194 



THE INDIVIDUAL ORGANISM 



common with the cells of higher plants, such as a cellulose cell wall. 

 These organisms, sometimes called the Protophyta, are classified as algae, 

 along with certain multicellular forms, and are placed in the lowest plant 

 group, the Thallophyta. As in the Protozoa, these unicellular plants 

 have, as single cells, to perform all the functions necessary for life — to 

 manufacture food, carry on respiration, excrete metabolic wastes, respond 

 appropriately to the stimuli from the environment, and reproduce their 

 kind. Again, as with the Protozoa, these unicellular plant cells must, in 

 general, be more complex in structure than the cells of 

 higher plants, in which there is cell specialization with 

 corresponding division of labor. 



For two reasons, the Protophyta do not constitute a 

 clear-cut and easily definable group of organisms. On the 

 one hand, they are closely tied in to the simple multi- 

 cellular plants through numerous transitional types, 

 starting with simple colonies made up of cells that are 

 all alike and almost indistinguishable from others that 

 are free-living, and progressing through colonies showing 

 incipient cell differentiation and division of labor to 

 the lowest multicellular algae. So numerous are the 

 intermediate types and so close the evident relation- 

 ships among many unicellular and simple multicellular 

 forms that it is not possible to separate the Protophyta as 

 a distinct plant phylum, as we do the Protozoa among 

 animals. 



On the other hand, the Protophyta are hard to 

 differentiate sharply from the Protozoa, and this 

 is the place, which we mentioned in introducing 

 the study of plants, at which the distinction between plant and animal 

 tends to become obscure. Some of the green unicellular organisms, 

 except for the presence of chlorophyll, are almost indistinguishable 

 from other free-living cells that lack chlorophyll and are clearly to 

 be placed in the Protozoa. But this is not the cause of the greatest diffi- 

 culty. There exist numerous transitional types, which combine plant 

 and animal features in a single cell. This is true, for instance, of Euglena — 

 a common fresh-water organism with a body composed of a long, flexible, 

 spindle-shaped cell, to one end of which is attached a lashing flagellum 

 that pulls the organism through the water. Near the end bearing the 

 flagellum there are a reddish, light-sensitive eyespot, and a small, some- 

 times rudimentary "cell mouth" through which food particles can be 

 taken in. Under ordinary circumstances, Euglena seldom feeds; its body 

 is filled with chloroplasts containing an abundance of chlorophyll, by 

 means of which it carries on photosynthesis. 



Fig. 13.11. Eu- 

 glena, an exam- 

 ple of the Pro- 

 tista. (Courtesy 

 General Biologi- 

 c a I Supply 

 House, Inc.) 



