77ie Plant Kingdom - 597 



CHLAMYDOMONAS 



DESMIDS 



Fig. 31-2. Some green algae (Chlo- 

 rophyta). Note the varied form, 

 which may be unicellular, colonial, 

 or multicellular. Also notice the 

 nuclei and chloroplasts in the cells. 

 Most green algae reproduce both 

 sexually and asexually. ULOTHRIX 



(chlorophyll a and b, carotene, and xantho- 

 phyll) are similar in the green algae and 

 higher plants; and true starch is usually the 

 intracellular storage product. 



The evolutionary status of the Chloro- 

 phyta is still quite primitive, however. Only 

 a few of the species can be regarded as multi- 

 cellular organisms. That is to say, most of 

 the species are either unicellular or merely 

 colonial. And even the multicellular forms 

 display a very limited differentiation of spe- 

 cialized cells and virtually no differentiation 

 of tissues. In the sea lettuce (Fig. 31-2), for 

 example, each large leaflike lobe of the thal- 

 lus, which resembles a piece of crinkled, 

 green waxed paper, consists of only two layers 

 of cells, all more or less alike; and the hold- 

 fast (and stalk) consists mainly of elongate 

 colorless cells arranged in the form of twisted 

 strands. And finally, a relationship between 



LOBE OF THALLUS 



HOLDFAST 



the unicellular and the colonial species is 

 clearly revealed by a study of the life cycle 

 in forms such as Ulothrix. Most of the time 

 this plant takes the form of a nonmotile 

 filamentous colony. But at the time of re- 

 production, when the zoospores are produced, 

 it reverts to a unicellular motile status, pend- 

 ing the formation of a new colony (Fig. 31-2). 



The Chrysophytes (Golden-brown Algae). 

 The most important and widely known 

 members of this group are the diatoms (Fig. 

 31-3). Most of the species are unicellular and 

 nonflagellated, although a few do have fla- 

 gella, and a few are colonial. Most of the 

 species reproduce both sexually and asexu- 

 ally; and generally the sexual reproduction 

 is isogamous. 



Diatoms display several unique features. 

 The cell wall is glassy and brittle, owing to 

 a high content of silicate. Also the cell wall 



