ASSIMILATION AND METASTASIS. 



703 



Sect. 5. — Assimilation and Metastasis (Stoffwechsel) \ The food-materials 

 absorbed by the plant are, with a few exceptions, compounds of oxygen containing 

 the highest possible proportion of that element. The assimilated substances, on 

 the contrary, which form the greater part of the dried substance contain but 

 little oxygen, some even none at all. It follows from this that assimilation must 

 be a process of deoxidation. The transformation of food-materials containing a 

 large proportion into the substance of plants containing but little oxygen must 

 necessarily be accompanied by elimination of that element ; and since we already 

 know that this takes place only in cells containing chlorophyll and under the in- 

 fluence of sunlight, we have at once the locality, the conditions, and the time of 

 the assimilation thus determined. No organs which are destitute of chlorophyll 

 can assimilate ; and in the dark or when the amount of light is small, even those 

 assimilating organs which contain chlorophyll lose the power of producing organic 

 substances out of water and carbon dioxide with the assistance of other food- 

 materials, — a process to which we shall henceforward exclusively apply the term 

 Asshm'latwn. 



The products of assimilation of the cells containing chlorophyll may undergo 

 various kinds of chemical metamorphosis either in these cells themselves or after 

 passing into other organs ; and the aggregate of these processes may be distin- 

 guished from assimilation as Metastasis. It is important to bear clearly in mind the 

 difference between these two processes, both in respect to their external conditions 

 and to their results, the following being the chief points : — (i) Assimilation takes 

 place only in those organs that contain chlorophyll ; metastasis in all alike. (2) 

 Assimilation occurs only under the influence of light ; metastasis equally well in the 

 dark. (3) Assimilation is necessarily accompanied by the elimination of a large 

 quantity of oxygen; metastasis is usually connected with the absorption of small 

 quantities of oxygen and the exhalation of small quantities of carbon dioxide. 

 (4) Assimilation increases the dry weight of a plant ; metastasis only alters the 

 nature of the assimilated materials, and these usually suffer a diminution of their 

 mass, the destruction of a part of the assimilated organic compounds being neces- 

 sarily associated with the inhalation of oxygen and exhalation of carbon dioxide 

 necessary for metastasis. (5) The increase in weight of a plant which contains 

 chlorophyll depends on the accession of assimilated substance in the organs that 

 contain the chlorophyll being greater during the time that they are exposed to 

 light than the loss in the dry weight connected with the exhalation of carbon 

 dioxide accompanying metastasis in all the organs and at all times of vegetation. 

 (6) Organs containing no chlorophyll and plants entirely destitute of it (parasites and 

 saprophytes) do not assimilate but absorb substances already assimilated ; no pro- 

 cess takes place in them except metastasis; and since this is associated with in- 

 halation of oxygen and exhalation of carbon dioxide, they decrease the entire store 

 of assimilated substances. 



Growth, i. e. the formation and enlargement of cells, always takes place at 

 the expense of substances already assimilated; and these therefore must be subject 

 to continual chemical change. 



^ See Sachs, Handbuch der ExperimentalPhysiologie, the section on the Transformation 

 of Food-material. 



