CHAPTER II. — CONSTITUENTS OF PLANTS. 225 



CHAPTER II. 



Constituents of Plants. — Food of Plants. — How the Food 

 is Absorbed. — Ascent of the Sap. — Gases in Plants. 



Constituents of Plants. By dessicating a plant at a tem- 

 perature too low to cause chemical decomposition, we find it 

 loses greatly in weight, owing to the evaporation of the free 

 water, which always forms a large part of the substance of the 

 living plant. The amount, however, varies greatly in different 

 plants, and in different portions of the same plant. In aquatics r 

 it often reaches 95 per cent., while in the wood of some trees, it 

 may fall as low as 20 per cent. The average for herbaceous 

 plants is probably not far from 75 per cent. It pervades all 

 parts of the organism, the protoplasm, the cell-walls, and, at 

 times, even the intercellular spaces. Protoplasm, as we have 

 seen, has the power of absorbing it in very large proportion. 

 By this means the cell-walls are kept distended, so that even 

 plants which are composed very largely of thin-walled cells are 

 turgid and resistant. This avidity of protoplasm for water has, 

 as will presently be shown, much to do with the circulation of 

 the sap, and, consequently, with the nutrition of the plant. Water 

 is also the best solvent in nature, and as such, plays a very 

 important part in the life of the plant. By virtue of this prop- 

 erty, it is the means by which solid matters are taken into the 

 interior to serve as food. It is the agent by which they are 

 conveyed to the appropriate organs to be digested, and it is the 

 means by which, when this has been accomplished, they are 

 carried to the various parts of the plant, where they are required 

 for the renewal of wastes, for the construction of new tissues, or 

 for storage against future requirements. It is also one of the 

 most important of the raw materials made use of by the plant as 

 food. 



If the dessicated plant now be burned, the larger portion 

 will pass off in the form of gases, consisting of watery vapor, 

 carbon dioxide, etc., while another portion, varying in amount 

 according to the nature of the plant and its age, will be left 



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