SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 193 



animals, but also enables us to see that the same problems are common to 

 all plants, more easily than was the case with animals. 



Although plants typically possess chlorophyll, there are a few that 

 do not produce this substance and must, therefore, obtain their food in 

 some other way than by photosynthesis. The distinction between the 

 "green plants" and those that lack chlorophyll is functionally a very 

 important one, and we shall use it as the basis for considering the major 

 plant patterns under these two heads. 



Types of Organization among the Green Plants 



As we have already noted, the plant kingdom can be separated into 

 four major groups, or divisions, which are, in order of increasing com- 

 plexity of organization, (1) the Thallophyta (algae and fungi), (2) the 

 Bryophyta (liverworts and mosses), (3) the Pteridophyta (ferns and fern 

 allies), and (4) the Spermatophyta (seed plants). In part, these divisions 

 are distinguished by the type and degree of individual organization 

 shown by their members, in part by differences in their life cycles and 

 methods of reproduction. Their characteristics and subdivisions are 

 discussed and illustrated in Appendix A. 



In a broad sense we may compare the thallophytes with the Protozoa 

 and the simplest Metazoa among animals, since this division contains 

 the unicellular and the simplest multicellular plants. Some of these may 

 be thought of as being on the protoplasmic level and others on the cellular 

 level of construction. All the remaining plants have their cells arranged 

 into definite tissues. The bryophytes are roughly analogous to the flat- 

 worms among animals — i.e., they possess definite tissues, plus the begin- 

 nings of organs. Beyond this point it is not profitable to carry the com- 

 parison. It is true that the two highest groups of plants (the pteridophytes 

 and spermatophytes) possess definite plant organs and even a suggestion 

 of organ systems in some instances, but they all function largely on 

 the tissue level. Nowhere in the plant kingdom do we encounter 

 anything so highly organized and functionally specialized as the organ 

 system as it exists among the higher animals. 



Unicellular green plants. Among the great numbers of minute or- 

 ganisms that inhabit the sea and bodies of fresh water, there are many 

 whose bodies consist of a single cell. Some of these are clearly animal- 

 like, in that they contain no chlorophyll and feed upon previously syn- 

 thesized organic material, which they take into the cell for digestion; they 

 are for the most part motile, and they lack cellulose cell walls. These are 

 the Protozoa, or unicellular animals, which constitute the lowest phylum 

 of the animal kingdom. Many of the other unicellular organisms are just 

 as clearly plantlike, making their own food from carbon dioxide and 

 water by means of chlorophyll, and possessing morphological features in 



