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 \\hose 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 esceapes 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 CO2 escaping from the 

 roots, converts the calcium carbonate (CaCOs) into calcium bicarbonate 

 [Ca(HC03)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 boihng, thus driving off the 

 CO2 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, arc 

 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. 



