578 PHYSIOLOGY. 



the substances found in the watery cell-sap, is generally such that they 

 may be regarded as consisting of carbon plus the elements of water ; but 

 it is by no means to be regarded as settled that they are secondary com- 

 pounds formed by the union of water with carbon. 



The formation of crystalline acids, such as oxalic acid &c., is theoreti- 

 cally accounted for by a process of deoxidation. A further deoxidation of 

 carbonic dioxide and water would result in the formation of the different 

 carbo-hydrates, cellulose, starch, sugar, &c. A still greater loss of oxygen 

 would account for the formation of the vegetable fats &c. 



The formation of the neutral ternary compounds being constantly in 

 relation to the absorption of carbonic dioxide and the passage of water 

 through the tissues, with the exhalation of oxygen, it has been assumed 

 that assimilation of carbon in the green parts of plants is the result of 

 decomposition of carbonic dioxide and of the combination of the carbon 

 with water. As Liebig, however, indicated, water is far more easily de- 

 composed than carbonic dioxide ; and perhaps the oxygen may be derived 

 from that, its hydrogen uniting with carbonic dioxide. 



There is no evidence to show which view is correct. In the next place, 

 Draper regards the decomposition of carbonic acid as a process resulting 

 from contact- action or fermentation excited by the nitrogenous protoplasm, 

 accompanied by a waste of the latter, in which nitrogen is liberated. 

 Mulder, on the other hand, believes that the carbonic dioxide enters into 

 combination with some substance existing in the protoplasm, and that the 

 oxygen is set free by the decomposition of this compound ; for example, 

 that chlorophyll is produced continually in sunshine, the wax associated 

 with this being formed from starch, accompanied by a separation of oxy- 

 gen, that this oxygen partly escapes and partly oxidizes the chlorophyll 

 substance and causes it to become green. 



Of these views, Draper's appears the most worthy of credit, as agreeing 

 best with the phenomena observed in the cell-contents. Chlorophyll does 

 not originate from starch, but usually vice versa ; and it is quite admis- 

 sible to assume a deoxidating contact-action of the protoplasm under the 

 influence of light, when we observe a distinct oxidizing contact-action of 

 the same part of the cell-contents in the dark, as in the decomposition 

 produced by the growth of the Yeast-plant (p. 552). 



Nitrogenous Constituents. As to the nitrogenous constituents of plants, 

 we know little at present beyond the fact that they originally exist in 

 the form of protoplasmic substance, which, according to Mulder, consists 

 of modifications of the substance called proteine, Known as vegetable 

 albumen, fibrine, caseine, c. They constitute the substance of the pri- 

 mordial utricle and the protoplasm, on which chiefly depend, in all pro- 

 bability, the vital and chemical activity of the cell-contents. These have 

 the power of decomposing organic compounds by contact-action^ and per- 

 haps of causing new organic combinations. How they originate them- 

 selves is unknown ; but it appears most probable that their source is either 

 ammonia in combination with organic substances, or in some cases 

 nitrates ; and it is most probable that there is ground for Mulder's opinion 

 that all actively vegetating cells (containing protoplasm) are capable of 

 directly assimilating organic matters to some extent, whether exposed to 

 light or not, as has been shown in the case of the carnivorous plants 

 before alluded to (p. 560). 



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