126 PHYSIOLOGY OF NUTRITION 



upon these plates, when they were exposed to the roots as was the marble men- 

 tioned above, and it therefore follows that acids belonging to the first list just 

 given are not noticeably present in root excretions. In other experiments by 

 the same writer Congo red was employed, which becomes brownish-red through 

 the action of carbonic acid and bright blue through the action of acetic, pro- 

 pionic and butyric acid. The roots turned the Congo red only brownish-red, 

 without any tendency toward blue, from which it appears that the corrosion 

 of the marble (in the experiment described above) and of soil particles, 

 is to be attributed to the action of carbonic acid excreted by the roots. 

 According to Stoklasa and Ernest 1 roots excrete organic acids only when 

 inadequately supplied with oxygen. 



The following examples indicate how much may be accomplished by plants 

 in dissolving soil particles. Lind 2 showed that the hyphae of certain fungi in 

 pure culture were able to penetrate through marble plates and bones. Nadson 3 

 described a considerable number of algae that penetrate somewhat deeply into 

 limestone and shells, dissolving the material. These forms experience severe 

 competition with many other algae on the surface of the substratum, but their 

 ability to grow in solid limestone, which is impenetrable to their competitors, 

 gives them a definite advantage in the struggle for existence. Nadson found 

 that these algae excrete oxalic acid. p 



It is also well known that parasitic fungi penetrate the cell walls of their 

 host plants. Miyoshi 4 found that fungus hyphae can pierce membranes of very 

 different kinds. The membranes to be studied were placed over nutrient gela- 

 tine and inoculated with spores. As germination took place the hyphae bored 

 through the membranes and reached the nutrient media below. 



Summary 



i. Materials Absorbed by Plants. — From the air the ordinary plant absorbs 

 carbon dioxide (and also oxygen sometimes, especially at night). From the soil it 

 absorbs water, and inorganic salts that contain nitrogen and the six essential ash 

 constituents (S, P, K, Ca, Mg, Fe) . As stated in Chapter III, free nitrogen is absorbed 

 by some lower forms and by the nodule bacteria in the tubercles of legume roots, etc. 

 Small amounts of oxygen appear to be absorbed from the soil by active roots. All 

 these substances, supplying the ten essential elements, and also many that supply 

 non-essential elements, are absorbed by diffusion in solution, generally in aqueous 

 solution. (When the transpiration rate is high, however, it appears that these sub- 



1 Stoklasa, Julius, and Ernest, Adolf, Beiträge zur Lösung der Frage der chemischen Natur des Wur- 

 zelsekretes. Jahrb. wiss. Bot. 46: 55-102. 1909. 



2 Lind, K., Ueber das Eindringen von Pilzen in Kalkgesteine und Knochen. Jahrb. wiss. Bot. 32: 

 603-634- 1898. 



s Nadson, G., Die perforierenden (kalkbohrenden) Algen und ihre Bedeutung in der Natur. [Abstract 

 in German, pp. 35-40. Text in Russian.] Scripta Botanica Hort. Univ. Imp. St. Petersburg 18: 1-40. 

 1900-1902. 



«Miyoshi, Manabu, Die Durchbohrung von Membranen durch Pilzfaden. Jahrb. wiss. Bot. 28: 

 260-289. 1895- 



p Also see: Diels, L., Die Algen- Vegetation der Südtyroler Dolomitenriffe. Ber. Deutsch. 

 Bot. Ges. 32 : 502-526. 1914. — Ed. 



