Re-productive Organs. 



CHAPTER III. 



REPRODUCTIVE ORGANS SPORES. 



A detailed account of the different spore-forms will be given in the sys- 

 tematic part in connexion with the vairious species ; but it will be convenient 

 here to take a preliminary view of some of the more essential general 

 features, such as distribution, germination, and infection, suppression or 

 omission, and repetition of spore-forms. 



DISTRIBUTION. 



The most important means of distribution of the rust-spores, as for 

 fungus spores generally, is the wind. They are usually exposed on 

 longer or shorter stalks, often powdery, and their immense numbers and 

 lightness all render them easily detached, and spread by the slightest 

 breath of wind. 



That the wind is an important factor in the distribution of rust-spores 

 is strikingly shown in an example given by Halsted 1 . The asparagus 

 rust (Puccinia asparagi, DC.) was very bad, but in one field the plants had 

 been cut over, and the rusty brush removed in order that the new growth 

 might escape. On examining this field about five weeks afterwards, he 

 found that the rust showed only on one side of the green plants, and that 

 was the side exposed to an old and very badly rusted! asparagus bed. That 

 this was the source of infection was obvious from the fact that a house with 

 a few trees around it intercepted a portion of this field, and there was less 

 rust upon that portion of the new bed in line with the house. 



The dust-storms which occur will also be a fruitful source of spreading 

 them, and in our northern areas where the dust is sometimes carried in 

 such quantities as to obliterate fences, it can easily be understood how even 

 in virgin soil the spores of wheat rust may be found. 



The rain will also help to scatter them, particularly over individual 

 plants, and on the surface of the soil, for it is a common observation how 

 rusty plants are cleared after a heavy shower. 



Insects as well as other animals serve the same purpose. I have often 

 observed the larvae of a species of Cecidomyia feeding on uredo and 

 aecidio-spores, and at the same time spreading them while crawling along. 

 Lindroth 1 has observed in Finland the occurrence of these larvae on no 

 less than sixty-one species of rusts, and similar larvae have been found feed- 

 ing on the conidia of various species of Oidium (Salmon 1 ). In some pot 

 experiments with wheat, I found the larvae associated with the Oidium of 

 E. graminis, and feeding on the conidia. At first sight it might seem as if 

 this devouring of the spores would tend to reduce the fungus, but the 

 wholesale way in which the spores are carried about more than counter- 

 balances any decrease from this cause. 



GERMINATION. 



As a rule, uredo and aecidio-spores germinate easily in a damp chamber, 

 and I have found the method recommended by Plowright to be very con- 

 venient. A gardener's propagating glass is used, placed on a plate of 

 water, and a simple stand is made with two flat oblong pieces of gutta- 

 percha. Holes are easily bored in them, and glass rods placed parallel 

 !>ehveen them, so that the microscopic slides may be laid across the rods. 



