THE MATERIAL OUTGO OF PLANTS 353 
the latter is a by-product of food making, but is used by all live parts in 
respiration. Carbon dioxid is continually produced in all live parts; 
but in green parts, when adequately lighted, it can be used for making 
food, and therefore in these parts under such conditions it never accu- 
mulates to an amount which permits it to diffuse out. Oxygen is only 
intermittently produced. When the green parts are making certain 
foods, its production is a measure of their activity; but that takes place 
only in the light. Since, therefore, the leaves are the green parts par 
excellence, oxygen escapes chiefly from them, because the amount pro- 
duced is in excess of that used in their respiration. When it has accu- 
mulated in the cell sap to a concentration whose osmotic pressure is 
greater than its pressure in the air (i.e. about 0.2 of an atmosphere, or 
152 mm. of mercury), it will fly off as a gas from the surface of the cell into 
the internal atmosphere of the aerating system. Likewise when carbon 
dioxid has accumulated to a suitable pressure (less than 0.0003 A., or 
about 0.22 mm. Hg.), it begins to diffuse into the air. 
Diffusion from the root. Oxygen can be formed only in green parts 
and hence escapes only from aerial parts; carbon dioxid, being formed 
in all live cells, can also escape through the other permeable region, the 
root. Its escape there may be directly into the soil water, whenever it 
has accumulated to a greater pressure in the cell sap. To demonstrate dif- 
fusion it is only necessary to grow the roots in contact with a polished mar- 
ble plate (calcium carbonate), whose surface will be etched along the lines 
of contact because water, " carbonated " by the CO 2 escaping from the 
roots, converts the calcium carbonate (CaCO 3 ) into calcium bicarbonate 
[Ca(HCO 3 ) 2 ], which is readily soluble. Or by growing seedlings in 
water with phenolphthalein (an indicator which is rose red in weak 
alkaline and colorless in acid solution), the water will be decolorized by 
the roots; but the color will return upon boiling, thus driving off the 
CO 2 which had united with the indicator. Were any mineral or organic 
acids the cause of the decoloration, the color would not return. 
But besides CO2 other substances may leave the plant by way of the roots. At 
present these are not accurately known. Water cultures made with soil extracts 
indicate that organic compounds, often very deleterious to the culture plants, are 
frequently present. These may have come into the soil by diffusion from roots 
(see p. 315). Acid salts, such as hydrogen potassium phosphate, are probably 
not among the exudates, as once they were believed to be. Yet any substance in 
the root cortex, to which the cells are permeable, may escape ; and when the matter 
is studied further, many compounds, now unsuspected, may be found diffusing 
into the soil water. 
C. B. & C. BOTANY 23 
