EUSTS. 97 



go, it may he treated as a distinct parasite, though advanced 

 botanists may look upon it as offering a case of what they call 

 dualism, which may be easily understood on a comparison with 

 such plants as marigold and orache, which produce seeds of 

 two different forms. Its ultimate effects may, indeed, be inju- 

 rious, as, if it be really a second form of mUdew, it is capable 

 of propagating that pest. The case is quite different with 

 Trichobasis linearis, which is in fact merely the young state of 

 the mUdew. When once that supposed species makes its 

 appearance, it is quite certain tliat there will be mildew to a 

 greater or less extent. While the spores of the one remain 

 unaltered, though intermixed with the true bipartite spores of 

 the mUdew, the other exhibits every intermediate state of form 

 and colour. 



M. Tulasne, to whom we have already alluded as 

 a high authority on this subject, is of opinion that 

 all the species enumerated in this group or genus, 

 are mere conditions of other species, and therefore 

 excludes it altogether. Another genus (Lecythea), 

 for the same reason, has shared the same fate. 



It must not be forgotten that above thirty species 

 of rust belonging to this genus are now recorded 

 as occurring in Great Britain. Some of these have 

 yellow or yellowish spores ; the rest have the spores 

 of a decidedly brown colour. One of the most at- 

 tractive of the yellow-spored species is that which 

 appears on the groundsel* (plate VII. fig. 145). It 

 is not always to be found without searching closely, 

 for in some localities we have found it very spar- 

 ingly, whilst elsewhere — as, for instance, on Wands- 



* Now regarded as a species of Cohosponum. 



H 



