88 SYMBIOSIS. 



symbiosis, in which the l'un<4us becomes the nurse or feeder, I 

 distincruish as Xntricism. Between the case just cited and 

 that in which the fungus is a pronounced root-parasite on 

 greeu plants, there exists every possible intermediate stage. 



l>efore nutricism is considered in detail it would be well to 

 exemplify briefly from the ranks of plant-parasites, that pheno- 

 menon of individuation so sharply defined in the lichens. 

 A large number of parasitic fungi cause local cell-enlargement and 

 cell-increase, with the frequent result that an attacked plant- 

 organ becomes very much enlarged and its form much changed. 

 One speaks in such cfises of hypertrophy and hypertrophied 

 organs. It is quite evident that in cases of hypertrophy the 

 attacked part must be better nourished, otherwise it could never 

 sustain the great increase in number and size of its cells. The 

 hypertrophied organ is, in fact, indebted to the surrounding 

 healthy parts for its additional nourishment ; in other words, 

 the place of demand draws to itself the materials it requires. 

 Tliis is all the more necessary when the region of increased 

 growth is deficient in, or altogether devoid of, chlorophyll, and 

 thus quite dependent on the assimilating green parts. This 

 is frequently the case, as in the scales of alder catkins attacked 

 by E.i'onscus alni incanae, in the needles of silver fir deformed 

 by Accidmm elatinum, or in the yellow needles on spruce 

 resulting from Arcidium coruscans. So also must the woody 

 swellings of branches attacked by Accidium elatinum, Gymno- 

 i^poranfjium mhinac, and other fungi, be produced at the cost of 

 neighbouring parts of the host. The hypertrophied organs be- 

 have, in fact, like these plant-organs — flowers, roots, etc. — 

 which are normally deficient in chloropliyll, and to which 

 plastic material must be supplied. 



In other cases the part of a plant attacked by fungi behaves 

 like a specialized organ, and, in combination with the fungus, 

 attains to a certain degree of independence. The so-called 

 "witches' brooms" furnish an interesting example. It is a well- 

 known fact that the direction of growth of the main axis of 

 plants is negatively geotropic, whereas that of the lateral branches 

 is only a modified form of this condition. If the terminal bud 

 of a tree {e.g. a spruce or fir) be removed, then one or more 

 lateral branches, or even buds of those branches, will exhibit 

 an increased negative geotropism. This is very marked in 



