242 TEXTBOOK OF PLANT PHYSIOLOGY 



enzymes, which make it easier for tne parasite to penetrate into the 

 cell and use the food found therein. The most important of these 

 enzymes is cytase, which dissolves the cell membrane in the path 

 of the parasite. Closely related to the enzymes are the toxins, 

 special substances which poison the host, thus lowering its resist- 

 ance to the attack of the parasite. 



In general, parasites are rather narrowly adapted to particular 

 nutritive substances, which they find only in certain plants. The 

 cause for this specialization is not yet understood. It is possible 

 to change many of them to a saprophytic life by cultivating them on 

 broth or infusions from tissues of the host plant. 



The relationship between many trees and shrubs and certain 

 fungi thickly enveloping their roots and even penetrating into their 

 cells, is an odd phenomenon and very little understood at present. 

 These fungal formations are called mycorrhizae. When the 

 hyphae of the fungus cover the root and penetrate only into the 

 intercellular spaces of its bark, we have an ectotrophic mycor- 

 rhiza. These are distinguished from the endotrophic form, in 

 which the hyphae of the fungus penetrate into the interior of the 

 cells of the root and become firmly established therein. The 

 endotrophic mycorrhizae are widespread among the orchid family, 

 where they form such a permanent integral part of the host plant 

 that some authors are inclined to compare orchids to lichens and 

 view them as complex organisms, composed of a fungus and a 

 higher plant, just as the lichens are composed of a fungus and an 



alga. 



In some cases the symbiotic relation is even hereditary, as the 

 fungus hyphae penetrate into the seeds and at germination begin 

 to grow with the growth of the first rootlet. Notwithstanding such 

 a close tie between fungi and roots, the significance of mycor- 

 rhizae is not clear to us. But since the majority of the plants sup- 

 plied with mycorrhizae grow on soils rich in decayed material, 

 where saprophytic fungi are the dangerous competitors of the 

 higher plants in the use of nitrogenous and mineral substances, 

 many authors suppose that the fungous mycelium entering into the 

 composition of mycorrhizae is used by the host as an intermediary 

 in the task of getting nitrogenous and mineral substances. The 

 host plant, usually supplies these fungi with non-nitrogenous com- 

 pounds. From this point of view mycorrhizae may be considered 

 as examples of symbiosis between fungi and higher plants. The 



