CHAP, i.] PLANT ARCHITECTURE. 9 



the differences exhibited between a forest tree on the 

 one hand and a human being on the other. 



In addition to the animal and plant lines of departure 

 from the primitive stock, several other types of departure 

 are known to scientists, which for some reason or other 

 never made much progress, that is, did not by constant 

 modifications or emendations of the idea they started 

 out with, keep improving their position in the struggle 

 for life, and never gave origin to higher developments, 

 and consequently either disappeared or still remain as so- 

 called terminal groups. For example, on the plant side 

 we have at least three such departures, one including 

 the minute plants called diatoms, remarkable for having 

 the cell-wall rendered rigid by a deposit of silica or 

 flint, and in having the green chlorophyll the central 

 idea of plant-life masked by the presence of a brown 

 colouring matter. 



In the simplest known plants belonging to the Algce 

 or Seaweed family, such structures as root, stem, leaf, 

 flower, etc., are entirely absent, the various kinds of 

 work performed in the higher plants by the organs 

 above enumerated, being in the simplest plants fre- 

 quently performed by a single microscopic cell, which 

 in numerous instances constitutes an individual. Such 

 one-celled plants are generally called in botanical lan- 

 guage, unicellular. As an illustration of the mode of 

 life of such simple organisms we may select the well- 

 known Pleurococcus viridis, a minute plant belonging to 

 the AlgaB, that forms green stains on the trunks of trees, 

 old palings, stones, etc., in fact on everything that has 

 been exposed for any length of time to the air. If a 

 very small portion of the green substance is placed in 



