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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