SIMPLE THALLOPHYTES WITH SPERMATOPllYTES 293 



some plants that are not true Ferns. The Pteridophytes made 

 much advancement in developing tissues and organs. They 

 have roots, stems, and leaves, and for this reason are regarded as 

 more highly developed than the Bryophytes. 



The Spermatophytes are the Seed Plants. With this group we 

 are most familiar, since to this group belong the trees, shrubs, 

 and most of the familiar herbaceous plants. It is the seed, which 

 is one of their contributions to evolution, that makes many of 

 them so useful. In this group occurs the greatest display' of 

 tissues and organs. 



The Spermatophytes consist of two subdivisions, Gymnosperms 

 and Angiosperms: 



The Gymnosperms (Gymnospermae), as the term signifies, do 

 not have their seeds enclosed. These are the evergreens, such 

 as Pines, Cedars, Spruces, Hemlocks, Firs, etc. 



The Angiosperms (Angiospermae), as the term signifies, have 

 their seeds enclosed. This refers to the enclosing of the seed in 

 an ovary. Nearly all of the cultivated plants belong in this 

 group. They contribute the fruits. 



A Comparison of Simple Thallophytes with Spermatophytes. — 

 One striking difference between the simplest Thallophytes and 

 the Spermatophytes is in the number of cells of which the plant 

 is composed. The simplest Thallophytes are unicellular, while 

 the Spermatophytes are extremely multicellular. A second 

 striking difference between the plants of the two divisions is in 

 the differentiation and specialization of cells which are thereby 

 fitted to perform special functions. In unicellular Thallophytes 

 one cell performs all of the different kinds of work that the plant 

 has to do, while in Spermatophytes there is a division of labor 

 among the cells; that is, Spermatophytes have tissues, which 

 are groups of cells especially adapted to do particular kinds of 

 work. 



As the cells of multicellular plants become differentiated into 

 tissues and thus specialized in function, they lose the ability to 

 exist independently. Many unicellular plants can live inde- 

 pendently of other cells, but in Spermatophytes, the life of a 

 cell in most cases depends upon the proper adjustment of the cell 

 to the vital processes of other cells of the plant body. Thus the 

 ability of a cell to perform many functions is lost in becoming 

 adapted to perform one function well. 



