ii 4 THE FACTORS [Part I 



quantities of definite mineral substances, such as common salt or carbonate 

 of lime. Mutatis mutandis, the same holds good, but with a greater 

 diversity, for plants growing on an organic substratum. 



We found in particular among lime-plants the phenomenon, at first 

 sight puzzling, that one and the same species in different localities made 

 quite different demands as to the chemical nature of the substratum. The 

 same phenomenon also occurs in relation to many parasites. The mistletoe 

 in many districts attacks only the silver-fir, in others only broad-leaved 

 trees. Loranthus europaeus in Bohemia grows only on the oak, in the 

 East on the chestnut also. Puccinia sessilis on Convallaria majalis. P. 

 Digraphidis on Polygonatum multiflorum and Maianthemum Convallaria, 

 P. Paridis on Paris quadrifolia, are in many regions strictly confined to 

 their usual hosts ; in other regions, however, they grow indiscriminately 

 on Convallaria, Polygonatum, Maianthemum, or Paris, and are thus in- 

 different as to substratum (Magnus). A similar condition holds good for 

 many other fungi. There cannot be a doubt but that, as in the case of 

 plants in relation to lime and other mineral salts, here too differences in 

 organization come into play, which differences in turn correspond to dis- 

 similar requirements as regards the conditions of life. Such changes in 

 organization are not always open to ocular demonstration, as in many cases 

 they are confined to the most minute structure of the protoplasm and are 

 beyond the reach of our means of observation. There are, however, species 

 of rust-fungi that in certain stages of their development agree with one 

 another completely, but in other stages distinctly and constantly differ and 

 are purely ' physiological ' species ; they can be distinguished from one 

 another by no morphological character, and yet show a decidedly specific 

 character in that they are connected with different host-plants and lack any 

 power of reciprocal interchange (Eriksson). 



SELECT LITERATURE. 



i. The Physical Properties of the Soil. 



Gain, Ed. I. Action de l'eau du sol sur la vegetation. Revue generale de botanique. 



Tome VII. 1895. 

 II. Recherches sur le role physiologique de l'eau dans la vegetation. Annales 



des sciences naturelles. y e serie. Tome XX. 1895. 

 Hoffmann, H. Untersuchungen zur Klima- und Bodenkunde mit Riicksicht auf die 



Vegetation. Botanische Zeitung. Bd. XXIII. 1865. Beilage. 

 Mayer, Ad. Die Bodenkunde. 4. Auflage. 1895. (Lehrb. der Agriculturchemie. 



Zweiter Theil. 1. Abth.). 

 Sachs, J. v. Ueber den Einfluss der chemischen und physikalischen Beschaffenheit 



des Bodens auf die Transpiration der Pflanzen. Landw. Versuchsstationen, 



1859. (Ges. Abhandl. Bd. I. p. 417.) 

 Thurmann, J. Essai de phytostatique appliquee a la chaine du Jura. Berne, 1849. 



