ABSORPTION OF ASH-CONSTITUENTS 



97 



These results support the idea that partial replacement of sodium nitrate by 

 ammonium salts renders the phosphoric acid of the phosphate rock available 

 for oats; when one-fourth or one-half of the NaN0 3 was replaced by(NH 4 ) 2 

 SO4 the yield did not fall below that of the control, as it did in the other cases. 

 It is clear that the nutrient materials in the soil are utilized to unequal degrees 

 by different plants. As we shall see later, roots excrete acid substances that favor 

 the solution of soil materials otherwise practically insoluble in water. Further- 

 more, many plants are characterized by having their roots covered with fungus 



hyphae, a fact discovered by Kami- 

 enski. 1 Frank 2 gave the name myco- 

 rhiza to this weft of fungus hyphae growing 

 upon roots, and emphasized the impor- 

 tance of this whole phenomenon in the 

 physiology of nutrition. Plants that have 

 mycorhiza are said to be mycotrophic. We 

 owe extended investigations upon the 

 physiological importance of mycorhiza to 

 Stahl. 3 In some cases the fungus hyphae 

 cover the surface of the roots (ectotrophic 

 mycorhiza), as is shown in the case of 

 beech roots (Fig. 61). The tip region of 



Fig. 61. — Ectotrophic mycorhiza of the 

 beech; a, humus particles; b, strands of 

 fungus hyphae penetrating the soil. 



Fig. 62. — Endotrophic mycorhiza in epider- 

 mal cells of the root of Andromeda polifolia, 

 the root shown in cross-section. 



the root is covered with hyphae some of which branch out into the soil and 

 attach themselves to particles of humus. In other cases the fungus hyphae are 

 found within the cells of the root (endotrophic mycorhiza), as in the case of 

 Andromeda polifolia (Fig. 62). Here the hypae occur in the large cells of the 

 root epidermis. 



Mycorhiza is of common occurrence, being found on the majority of vascular 

 plants, not only trees, shrubs and herbs, but even mosses. Some plants cannot 



1 Kamienski, Fr., Die Vegetationsorgane der Monotropa hypopitys L. Vorlauf. Mitth. Bot. Zeitg. 

 39= 457-461. 1881. 



2 Frank, B., Ueber die auf Wurzelsymbiose beruhende Ernahrung gewisser Baume durch unterirdische 

 Pilze. Ber. Deutsch. Bot. Ges. 3: 128-145. 1885. 



3 Stahl, E., Der Sinn der Mycorhizenbildung. Ein vergleichend-biologische Studie. Jahrb. wiss. Bot. 

 34: 539-668. 1900. 



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