ARTIFICIAL CULTURES 63 



of a genus may have totally distinct rusts upon them, as in the 

 genus Galium. So far there is nothing that would contravene 

 one's expectations, but it is impossible to avoid a little in- 

 credulity when one is told that on Hieracium there is a 

 Puccinia which is confined strictly to a single form of a variety 

 of a subspecies of a species of that genus (Probst, 1909). 



These specialisations can be proved only by artificial cultures. 

 Certain experimenters have developed very successful methods 

 of infecting given plants with the spores. The ascidio- or uredo- 

 spores are the handiest for this purpose. A sorus of mature 

 uredospores is removed from the leaf, placed in a drop of water 

 and broken up with a needle ; the spores are then thoroughly 

 shaken up with a suitable quantity of distilled water. The soil, 

 in which the plant to be used is growing, should have been 

 well watered before the experiment begins. The leaves to be 

 inoculated are first sprayed with distilled water from an atomiser, 

 and then the liquid containing the spores is similarly sprayed 

 upon them, naturally upon a surface which possesses stomata. 

 The plant is then placed under a bell-glass for 24 — 48 hours or 

 longer, and afterwards kept in a greenhouse at a suitable tem- 

 perature, protected if required by a larger glass shade with 

 sufficient ventilation. A similar uninoculated plant should be 

 kept near it as a control. In spraying, too great a quantity of 

 moisture should be avoided ; in nature it is observed that the 

 germination of spores succeeds best in a layer of dew, not of 

 rain. The keeping under a closed bell-glass is for the purpose 

 of allowing the germ-tubes to enter the stomata ; after that 

 nothing more is required but to grow the plant in the ordinary 

 way. The result of the infection will begin to appear in about 

 10 — 20 days or more, according to the species. 



The inoculation with teleutospores which are ready to 

 germinate may be effected in the same way ; or a leaf with 

 mature teleutospores may be tied, spores downwards, on the 

 plant at the selected spot, and left to itself or protected for a 

 time with a layer of wet cotton wool. If it be required to 

 discriminate, under special circumstances, between the artificial 

 infection and any accidental one that might occur, the leaf to 

 be used may be marked with lines in waterproof ink, and the 



