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3 ri BOM ate cea y as Pan . - ty ait t;) 
SURG y % al + 4, \e die 
| : 172 ‘: iain bie Piystilie ical B otany. 
the canis, undergo important trinsforinatione: and be- t 
come changed into organic compounds suitable for nutrition. 
This process of assimilation may be compared to thes 
digestion of food by animals, ze. to its conversion into — 
-blood ; but still differs greatly from that process, since the 
‘ etinial- takes up organic compounds already existing as such, 
which have only to undergo change, while plants have to 
construct more highly complicated out of simpler com- 
pounds, or, possibly, even out of their constituent elements. 
‘The fact that the compounds which are absorbed into the 
plant are usually very rich in oxygen, while the substance of 
the plant itself contains only a small proportion of that ele- 
ment, shows that assimilation must be a process of deoxidation 
or of elimination of oxygen. It can, therefore, take place 
only in those cells which contain chlorophyll, and under 
the influence of light ; because it is only in those parts of 
the plant, and only under the action of sufficient light, that 
oxygen is given off. The leaves are, therefore, the principal 
organs of assimilation. : 
_ It is not at present possible to follow step by step the 
process of assimilation ; it seems to depend mainly on the 
following forces : (1) the evaporation of the water contained 
in the nutrient sap by transpiration ; (2) the decomposition 
of carbon dioxide and the fixation of carbon; (3) the for- 
mation of albuminoids which are essential for the produc- 
tion of protoplasm and chlorophyll ; (4) the formation of the 
substances from which cellulose is produced, zzz. starch, the 
different kinds of sugar, oil, and inuline. 
_.. Since the growth of plants can take place only at the 
expense of assimilated substances, it is of the highest im- 
portance that a number of the products of assimilation 
should be transported from the places where they are pro- 
duced, and made to take their part in the vital processes of 
the plant ; and they have therefore to be stored up at cer- — 
tain points, in order to serve, as required, as reserve-matertals. 
_ Some of these reserve-materials are, for example, again dis- 
