214 ESSENTIALS OF BOTANY 
From the types discussed it is evident that in the case 
of yeast and Plewrococcus a cell is an individual plant. In 
Cladophora and many other thread-like genera either one 
cell of a filament or a straight or branching row of similar 
cells is an individual. In the mosses, ferns, and still more 
highly developed types the plant is composed of many 
layers of cells, which have assumed various forms to fit 
them for their several duties. 
270. Series of Spore-Plants. — The spore-plants are 
grouped in three grand divisions, or series, as follows:1 
Smries J. Thallophytes.2 One-celled or several to many- 
celled plants of very simple organization, usually without 
separate root, stem, and leaves, always without fibro-vas- 
cular bundles. 
Serizs II. Bryophytes, or liverworts and mosses. Small 
plants which sometimes consist merely of a nearly flat mass 
of cellular tissue sometimes have a leafy stem, but are 
always without true roots or fibro-vascular bundles. The 
method of reproduction is of a far more advanced char- 
acter than in Series I. 
Series III. Pteridophytes, or ferns, horsetails, and club 
mosses. Plants, some of them small, others large, with 
true roots, stems, and leaves, and with fibro-vascular bun- 
dles (although the vessels have not exactly the structure of 
those in seed-plants). The method of reproduction is still 
more advanced than in Series IT. 
1 See p. 210. This section should not be studied in detail until the discus- 
sion of pteridophytes (Chapter XXVII) is completed. 
2 This is a rather miscellaneous group and the name is somewhat misleading, 
as some bryophytes also have a plant body consisting only of a flat thallus. 
