98 THE SCIENCE OF LIFE. 



CHAPTER VII. 



THALLOGEN3. 



Thus Nature varies ; man and brutal beast, 

 And herbage gay, and silver fishes mute, 

 And all the tribes of heaven, o'er many a sea, 

 Through many a grove that wing, or urge their so:ig 

 Near many a bank of fountain, lake, or rill, 

 Search where thou wilt, each differs in his kind, 

 In form, in figure differs. LUCRETIUS. 



1. THALLUS-PLANTS, called also Thallogens, or Thai- 

 lophytes, (from thallos, a frond, or green leaf; genein, 

 to produce, phyton, a plant,) have no true vascular sys- 

 tem, but are composed of cells of various sizes, forming 

 membranous expansions, or filaments more or less sim- 

 ple, branched, or interlacing. They differ from Proto- 

 phytes by the more intimate union of cells in their 

 structure. In some of the Protophytes there is an ad- 

 hesion of the cells by a fusion of their gelatinous invest- 

 ment, yet each cell is a repetition of the former one, and 

 is capable of living independently if detached, so that 

 each answers to the designation of a unicellular, or 

 single-celled plant. In the Thallogens the cells are not 

 only closely united, but exhibit a differentiation in struct- 

 ure or function, and a relation of mutual dependence, 

 constituting each plant (not each cell) an individual. 



2. The higher Alga, or Sea-weeds, the Lichens , and 

 the Fungi may be regarded as Thallophytes, although 



