14 



PLANT MORPHOLOGY 



2. EUGLENOPHYCEAE 



Flagellates are unicellular organisms combining characters of both 

 plants and animals. Zoologists regard them as one-celled animals, 

 while botanists consider at least those with chlorophyll as plants, as well 

 as certain colorless ones evidently derived from them. Nearly all 

 flagellates are solitary and free-swimming, but some form loose gelatinous 



colonies and some are attached. Most of them live in 

 fresh or salt water, some occur on damp earth, and 

 some are parasitic. Formerly they were placed in a 

 separate group, the Flagellatae, but they show so 

 many differences among themselves that they are 

 now distributed, so far as possible, into other groups. 

 One of these, the Euglenophyceae, includes about 350 

 species of green or colorless, mainly fresh-water 

 flagellates. The best-known genus, Euglena, is widely 

 distributed and common in stagnant pools and ditches, 

 often occurring in such abundance as to color the 

 water a deep green. 



Cell Structure. Euglena is somewhat pear-shaped, 

 being blunt at its anterior end and gradually tapering 

 behind (Fig. 5). As in other flagellates, there is no 

 cell wall, each cell consisting of a naked protoplast. 

 In Euglena the outer part of the protoplast is differ- 

 entiated into a thin pellicle that is somewhat firm but 

 flexible enough to permit the cefl to undergo frequent 

 changes in shape. In some flagellates the pellicle is 

 more rigid, giving the cell a constant form, while in 

 others it is wanting. Some flagellates that lack a 

 pellicle are amoeboid, putting out slender pseudopodia. 

 Flagellates are characterized by having, in the vege- 

 tative condition, one or two (rarely more) cilia — slender 

 protoplasmic threads that lash back and forth in the water. Long whip- 

 like ciHa are called flageUa,! the possession of which gives the flagellates 

 their name. The flagella are generally borne at the anterior end of the cell 

 and, where two are present, they are either equal or unequal in length. 



Euglena has a single flagellum attached anteriorly. Near its base 

 is a conspicuous red eyespot, which is thought to be sensitive to fight. 

 Although avoiding direct sunlight, the organism tends to swim toward 

 the best-illuminated part of the water. At the anterior end of the cell 



1 If a distinction is to be made between cilia and flagella, the latter are not only 

 longer than the cell that bears them but coarser and fewer in number. Cilium means 

 "eyelash"; flagellum means "whip." 



Fig. 5. Euglena 

 viridis, X750. The 

 cell contains a large 

 nucleus and a num- 

 ber of chloroplasts. 

 At the anterior end 

 is a long flagellum, 

 a narrow gullet lead- 

 ing to the reservoir, 

 a contractile vacu- 

 ole, and an eyespot. 



