HOW PLANTS OBTAIN FOOD. gi 



4. Mycorhiza. 



191. While such plants as the Indian pipe (Monotropa), some of the 

 orchids, etc., are humus saprophytes and some of them are possibly able to 

 absorb organic food from the humus, many of them have fungus mycelium 

 in close connection with their roots, and these fungus threads aid in the 

 absorption of organic food. The roots of plants which have fungus myce- 

 lium intimately associaVed in connection with the process of nutrition, are 

 termed mycorhiza. There is a mutual interchange of food between the 

 fungus and the host, a reciprocal symbiosis. 



192. Mycorhiza are of two kinds as regards the relation of the fungus to 

 the root; ectolrophic (or epiphytic), where the mycelium is chiefly on the 

 outside of the root, and endotrophic (or endophytic) where the mycelium is 

 chiefly within the tissue of the root. 



193. Ectotrophic mycorhiza. Ectotrophic mycorhiza occur on the roots 

 of the oak, beech, hornbean, etc., in forests where there is a great deal of 

 humus from decaying leaves and other vegetation. The young growing 

 roots of these trees become closely covered with a thick felt of the mycelium, 

 so that no root hairs can develop. The terminal roots also branch pro- 

 fusely and are considerably thickened. The fungus serves here as the 

 absorbent organ for the tree. It also acts on the humus, converting some 

 of it into available plant food and transferring it over to the tree. 



194. Endotrophic mycorhiza. These are found on many of the humus 

 saprophytes, which are devoid of chlorophyll, as well as on those possess- 

 ing little or even on some plants possessing an abundance, of chlorophyll. 

 Examples are found in many orchids (see the coral root orchid, for exam- 

 ple), some of the ferns (Botrychium), the pines, leguminous plants, etc. 

 In endotrophic mycorhiza the mycelium is more abundant within the tissues 

 of the root, though some of the threads extend to the outside. In the case 

 of the mycorhiza on the humus saprophytes which have no chlorophyll, or 

 but little, it is thought by some that the fungus mycelium in the humus 

 assists in converting organic substances and carbohydrates into a form 

 available for food by the higher plant and then conducts it into the root, 

 thus aiding also in the process of absorption, since there are few or no root 

 hairs on the short and fleshy mycorhiza. The roots, however, of some of 

 these humus saprophytes have the power of absorbing a portion of their 

 organic compounds from the humus. It is thought by some, though not 

 definitely demonstrated, that in the case of the oaks, beeches, hornbeans, 

 and other chlorophyll-bearing symbionts, the fungus threads do not absorb 

 any carbohydrates for the higher symbiont, but that they actually derive 

 their carbohydrates from it.* But it is reasonably certain that the fungus 



* Evidence points to the belief that certain cells of the host form substance.-, 

 which attract, chemitropically, the fungus threads, and that in these cells the 

 iungus threads are more abundant than in others. Furthermore in the vi 

 cinity of the nucleus of the host seems to be the place where these activities 

 are more marked. 



