84i ANATOMY AND PHYSIOLOGY OF 



speak ia favour of tMs. Wlien. cut leaves of water-plants, sucli as YalUs- 

 nerla, Potamogeton, Nymphma, Hydrocharis, &e., lilie tissue of which is 

 traversed by wide air passages, are exposed to light under water, the 

 oxygen does not flow from the surface of the leaves, but from the cut 

 surfaces. It is therefore evident that the gas has to overcome a certain 

 resistance to penetrate the epidermis, and we may fairly conclude that 

 ill many uninjured leaves, a portion of tlie oxygen excreted in the green 

 substances is carried by the intercellular passa-ges and vessels into the 

 Su'^m and roots of the plant, and consequently arrives at parts not green^ 

 which as will appear presently absorb oxygen; consequently a portion of 

 the oxygen must be deficient, on the determination of the amount formed. 

 For this process bpeaks Dutrochet's observation ("ilfe??2oir" i 340), that in 

 Nifmiyh(Ba lutea the air contained in the interior of the plant contains 

 less oxygen the further from the leaves it is taken ; in the roots, eight per 

 cent ; in the stem, sixteen per cent. , in the leaves, eighteen per cent. In 

 accordance with this, stands the fact, that the vessels of the stem of the 

 gourd contain 27*9 to 29 8 per cent, of oxygen by day (Bischoff '^de mra 

 vas spir. naturaj^' 83), while by night no oxygen but much carbonic 

 acid is formed in them (Focke, " de respirat. 'oeget ^' 21). 



It may be mentioned as a curiosity, that, according to Schultz's state- 

 ments Q^Dle Entdeckung der wahren Pflanzennahmng^^), the whole theory, 

 that plMits exhale oxygen in place of the carbonic acid taken tip, rests 

 upon an error, for the green parts of plants do indeed decompose vegeta- 

 ble acids, and salts of these acids, under the influence of light, but carbonic 

 acid forms an exception to this. Wonderful to relate, hydro-chloric 

 acid; which contains no oxygen, is named among the acids yielding most 

 of it. It is unnecessary to remark that the repetition of the experi- 

 ments by Boussingault, G-risebach, and GrischoWj fully sustain the experi- 

 mental skill of a Saussure against that of the Berlin physiologist. 



The absorption, of carbonic acid, and exhalation of oxygen by 

 the green parts of plants, nnder the influence of light, are but a 

 part of the complicated relations in which plants stand to atmo- 

 spheric air. In order to form a conception of these, we must at 

 the same time investigate the behaviour of the green parts in 

 darkness, and of organs not of a green colour, Saussure is again 

 the chief guide here. 



As soon as green-coloured parts are withdrawn from the in- 

 fluence of light, their action upon the surrounding air is converted 

 into the opposite, they now absorb oxygen, and exhale carbonic 

 acid. The amount^ of oxygen taken up varies in the leaves of 

 different plants : within twenty-four hours, from half to eight 

 times the volume of the leaves. The volume of the carbonic 

 acid exhaled, is somewhat smaller than the quantity of oxygen 

 taken up; wlxen the leaves are again brought to the light, they 

 again exhale the oxygen which had disappeared. 



All parts not coloured green (Fungi, roots, stems, flowers, &c.), 

 whether exposed to light or not, take up oxygen and exhale 

 carbonic acid. 



It is usual to apply to this absorption and exhalation of parti- 



