175 



CHAP. 'II. 



FUNCTION OF NUTRITION Periods 1, 2, 3, 4. 



ABSORPTION (160.). ASCENT OF SAP (163.) CAUSES OF 



PROGRESSION (165. ). EXHALATION (169.)- RETENTION 



OF SAP (172.). RESPIRATION (173.). FIXATION OF CAR- 

 BON (176.). ORGANIZABLE PRODUCTS GUM (177.). 



ETIOLATION (179.). COLOURS AND CHROMATOMETER (182.). 



RESULTS OF RESPIRATION (189.). 



(159-) Function of Nutrition. THE first of the two 

 general functions (art. 152.), that of nutrition, may 

 be conveniently subdivided into about seven distinct 

 processes or subordinate functions, which are all car- 

 ried on simultaneously in different parts of the vege- 

 table structure, more especially during those seasons 

 of the year in which the powers of vegetation are 

 the most active. Sometimes, only one of them is 

 in activity, whilst the rest are either partially or 

 entirely suspended. But as the whole of the materials 

 which serve to nourish the plant must have been 

 subjected to these several processes in succession, 

 we may consider the function of nutrition to be 

 carried on during as many successive periods, be- 

 fore it is completed. We will briefly mention what 

 these successive processes are, before we enter upon 

 the details necessary for the more accurate description 

 of each of them. In the first place, plants absorb 

 their nutriment by the roots ; this nutriment is then 

 conveyed through the stem into the leaves ; there it 

 is subjected to a process by which a large proportion 

 of water is discharged; the rest is submitted to the 

 action of the atmosphere, and carbonic acid is firA 

 generated, and then decomposed by the action of light : 



