16 AERATION AND AIR-CONTENT. 



quantities, and they occur in the form of primary potassium phos- 

 phate. Acetic acid and lactic acid are not found in the root excre- 

 tions, but formic acid in the form of potassium salts is not at all rare. 

 This diffuses out of the living cells of the root-tips, and is, therefore, 

 not a product of decomposition. Oxalic acid, as calcium oxalate, was 

 found but once, in the excretions of Hyacinthus orientalis. The cor- 

 rosion phenomena produced by roots are due in the largest degree to 

 the excretion of CO2. The reddening of litmus paper and the corrosion 

 of rock surfaces is due to two different substances, carbon dioxid and 

 monopotassium phosphate. No acid other than CO2 is regularly ex- 

 creted by the roots of higher plants. The excretion of diastatic or 

 inverting ferments by the roots of higher plants is not physiologically 

 impossible, but a critical repetition of the experiments of Molisch, 

 who assumed the regular occurrence of these ferments in the root- 

 secretions, gave only negative results. 



Richards (1896 : 551) determined that a greatly increased respira- 

 tion results after injury to plant-tissue, varying in intensity and dura- 

 tion with the character of the tissue and the extent of the wound. 

 This increased activity usually reaches a maximum within two days 

 and then falls gradually to the normal as the wound heals over. 



Palladin (1897:827) concluded, since completely immersed etiolated 

 leaves do not become green, that oxygen, to an amount greater than that 

 freed in assimilation, is necessary for the production of chlorophyll. 



Wacker (1898 : 70) has studied the effect of soil and water upon 

 water and land plants. He found that Vicia faba, Lupinus albus, 

 Helianthus annuus, and Cucurhita pepo undergo a retardation in the 

 growth in length of their main roots when they are cultivated in 

 water. On the other hand, such water plants as Lernna viinor and 

 trisulca, Azolla filiculoides, and Hydrocharis viorsus-rancB, in a nor- 

 mally moist garden soil, show almost no root-growth. In both cases 

 this difference in growth is not a consequence of the different amount 

 of oxygen in the two media, or of the greater density of the soil so- 

 lution. The almost complete cessation of root-growth in Lemna 

 minor in soil seems to indicate that these plants do not have the 

 ability to draw water from the soil particles in sufficient amount. 

 Roots are only formed when the soil is saturated with water, so that 

 the plant may come in intimate contact with the water in the soil. 

 In mud, the roots of land plants, Vicia faba and Lupinus albus, died 

 off, either due to the absence of free oxygen, to the presence of vari- 

 ous decomposition products, or to both of these factors together. 

 While swamp plants are able to secure a sufficient amount of oxygen 

 for the roots from the aerial parts, land plants are unable to do this, 

 owing to the absence of aeration passages. 



Recent researches. — Maze (1900 : 350) found that a short immer- 

 sion of seeds in water restricted their further germination and that 



