CHAPTER XV 



SUMMARY AND CONCLUSION 



All plants agree closely in their essential cell struct- 

 ure, the typical cell having a cellulose membrane and 

 a single nucleus. This simple type of cell constitutes 

 the whole plant in many low forms and makes up the 

 young parts of the higher plants. From it are derived 

 the variously modified cell forms constituting the spe- 

 cialized tissues of these higher plants. In the lowest of 

 all plants, the Bacteria and their blue-green allies the 

 Schizophycese, the cell does not always show a cellulose 

 membrane and the nucleus is imperfectly developed. 



The lowest plants are mainly aquatic, and it is ex- 

 ceedingly probable that this is the primitive condition 

 for plant life. Leaving aside the Schizophytes, whose 

 affinities are somewhat doubtful, the peculiar group of 

 motile green algse, the Volvocinese, probably represents 

 more nearly than any existing forms the ancestral type 

 of all the higher green plants. These ciliated algse 

 are also probably related to certain colorless flagellate 

 Infusoria, which in turn may represent the starting- 

 point for the whole group of Metazoa among the ani- 

 mals. It is not unlikely that the separation of the two 

 great branches of organisms, plants and animals, took 

 place among the Flagellata. 



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