232 BOTANY 



PART I 



a mycorliiza could utilise atmospheric nitrogen. It is thus not improbable, though 

 as yet unproved, that other mycorhizas may have a similar significance. Tlie 

 roots not only of the phanerogamic plants without chlorophyll, referred to on 

 p. 222 ; but also of most green plants living in the humus soil of woods and heaths, 

 especially the trees, stand in close relation to Fungi (*"). 



The fungal hyphae are sometimes found within the root occurring in tangled 

 groups in the cells of definite cortical layers while individual filaments extend into 

 the soil. In other plants the hyphae invest the outer surface of the young roots 

 with a closely woven sheath. The former is called endotrophic the latter ecto- 

 trophic mycorhiza but the extreme forms are connected by intermediate conditions. 

 Tlie fungi of the endotrophic mycorhiza are in part digested by the cells of the 

 root, and thus all the substances liberated will be available for the phanerogamic 

 plant. This is not known in the case of ectotrophic mycorhiza. Stahl regards the 

 significance of fungal infection of the flowering plant to lie in the active absorption 

 of nutritive salts from the soil by the fungus. The advantage to the fungus is 

 obviously, at least in the cases in which it infects green plants, the provision of 

 carbohydrates which it obtains. Whether the consortia of Fungi and Algae which 

 are called Lichens can be ranked here as regards their physiology of nutrition is 

 still doubtful. 



C. Assimilation of the other Nutrient Salts 



Sulphuric acid most nearly resembles nitrogen since it also is used 

 in the construction of proteids which contain about A - Ih per cent 

 of S. It is still uncertain where and under what conditions its 

 assimilation occurs ; we only know that a reduction of acid radicals 

 must take place in the process. In some plants sulphur is combined 

 in other substances besides proteids. 



Phosphoric acid is connected with sulphuric acid in so far as it 

 is employed in the construction of at least some proteid substances, 

 especially the nucleo-protein of the cell-nuclei ; it forms from 0"3 - 3 

 per cent of this. In entering into the molecule of this substance the 

 phosphoric acid, unlike sulphuric acid, is not reduced. Lecithin, which 

 is present in all plants, also contains phosphorus, and this is also the 

 case for pliytin, which occurs especially in seeds. 



The Metals. — As may be shown by the method of water-ciilture, 

 potassium, calcium, magnesium, and iron are just as essential as any 

 of the substances hitherto mentioned. It is very probable, at least 

 for potassium and magnesium, that they take part in the construction 

 of certain compounds that are essential for the existence of the plant. 

 Probably i)rotoi3lasm contains these elements. Other substances also 

 may contain them ; thus, for instance, a considerable amount of 

 magnesium has been shown to exist in chlorophyll. It was formerly 

 believed that chlorophyll contained iron because the chloroplasts 

 remained yellow when iron was omitted from the food-solution, but 

 this idea has become very improbable. On the other hand it has been 

 shown that iron is also necessary for i)lants that are not green, and 

 this supports the assumption that protoplasm itself contains iron, and 



