KXl'KIJlMF.N'r STATION KKPOlM'. r,21 



certain of tlu' most sustri)til)k' of the tender roses tlie <;ro\vth is so 

 rapid that the younjr leaves are nearly covered with the mildew in a 

 day or two after it i- once started. Some rose-growerg associate its 

 appearance with an ini])roper ventilation of the greenhouse, and 

 claim that it can he ])roduccd with certainty by drafts of outside air 

 striking the plants when all the other conditions are favorable for the 

 spread of the mildew. However this may be, the fact must not bo 

 lost from sight that it is a germ disease and the source of contagion 

 consists in spores that fall u])on the surface-of the suscei)til)le plant. 

 Spores, like seeds, require certain conditions for their germination, 

 and when these obtain they grow and sometimes with wonderful 

 rapidity. 



Soon after the spore has germinated it sends suckers into the host 

 'vvhereby the mildew ])lant derives its nourishment and is able to 

 spread its cobwebby threads along the surface. Shortly it begins to 

 produce uj)right branches which bear spores at their ti])s. The time 

 from the germination of the spore and the beginning of an infection 

 spot to the production of a crop of spores is measured by only a very 

 few days, and this helps to exi)lain the rapidity with which the 

 fungus covers a whoJe ])lant or fills a rose-house with the mildew. 



The production of the spores, as above mentioned, continues in- 

 definitely and in numbers beyond computation. They are the '•sum- 

 mer spores," and designed for rapid germination during the grow- 

 ing season. A second form of spore is very different from this and 

 arises slowly within the perithicia before mentioned. These are 

 called the "winter spores," and are designed to pass the winter as 

 such and germinate the following spring or summer. The spore- 

 case arises from a union of threads, and, after producing a nearly 

 spherical shell, the spores are developed in sacs within. Situated as 

 they are witliin the sacs, which in turn ari' inclosed by the thick, 

 brown wall of the perithicium, the spores are protected from the de- 

 structive elements, like peas in their pods, the pods being in. for 

 example, a tightly-covered basket. 



r|)on some plants only the summer form of si)ore is found, but 

 as a rule the perithicia are formed late in the season, and l)y the time 

 the leaf is ready to fall the white of the early appearance of the 

 mildew is replaced by the brown of the jierithicia. which in some 

 instances are so numerous as to almost cover the whole infested sur- 

 face of the host. This thick, l)rown coating of the leaf in autumn 

 is suggested in the ])ieture of the phlox Icnf nt I. in rintr XII. 



