ORGANIC SUBSTANCES AS PLANT FOOD 243 



absence of root hairs in mycotrophic plants is counted as one of the 

 proofs in support of this theory. 



From isolations in pure cultures it has been found that the 

 mycorrhizal fungi belong to the group of root parasites, many of 

 them being unable to exist without the host plant. The recent 

 detailed investigations by Melin have shown that many of the 

 edible mushrooms, for example, birch, aspen mushrooms 1 and others, 

 the very names of which indicate their adaptation to certain trees, 

 likewise are mycorrhizal fungi, and that they can be cultivated 

 only by growing with them seedlings of the proper species of 

 trees. On the contrary, it is possible to grow mycotrophic trees 

 and other plants without the fungus component. Therefore, 

 some consider mycorrhizae to be in a condition of balanced para- 

 sitism. In many of their characteristics, mycorrhizae are similar 

 to the tubercles of leguminous plants; hence, the opinion has fre- 

 quently been expressed that with their help fixation of molecular 

 nitrogen of the atmosphere likewise may take place. These sup- 

 positions, however, have received no exact experimental substan- 

 tiation. 



75. Translocation of Organic Substances. The Deposition of 

 Food Reserves. — The organic substances produced in the leaf 

 by photosynthesis and those stored as food reserves in seeds and 

 other storage organs must be translocated to the place of their 

 consumption. We have already discussed one of the first pre- 

 requisites for their translocation, namely, their hydrolysis into 

 more movable compounds. The more complex transformations 

 which fats and proteins undergo also has been noted. The sub- 

 stances most used by a plant and most easily translocated in its 

 body are the sugars and amino acids, especially asparagine. 



The initial synthesis of all these compounds takes place in the 

 leaves, and it is not difficult to show by simple experiments that 

 this is followed by an uninterrupted movement of these substances 

 from the leaves. It is possible to demonstate this by Sachs' dry- 

 weight method by which one may also study the accumulation of 

 substance during the process of photosynthesis (Art. 2). If one- 

 half of a leaf is cut off and dried in the evening, and the other half 

 in the morning, it will be found that in the latter case the weight 

 per unit leaf area will be considerably less. Thus, Sachs noted 

 that a square decimeter of leaf area of the sunflower weighed 0.8 g. 



1 Boletus scaber and B. aurantiacus. 



