rfOb STUDIES IN SYMBIOSIS. 1., 



having like the mycelium of an ordinary fungal parasite, depriving the host- 

 cell of certain nutritive substances. 



The early coiling of the hyphae around and in contact with the nucleus of 

 a cell, the absence of the fungus from raphide-cells, and from the active meris- 

 tematic region of the root tend to confirm the hypothesis that the growth of 

 the fungus from cell to cell and towards the cell-nucleus is controlled by ohemo- 

 tropie substances in the cells, and particularly in the region of the nucleus. The 

 absence of the fungus from the meristematic zone may be explained by the fact 

 that the chemotropic substance or substances which accumulate in the cortical 

 cells (essentially a storage zone in orchid roots) are not allowed to do so in 

 cells which are actively dividing or growing. The substances are consumed in 

 the process of cell-division and cell-growth, in the production of new cell-walls, 

 new protoplasm, in the provision of energy, etc., as fast as it is received from the 

 storage cells. In the ease of the raphide-cells it is probably the excess of oxalic 

 acid which inhibits the infection of the cell by repelling the fungal hyphae. 



After a period of active nutrition, when the fungal hyphae absorb from 

 the host-cell (probably the chemotropic substance — sugar*) and increase their 

 protoplasm or proteid as indicated by the densely-staining granular contents of 

 the hyphae, they become gradually disorganised. No hyphal structure can be 

 seen in the disintegrating mass; certain free hyphae around the periphery of 

 this mass and passing out from it spread into neighbouring cells (Text-figs. 3, 26) 

 as if to avoid destruction also. A few starch grains appear in the cell-cytoplasm, 

 but most of the carbohydrate which is liberated during the disorganization and 

 digestion of the fungal hyphae is probably conducted directly to the flowering 

 axis. At any rate it does not appear in the same quantity as exists in normal 

 uninfected cells. The migration of the free peripheral hyphae from the dis- 

 organizing central mass is probably due to the greater chemotropism of the 

 contents of neighbouring uninfected cells. For the attraction of a fungal hypha 

 there is a definite optimum concentration of a chemical substance, and Miyoshi 

 (1894) has proved that hyphae will always tend to grow towards the solution 

 which approaches most nearly to the optimum concentration. 



The progxessive development of the hyphae from the periphery of the root 

 towards the cortical cells, and from one cortical cell to another, may be explained 

 on the assumption that the chemotropic substance required by and attracting 

 the fungus increases towards the inner tissues. 



In previous studies on mycorhiza, very few facts have been ascertained in 

 regard to the reciprocal interchange of nutritive substances between the host-cells 

 and the endophytic fungTis. Several hypotheses have been put forward to ex- 

 plain the significance of both endophytic and exophytic mycorhiza. It seems to 

 me that each case of endophytic mycorhiza in a plant or group of related plants 

 may have its own peculiarities, which can only be revealed in a close study. 

 Frank gives different interpretations of ectotrophic and endotrophic mycorhiza 

 — the ectotrophic fungus assimilates, partly for the benefit of the host, humus 

 constituents; while the endotrophic fungus is an organism captured by the host. 

 There are probably numerous cases of such physiological relations between the 



•"Sections of fresh material were mounted in a few drops of the following 

 solution: — 2 grams phenylhydrazine, with 2c.c. of 50% glacial acetic acid and 

 lOc.c. of HoO, which had been shaken till clear. They were then warmed for an 

 hour in a bath at a temperature of 100°C. Fine yellowistti crystals separated out 

 in cells free from the fungus, and a few in the infected cells. The starch is con- 

 verted into sugar, which is precipitated by the above solution as a phenylhydrazone. 



