722 CHEMICAL PROCESSES IN THE PLANT. 



but if this happens only for a short time the motility returns when the oxygen is 

 again restored. 



The respiration of plants is, like that of animals, associated with a loss of 

 assimilated substance, this loss being always a great deal smaller in assimilating 

 plants than the gain of substance by the activity under the influence of light of 

 the cells which contain chlorophyll ; but when, as in the germination of seeds, an 

 energetic growth is combined with powerful respiration, no new products of as- 

 similation replacing the loss, the loss in weight of the growing plant may be very 

 considerable. Seeds which germinate in the dark may in this way lose almost 

 one-half of their dry weight, and it would seem that this loss is occasioned ex- 

 clusively by the decomposition of the non-nitrogenous reserve material^ and its 

 combustion into carbon dioxide and water. If the non-nitrogenous reserve-material 

 consists of oil, i.e. of a substance containing very little oxygen, a portion of the 

 inhaled oxygen remains in the germinating plant, carbo-hydrates containing a large 

 quantity of oxygen such as starch and sugar being formed at the expense of 

 the oil. 



The loss of assimilated substance caused by respiration would appear purpose- 

 less if we had only to do with the accumulation of assimilated products ; but these 

 are themselves produced only for the purposes of growth and of all the changes 

 connected with life ; the whole life of the plant consists in complicated movements 

 of molecules and atoms ; and the forces necessary for these movements are set 

 free by respiration. The oxygen, while decomposing part of the assimilated sub- 

 stance, sets up important chemical changes in the remaining portion, which on their 

 part give rise to diffusion-currents, and these bring into contact substances which 

 again act chemically on one another, and so on. The dependence on respiration of 

 the movements in protoplasm and motile leaves is very evident, since, as has been 

 mentioned, they lose their motility when oxygen is withheld from them. These 

 considerations lead to the conclusion that the respiration of plants has the same 

 essential significance as that of animals ; the chemical equilibrium of the substances 

 is being continually disturbed by it, and the internal movements maintained which 

 make up the life of the plant. Respiration is, it is true, a source of loss of sub- 

 stance; but it is also in addition the perpetual source from which flow the forces 

 necessary to the internal movements ^. 



^ [According to Borodin (Ueb. die physiol. Bedeutung des Asparagins im Pflanzenreiche, Bot, 

 Zeitg. 1878) this is not the case. In the process of respiration the nitrogenous substances constituting 

 the protoplasm become oxidised, and of this oxidation asparagin is one of the products. The non- 

 nitrogenous materials are used up in supplying plastic material to the protoplasm. 



Asparagin is regarded, from this point of view, as a nitrogenous waste- product (metabolite), and 

 it therefore corresponds physiologically to the urea formed in the animal body, a comparison which 

 was long ago suggested by Boussingault.] 



^ [M. Corenwinder, from a series of observations on the Maple and Lilac, has confirmed the 

 view to a certain extent held by Mohl, that the process of respiration is always going on in a plant 

 even when concealed by the greater activity of the decomposition of the carbon dioxide by the 

 parts containing chlorophyll. He distinguishes two periods in the vegetative season of the plant : — 

 the first period, when nitrogenous constituents predominate, is that during which respiration is most 

 active ; the second, when the proportion of carbonaceous substance is relatively larger, is the period 

 when respiration is comparatively feeble, the carbon dioxide evolved being again almost entirely 

 taken up by the chlorophyll, decomposed, and the carbon fixed in the process of assimilation. He 



