OF PI.AXTS. 



^89 



Aiinal. Transalp. i. p. 414. ; Meidinger in Beschaftiguiigen der 

 Berlin Gesellschaft naturforschender Freiinde.) 



417. 



We now come to what are called the Diseases of Plants, or 

 to those varieties in the form and in the composition of their 

 parts, which are injurious to the fulfilment of their functions, 

 and to the continuance of their life. As plants are organic 

 bodies endowed with vital activity, the same causes which af- 

 fect animal bodies must produce similar effects upon vegetation. 

 They must, therefore, be affected in the same manner as ani- 

 mal bodies, by heat and by cold, by moisture and drought, 

 by a deficiency and by a superfluity of the ingredients of the 

 atmosphere. We have partly stated above (369, 370, 372, 

 376.), the effects of the great natural agents upon the vege- 

 table world. Among other effects, we have mentioned the 

 blighting of corn during severe storms of lightning. The 

 aerial smoke or dry-fog, which Pfaff (Ueber den heissen Som- 

 mer von 1811, s. 52.) calls a dry electrical vapour, and which 

 is apparently an impregnation of the atmosphere with sul- 

 phurous acid gas, is likewise very hurtful to plants, because, 

 by means of it, an over excitement and parching are produced, 

 and the lively green of the leaves is changed into a dirty 



brownish-vellow 



t/ 



418. 



But vegetables are subjected to a still greater number of 

 causes of disease than animals, because an innumerable crowd 

 of small parasitic plants and insects beset them, suck out their 

 juices, disturb their functions, and are injurious to their life. 

 In many cases, a diseased tendency in the plant seems to 

 favour the production, or at least the increase of these ene- 

 mies. We have already (321.) stated the production of 

 what is called Mildew, or of the sprinkling of plants with 

 aphides, to be a consequence of the unnaturally increased 

 evaporation of saccharine matter. In the same manner, 

 we observe a much more rapid and general production of 

 Lichens on gooseberry bushes which grow upon an vmfruit- 



T 



