344 PROCEEDINGS OF SECTION D, 



province of Marn, where — as in the neighbouring arrondisements — 

 the soil reposed on a great bed of chalk, and though rich in nitro- 

 genous matter, was accordingly largely composed of carbonate of 

 lime, the wheat crop was always very subject to rust. With 

 ]\I. Gneyraard's opinion before him, he was now able to explain 

 this, as also the fact, which he too had noticed, that all varieties 

 of wheat did better on land which contained silica, that all wheats 

 seemed to requii'e this mineral — even the short-bearded varieties 

 Avhich could do with the least amount of any ; and lastly, that 

 VI hen wheat growing in such a soil as he had described, was 

 manured with compost rich in nitrogenous plant food-substances, 

 the crop, as far as immunity from rust was concerned, did best 

 which received the manure which, though in other respects the 

 poorest, contained the largest amount of silica. 



The opinion too receives further corroboration from tlie expe- 

 rience that, of so-called rust-resisting wheats, the hard wheats, 

 i.e. the pericarp of whose seed contain an unusually large 

 amount of silica, are those which withstand the attacks of this 

 fungus in the greatest degree, and the unusually large amount of 

 silica contained in the skin of these hard wheats is indicative of 

 the fact that the whole plant is unusually rich in this mineral 

 also. 



No attempt to arrive at an explanation of the role performed 

 by silica in this connection has, as far as we can learn, ever been 

 made. Now it may be assumed that, the rust-fungus lives and 

 grows at the expense of bodies genetically related to sugar, i.e. the 

 glucosides, or soluble forms of starch, which it finds in the 

 perrenchymatous tissue of the leaf, where it is located either within 

 the cells, or the cell walls, or as most frequently, in the intercellular 

 spaces. And that it does so subsist at the expense of the soluble 

 forms of starch and gluten, is plainly indicated by the fact that 

 these bodies are almost absent in " rusted" wheat plants. This 

 food-material is derived from the starch of the neighbouring 

 chlorophyl-containing tissue, from which it passes by a process of 

 osmosis, glucosides possessing the greatest power of endosmosis of 

 all vegetable substances of equal density with them. The degree 

 of osmosis, however, for the same body, varies with the same 

 composition or nature of the membrane through which it takes 

 place, and nothing so much determines its amount as the presence 

 of silica to a greater or less extent, vegetable substances especially, 

 as is a well ascertained fact— being permeable by osmosis in 

 indirect proportion to the amount of silica which they contain. 

 The formative substance then of the cell walls of the tissue of a 

 wheat-plant when largely composed of silica, would, therefore, 

 form a much greater hinderance to the passage of these food 

 materials of the Puccinia to the tissue iu which this fungus grows, 

 than would one in which silica was little developed; in other 

 words, non-siliceous wheats would be those which were most favour- 

 able for the support of the Rust-fungus. 



