838 REPORT— 1901. 



as negative all cases where no pustules appear — distinct Hecks were formed ou the 

 leaves ; eight j-four plants of the same species {B. mollis) were inoculated with spores 

 derived from B. sterilis but none succeeded, and eight were tried with spores from 

 B. secalimis, of which three (.37"5 per cent.) succeeded and five failed. 



Attain, looking at B. sterilit', we find that eighty-six out of ninety attempts to 

 infect this species with spores from B. mollis failed, whereas sixty-eight out of 

 eighty-four (81 per cent.) attempts to inoculate the same species succeeded when 

 the spores nsed were derived also from B. sterilis. All the eighteen plants inocu- 

 lated with spores from B. secaliims proved immune. 



Having regard to the morphological groups of these Bromes, it is found that 

 any given species or variety is most easily infected by spores which have been 

 grown on the same species or variety, less certainly by spores from allied species, 

 and not at all successfully by spores from a species in another group. Some in- 

 teresting details regarding the relations between host and parasite in infection are 

 also to hand. 



Three stages of development on the part of the uredo must be distinguished : 

 (1) The germination of the spore and development of the germ-tube ; (2) the en- 

 trance of the latter as an infection-tube through the stoma of the grass ; and (3) 

 the growth of the latter into a branched mycelium in the intercellular spaces of 

 the host, into the cells of which it sends haustoria, and liually — about the tenth 

 day after sowing — again puts forth spores as it breaks out through the stomata in 

 the form of the well-known rust-pustules. This last period may be termed the 

 incubation period. 



The author finds that various exigencies, especially of the weather, affect the 

 fungus during each of these three periods. 



Infection may fail because the temperature is too high or too low during the 

 germination period, or the germ-tubes may dry up, or be killed in other ways. 



On reaching a stoma the successful entrance of the germ-tube, as an infecting 

 tube, depends on various factors, of which the specific nature of the Brome attacked 

 is an important one. Taking spores derived from B. mollis, for example, their 

 germ-tubes appear to so corrode and destroy the tissues of B. sterilis that the spot 

 where the sowing is made turns black and dies, and no successful infection occurred ; 

 on B. ma.rimus, B. inennis, and others, on the other baud, no successful attack is, 

 as a rule, established at all. 



Even when the infecting tube has established an entrance, several events maj' 

 intervene to prevent successful infection; i.e., the formation of a normal inter- 

 cellular mycelium which dominates the tissues and ultimately breaks forth from 

 the stomata again as pustules with fresh crops of uredo-spores. 



If the host is starved of carbohydrates by partial etiolation, or of minerals by 

 lack of supplies in the soil, or by interference with the transpiration, &c., the 

 mycelium — even in a species normally quite suitable to the parasite — only drags 

 on a miserable existence and has not strength to form spores. In such cases 

 nothing further results than the development of pale, feeble fiecks on the leaf. 

 The same thing occurs in some partially immune species, even though flourishing, 

 evidently owing to the refusal of the cells to allow the mycelium to dominate 

 their life. 



These antagonistic reactions of the host-plant are not due to any structural 

 peculiarities discoverable by the microscope ; nor is it a simple matter of the 

 excretion of any poisonous soluble constituent of the sap, judging from the experi- 

 ments in which uredo-spores derived, for example, from B. mollis germinated 

 satisfactorily in both boiled and unboiled aqueous extracts of the leaves of B. sterilis, 

 which had been previously filtered through stone filters under pressure. In addi- 

 tion to the case of successful and normal infection, therefore, three distinct cases 

 of failure to infect can be distinguished : (1) in which the preliminary establish- 

 ment of an infecting mycelium is assured, but this remains dormant, i.e., fails to 

 dominate the living cells of the leaf, and only a pale yellowish fleck results ; (2) in 

 which the attack of the germ-tube is so vigorous that it kills the guard-cells and 

 tissues, and produces a black corrosion spot in which the parasite can make no 

 progress ; (.3) complete immunity ; the parasite fails to get any hold on the leaf 



