CHAPTER XXIII 

 FROM ALGA TO FERN 



327. Progressive Development. — The largest fact that 

 stands out in a hasty review of the plants we have studied 

 is the increasing simplicity of form and organization from 

 fern to alga, or in reverse order, the increasing com- 

 plexity from alga to fern. The Pleurococcus is a simple 

 globule of living matter. Its organs are ail reduced to 

 their lowest terms — cell-wall, cytoplasm, limiting surface 

 (or membrane), nucleus, nuclear membrane, chromato- 

 phore — the parts of a cell. The one protoplast performs 

 all the functions of life — takes in raw materials, elaborates 

 food out of these raw materials, digests the food thus 

 made, assimilates it, respires, and reproduces itself. It is 

 difj&cult to imagine the fundamental life-functions per- 

 formed under simpler circumstances of structure. 



But as soon as plant cells begin to remain united after 

 cell-divison they begin to be differentiated^ The single 

 Pleurococcus cell is globular, but when two or more 

 remain attached they are flattened at the surfaces in 

 contact (Fig. 183). This is a simple illustration of 

 morphological differentiation. When a cell-mass is 

 formed the outer cell-walls, in contact with the air, 

 become covered with a layer of cuticle, which retards 

 the loss of water. Cell-walls in contact with each other 

 do not possess the cuticle. Thus, by gradual steps the 

 plant body becomes increasingly complex, so that we 



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