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 all 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 

 difficult 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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