sexual qualities, or, stated differently. lacking the power to develop independently into new plants. It has been thor oughly established by many experimental studies that these peculiarities are determined by envi ronmental factors. As an example, almost all Ilydrodietyon plants will produce sex cells after cultivation in a solution of cane sugar and under subdued light. Asexual spores will be devel oped by the same plants when cultivated in a nutrient salt solution with bright illumination. As would be expected in a group where sex begins, there are a great ninny illustrations of partheno genesis among the Alga: that is, sex cells very frequently develop new plants asexually (without fusing). There are instances of parthenogenesis in almost all large groups of the Alga, and the phenomenon is frequently related to seasonal and other environmental conditions. See PAR THENOGENESIS.
The Alga, as a whole, must be considered as a complex of divergent lines of development, very few of the living types being near the theoretical main line of ascent to the Bryophytes (liverworts and mosses). The various lines have frequently worked out similar vegetative conditions, and, what is most interesting, several groups have arrived independently at the same condition of sexual differentiation. For illus trations, see articles CHLOROPHYCE.E; CYA NOPITYCE.E ; PFLEOPHYCEA, and ROODOPHYCE2E.
For general description of Alga, consult: Eng ler and Prantl, Die natfirlichen Pllanzenfarnilien. (Berlin, 1899, et seq.) ; Murray, Introduction, to the Study of Sealceeds (London, 1895) ; Far low, Marine of New England (Salem, 1881) ; Cook, British Fresh Water Alyce (London, S3) ; Kirchner, "Kryptogamen-tlora von Schle sien," in Schlcsische Gesellschaft fir vaterliind ische !intim- (Breslau, 1870-89).