628 TEXTBOOK OF BOTANY 



plasts in which yellow pigments predominate over the green; by motile 

 spores having two flagella of unequal length; by the presence of oils, but 

 never starch; and by the rare occurrence of sexual reproduction. 



Most of the yellow-greens occur free-floating in temporary ponds and 

 pools, or as epiphytes upon other algae and upon the stems and leaves 

 of submerged plants. A few species grow on small crustaceans. The aerial 

 yellow-greens may be found on tree trunks or damp walls, or growing 

 with liverworts and mosses. A common yellow-green alga on prairie and 

 bottom-land soils is the balloon-shaped Botrydium. 



The cells of the yellow-greens may be solitary, or united into colonies 

 and filaments. The cell walls are chiefly pectic compounds (pectose or 

 pectic acid) often impregnated with silica, with carbonates, and some- 

 times with small amounts of cellulose. The cells of several genera have 

 walls made up of two overlapping halves that fit together like the two 

 parts of a gelatine capsule used by druggists. Some of these characters 

 of the yellow-greens resemble those of the diatoms. 



Reproduction occurs through the formation and subsequent growth of 

 motile and non-motile spores. Sexual reproduction has been observed 

 only a few times, but seems to involve the union of motile gametes. 



The Brown Algae: Phaeophyceae 



The predominant color of brown algae is due to the presence of 

 fucoxanthin, although green and yellow pigments are also present. Brown 

 algae are almost entirely marine, and the larger ones are often referred 

 to as seaweeds (Fig. 287). The largest ones, which are said to exceed 

 200 feet in length, are in the colder waters of temperate continental 

 shores. The motile spores and gametes have two laterally placed flagella, 

 one of which extends forward and the other backward. The cells are 

 uninucleate and have from one to many chloroplasts. Oil, sugar, and 

 complex polysaccharides accumulate in them. Some 1100 species are 

 recognized. 



Most species of brown algae are found on rocky shores and do not 

 ordinarily grow at much greater depths than 50 to 75 feet, although 

 some of the kelps are attached to rocks at depths of 200 feet. Many of 

 the plants are completely submerged only at high tide, being exposed to 



vegetative material alone. In hot concentrated HCl the chloroplasts of the yellow-greens 

 become blue-green, whereas those of the greens remain green or become yellow-green 

 when similarly treated. This may be taken as evidence of a difference in chemical com- 

 position of the two groups. 



