178 31ississippi Valley Horticultural Society. 



kind, aiie or size is exempt, though some varieties upon the whole sutier less 

 than others, and on some kinds of soils and exi)0sures less damage occurs. 



In studying the matter somewhat closely, four principal kinds of injury 

 have been found, viz: by insects, rabbits and mice, by blight and by frost. 

 Of the tirst and second nothing is to be said in this place, except to mention 

 the wood borers and the so called " woolly aphis " as the insects speciallj' re- 

 ferred to. The third must also be dismissed with a few words. I have else- 

 where argued that the so-called "sun-scald'' is really blight, produced by 

 bacteria, and this opinion is here further insisted upon, a large number of 

 recent examinations making it impossible to do otherwise. In this case the 

 bark adheres firmly to the wood, at least until it is quite rotten. Not unfre- 

 quently new bark forms beneath the old, and so heals the wound and saves 

 the tree. The injury occurs on any part of the trunk and on any side, but is 

 much more common on the south or southwest side, and a tree that leans to 

 the northeast is much more liable to suffer. This is the reason that many have 

 attributed the damage directly to the sun, and have kept in vogue the pop- 

 ular name. In one sense the effect of the sun is a cause of the mischief; but 

 it may be confidently asserted that the injury now considered is in no sense 

 a scald, produced by heat. The same thing can be artificially brought about 

 by inoculation with blight bacteria — has been done — and the inoculations 

 succeed' as well on the north as on the south side. But the influence of the 

 Sim upon the outer bark of trees is well knowii to every one who has fre- 

 quented the woods without a compass, and to every one else who has been 

 sharp in his observations. The outside corky envelope, old and dead, is much 

 more deeply cracked and furrowed on the south than on the north side, and 

 it does not usually take long to discover if one looks, especially in early sum- 

 mer, that many of these cracks exte;id to the living cells, which are thereby 

 exposed to the contagion of blight. So long as the bark of young trees re- 

 mains smooth, and is otherwise free from wounds, we do not find this affec- 

 tion of the trunks. When trees lean from the sun the rays of heat fall 

 more nearly at right angles to the surface, and are thus more effective in de- 

 stroying the elasticity of the outer protecting bark. Kains, too, may more 

 readily wash fine material into the cracks and thus in numerous cases be the 

 means of infection. Beginning from without, it is not very uncommon for 

 blight to be confined to the living cellular layer outside of the bast. In case 

 the cambium layer is not invaded new bark may be formed, and the tree 

 saved as already said. Evidently keej)ing the tree erect or leaning to the 

 southwest, and providing a shield of some kind, winter and summer, to pro- 

 tect the bark from drying and cracking, are protective mejisures, and they 

 are the best we have. 



We i)jvss now to the fourth cause of injury, namely, freezing; and shall en- 

 ter more into detail. 



The injury due to frost, so far as the trunks of our trees are concerned, is 

 of two kinds. In one case the wood and bark is split so as to gape open 

 while frozen, though the crack may l)e closed after warm weather comes 

 again. Sometimes only the bark thus cracks, and then there is left a more 



