80 SYLVA AMERICANA. 



9. Suffocation. Sometimes it happens that the pores of the 

 epidermis are closed up and transpiration consequently obstructed, 

 by means of some extraneous substance that attaches itself to and 

 covers the bark. This obstruction induces disease, and the 

 disease is called suffocation. Sometimes it is occasioned by the 

 immoderate growth of lichens upon the bark covering the whole 

 of the plant, as may be often seen on fruit trees, which it is 

 necessary to keep clean by means of scraping of the lichens, at 

 least from the smaller branches. For if the young branches are 

 thus coated, so that the bark cannot perform its proper 

 functions, the tree will soon begin to languish, and will finally 

 become covered w T ith fungi inducing or resulting from decay, 

 till it is at last wholly choaked up. But a similar effect is also 

 occasionally produced by insects, in feeding upon the sap or 

 shoot. This may be exemplified in the case of the aphides, 

 which sometimes breed or settle upon the tender shoot in such 

 multitudes as to cover it from the action of the external air 

 altogether. Sometimes the disease is occasioned by an extrava- 

 sation of juices which coagulate on the surface of the stalk, so as 

 to form a sort of crust, investing it as a sheath, and preventing its 

 further expansion. Sometimes the disease is occasioned from 

 want of an adequate supply of nourishment as derived from the 

 soil, in which the lower part of the plant is the best supplied, 

 while the upper part of it is starved. Hence the top shoots 

 decrease in size every succeeding year, because a sufficient 

 supply of sap cannot be obtained to give them their proper 

 developement. This is analogous to the phenomena of animal 

 life when the action of the heart is too feeble to propel the blood 

 through the whole of the system. For then the extremities are 

 always the first to suffer. And perhaps it may account ajso for 

 the fact, that in bad soils and unfavorable seasons, when the ear 

 of barley is not wholly perfected, yet a few of the lower grains 

 are always completely developed ; which not only shows the 

 great care of Providence for the preservation of the species, but 

 points out also the efficient cause. 



10. Contortion. The leaves of plants are often injured by 

 means of the puncture of insects, so as to induce a sort of 



