DISPOSITION OF PLANTS TO DISEASE. 59 



Hartig ^ also found that Agaricus melleus, in penetrating into 

 stools of oak, only killed those cells which, as it were, rested, 

 whereas the cells of parts in communication with stool-shoots 

 are not attacked. Likewise, Schwarz states that the mycelium 

 of Cenangium abietis only extends through pine-shoots at a time 

 when there is little vegetative activity. 



Accidental disposition depends largely on the nature of the 

 epidermis enclosing plant-organs. The stems of many plants are 

 protected from intruding fungi from the time the epidermis is 

 replaced by a corky layer, still better after a bark is formed. 

 Hence young shoots are in a condition of greater disposition 

 than older ones. There are, however, various grades of dis- 

 position to be observed, even when a simple epidermis forms 

 the only covering, as is the case with most leaves, flowers, and 

 many fruits. The newly-formed epidermis is, as a rule, most 

 disposed while its walls are still delicate and uncuticularized, 

 hence many organs are exposed to attacks of fungi only in their 

 youngest condition. It is easy to infect and kill young leaves, 

 and shoots of conifers with Botrytis Douglasii, whereas older 

 needles will remain quite unharmed. Similarly with Ghrysomyxa 

 rhododendri on spruce-needles, Calyptospora Goeppertiana on 

 silver fir, and others. Flowers are also more easily infected 

 in the young stage, e.g. cones of spruce by Aecidium strdbilinum. 



-During early youth plants are insufficiently protected from 

 great cold and drought, and also from infection by parasitic 

 fungi. This may be be,cause the young non-cuticularized walls 

 offer less resistance to the germ-tubes and haustoria, or because 

 they are more permeable to any ferment excreted by the fungus. 

 Organs developed late in the vegetative season resemble those 

 in the spring-condition in that they have not as yet matured, 

 and are but poorly protected against extremes of temperature, 

 or attacks of parasites. 



The condition of disposition may be easily promoted for 

 purposes of artificial infection, by cultivating the host-plants in 

 a moist chamber, or under a bell-jar. The same condition 

 may easily arise in glass houses or hot-beds, hence one has, by 

 means of constant ventilation, to guard against it. 



Many diseases of seedlings {e.g. Phytoplithora omnivora, 

 and Pythium) are only to be feared so long as the stems of 

 ^ Forstl.-ncUurwi/is. Zeitschrift, 1894. 



