ECTOTROPHIC MYCORRHIZAS 105 



common members of the family, as toadflax, Linaria vulgaris, are 

 com])letel\- independent ])lants. Others, as some species of louse- 

 wort, Pedicularis, are mild root i)arasites. They contain chlorophyll 

 and look like independent plants but they are able to aiijjment their 

 supi)ly of water and minerals by robbing their neighbors. Often a 

 single plant has numerous contacts with the roots of other plants, 

 frequently of several species at the same time. Tozzia, another genus 

 of the same family, with one species occurring in the Alps and 

 another in the Carpathians, is much different. A plant of this genus 

 lives for two or three years in a holoparasitic, entirely subterranean, 

 condition. It then sends up an aerial shoot which becomes green, 

 changing the plant, therefore, to a partial parasite, and in a few 

 weeks produces flowers and fruits and then dies. Such a life history 

 is strikingly similar to that of a two-year cicada in the insect world. 

 Finally, there is the European genus, Lathraea, also belonging to the 

 Scrophulariacese, which completely lacks chlorophyll and is thus 

 holoparasitic throughout its life. 



The examples of holoparasites among the fungi and bacteria are 

 of course innumerable, their exact relations to the host plants and 

 the amount of injury they do being very variable. There are also 

 various kinds of holoparasitic animals, notably several species of 

 scale insects, that exhibit this same type of symbiotic relationship. 

 65. Ectotrophic Mycorrhizas.— A mycorrhiza, as the word sug- 

 gests, is a structure composed of root and fungus. An ectotrophic 

 mycorrhiza is one in which the fungus mycelium is found on the out- 

 side of the root and between its cells, as contrasted with endotrophic 

 mycorrhizas in which the fungus occurs inside the root cells. Ecto- 

 trophic mycorrhizas are found on various kinds of trees such as pines, 

 oaks, hickories, beech, etc. They occur in the upper layers of the 

 soil on the smallest rootlets. They usually form clusters of short, 

 stubby branches often described as "coral branching" rootlets 

 (Figs. 50 and 51). They vary in color from white to bright yellow, 

 brick red, or dark brown, the color depending upon the kind of 

 fungus. 



As many as seven different kinds of fungi have been found pro- 

 ducing mycorrhizas on the same species of tree and there is proba- 

 bly no arbitrary limit to the number that might be so found, just 

 as there is no arbitrary limit to the number of diseases to which a 

 species may be subject. There is considerable evidence, however, 

 that certain species of trees are more or less immune to the attacks 

 of many mycorrhizal fungi while other species lack such immunity. 



