SECT. II. DISEASES. 4Q9 



the case with the radicles of Secale cereale, and the 

 same result may also arise from poverty of soil. 



SUBSECTION Dt. 



Suffocation. Sometimes it happens that the 

 pores of the epidermis are closed up and transpira- 

 tion consequently obstructed, by means of some ex-* 

 traneous 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 in fruit trees, which it is ne- 

 cessary to keep clean by means of scraping off the 

 Lichens, at least from the smaller branches. For if 

 the young branches are thus coated, so as that the 

 bark can not perform its proper functions, the tree 

 will soon begin to languish, and will finally become 

 covered with 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. It may be ex- 

 emplified also in the case of Coccus Hesperidum and 

 Acarus tdlarius, insects that infest hot-house plants, 

 the latter by spinning a fine and delicate web over 

 2 K 2 



