XIIL] "DAMPING OFF" OF SEEDLING-TREES. 273 



only obtain, after a certain stage of germination, by 

 means of the normal activity of its own green coty- 

 ledons or leaves, properly exposed to light, air, &c. 

 At the same time, it is not to be forgotten that, as 

 conditions which favour the spread of tlie disease to 

 be described, the above factors and others of equal 

 moment have to be taken into account : which is in- 

 deed merely part of a more general statement, viz. 

 that, to understand the cause and progress of a disease, 

 we must learn all we can about the conditions to which 

 the organisms are exposed, as well as the structure, 

 &c., of the organisms themselves. 



First, a few words as to the general symptoms of 

 the disease in question. In the seed-beds, it is often 

 first noticeable in that patches of seedlings here and 

 there begin to fall over, as if they had been bitten or 

 cut where the young stem and root join, at the surface 

 of the ground : on pulling up one of the injured seed- 

 lings, the "collar," or region common to stem and 

 root, will be found to be blackened, and either rotten 

 or shrivelled, according to the dampness or dryness of 

 the surface of the soil. Sometimes the whole of the 

 young root will be rotting off before the first true 

 leaves have emerged from between the cotyledons ; in 

 other cases, the " collar " only is rotten, or shrivelled, 

 and the weight of the parts above ground causes them 



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