20 



MORPHOLOGY 



the connection with the next group cf green alga* 3 

 is to be sought in such forms as Chlamydomona 

 and Sphaerella. 



Flagellates. This problematical group of organisms is 

 so suggestive of a relationship to Volvocales that it should 

 be mentioned in this connection. They are one-celled, 

 active, aquatic forms, with one or two (sometimes more) 

 cilia (fig. 33). The cell is naked or with a distinct mem 

 brane, which rarely contains any cellulose. The protoplast 

 shows contractile and amoeboid movements, and contains 

 a nucleus, a pulsating vacuole, and in many forms distinct 

 green or brown chromoplasts. Some of them are very 

 animal-like in taking food, ingesting solid particles; and 

 their usual multiplication by longitudinal splitting is not 

 P^ntlike. No sexual reproduction is known, but the 

 frequent formation of thick- walled " resting spores " is 

 suggestive of plants. This combination of plant and 

 animal characters has suggested that the flagellates may 

 be regarded as the starting point for such one-celled thallo- 



phytes as the Volvocales on the one hand, and for Protozoa on the other. In 

 fact, by some authors Volvocales are included among the flagellates. 



FIG. 



showing a single cilium 

 (flagellum), a large nu- 

 cleus, and numerous 

 chloroplasts. 



(b) Protococcales 



General character. The group of forms included here under this 

 name is probably a very heterogeneous assemblage, and it has been 

 much broken up recently by the special students of algae, but it will 

 serve our purpose. The plants occur mainly in fresh water, ranging 

 from an extreme aquatic habit to occurrence in moist places, such as 

 tree trunks, shaded earth, etc. Some are endophytic, living in the 

 intercellular spaces of certain aquatic seed plants; others enter into the 

 structure of lichens; while still others give the green color to certain 

 animals, as fresh-water sponges, hydra, etc. The possible connection 

 of the solitary (one-celled) forms with such solitary Volvocales as 

 Chlamydomonas is apparent, the former being characterized by the 

 absence of cilia on the vegetative cells; in other words, there is an occa- 

 sional loss of motility by the vegetative cells of Volvocales, and this is 

 the permanent condition of the vegetative cells of Protococcales. 



Pleurococcus. This is an exceedingly common green slime found 

 on flower pots, damp bricks, tree trunks, etc., and it may be regarded 

 as a representative one-celled green plant. If there is any connection 

 between Protococcales and Volvocales, it is through such forms as 



