SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 



195 



It is evident, then, that among these lowest organisms, we can no longer 

 distinguish clearly between animals and plants. Because of this difficulty, 

 all the unicellular organisms are sometimes grouped under the name 

 Protista, thus avoiding the dilemma by not attempting to differentiate 

 between animal and plant. Since there are equally strong reasons for 

 associating the Protophyta with the multicellular algae, it will usually be 

 a matter of convenience as to which method of treatment is followed. 

 The important thing is not what the organisms shall be called but the 



IS* x 



Fig. 13.12. Simple multicellular plants (green algae). At left Spirogyra, a filamentous form; 

 at_right Volvox, a spherical colony of cells. {Courtesy General Biological Supply House, Inc.) 



fact that at the base of the divergent animal and plant series the gap 

 between these two types of life is at least partially bridged. 



Simple multicellular green plants. At the next higher level of organ- 

 ization above the single-celled and cell-colony types of plants, we find 

 the multicellular algae, represented by the seaweeds, pond scums, and 

 their like. These are aggregates of few or many more or less similar and 

 semi-independent cells, forming a plant body of the type called a thallus, 

 without roots, stems, or leaves and with little evidence of cell specializa- 

 tion and division of labor. The beginnings of the latter are found in some 

 of the higher forms, such as the brown and red algae, in which certain 

 cells form a rootlike holdfast for attaching the plant to some support, and 

 the body is divided into a stemlike portion and leaflike blades. The 

 resemblance of these structures to roots, stems, and leaves is, however, 



