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tlian to trace out the changes in every organ, from the 

 embryonic state to its full development. 



Each cell acts as an independent organism, absorbing 

 fluids and gases from without, and elaborating in its interior 

 new substances, some for the support of the plant, some 

 stored up for future use, either in the life of the individual 

 or of other orders of creation, some excreted as useless. In 

 the higher plants we have great diiferentiation of tissues and 

 great diversity in the functions which each is called on to 

 perform : one set constituting cambium is occupied solely 

 with the growth of the plant, while another is preparing 

 sugar or starch or chlorophyl, yet guided by the inherent 

 vital principle, each plays its proper part without interference 

 with its neighbour, and we cannot imitate the process. 



Nageli has attempted to explain the mechanism of endos- 

 mose and exosmose, by supposing that the walls of a cell 

 consist of inconceivably minute particles, each of which, 

 however, is surrounded on all sides by a fluid envelope ; 

 other fluids will readily pass through this, and the entry of 

 water would thus cause the molecules to be moved farther 

 from each other and distension would result, while the 

 passage of water outward would bring them closer together, 

 and we should then have contraction. 



For the germination or first process of growth of the 

 embryo plant which lies dormant in every fertile seed, certain 

 conditions are necessary, without which it fails to appear. 

 These are moisture, heat, and atmospheric air; water is 

 absorbed by the cells, and after softening their texture, it 

 next sets -about the solution of the nutritive material stored 

 up in the seed, at the same time the cells continue to swell, 

 and soon burst open the hard testa or pericarp by which 

 they had been protected ; the amount of heat required is 

 variable, depending on the climate of the countr}^ of which 

 the plant is a native ; and that air is necessary, is proved by 



