now PLAXTS OBTAIN FOOD. $1 



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 mvcelium 

 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 associated in connection mth the process of nutrition, are 

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

 fungus and ihc 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 mvcelium is 

 chiefly within the tissue of the root. 



193. Ectotrophic mycorhiza. — Kctotrophic 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 feU of the mvcelium, 

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

 fusely and are considerably thickened. The fungus scires 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 (jf the humus 

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

 ing httle or even on some plants possessing an abundance, of chlorophvU. 

 E.-<;,amples 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 mvcorhiza the mvcelium is more abundant within the tissues 

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

 of the m^'corhiza on the humus saprophytes which have no chlorophyll, or 

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

 assists ill con\erting 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 mvcorhiza. 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 actuallv derive 

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



* Evidence poijits to the belief that certain cells ol the host form substance^ 

 which attract, chemitropicalK', the fungus threads, and that in these cells the 

 fungus threads are more abundant than in others. Furthcrm(jre in the vi- 

 cillit^- (]f the micleus of tlie host seems to be the place where these activities 

 are more marked. 



