398 DISEASES OF PLANTS. 



bours, and the leaves being punctured by the rigid points, so that there 

 is an extravasation of resinous juices ; when this is often repeated the 

 terminal bud at length dies. Some coniferous trees imported into 

 Britain are suSering from the nature of the climate and soil. This is 

 especially seen in the case of the Larch. Some have supposed that 

 the destruction of the Larch is owing to dryness of the soil in conse- 

 quence of draining ; while others attribute it to the propagation of the 

 tree by means of badly matured seeds, taken from specimens grown in 

 Britain. 



Concerning the influence of atmospheric changes on plants, very 

 little has been determined. Many extensive epidemics seem to depend 

 on this cause. By some the late potato-disease has been referred 

 to an unknown miasma conveyed by the air, and operating over 

 large tracts of country ; the disease probably affecting some plants 

 more than others, according to their state of predisposition, and in its 

 progress leading to disorganisation of the textures, alteration in the 

 contents of the cells and vessels, and the formation of a nidus for the 

 spores of Fungi. In the early stage of the disease, as witnessed in 

 1845, Harting observed that a brown granular matter was deposited 

 in the interior of the cells, beginning with those near the surface. 

 For some time the cell-walls and starch-grains remained uninjured, but 

 were ultimately attacked, the former losing their transparency, and 

 the latter becoming agglomerated in masses. Subsequently to this, 

 parasitic organisms of various kinds made their appearance, cavities 

 were formed, and rapid decay took place. Among the vegetable 

 parasites were detected species of Fusisporium, Oidium, Botrytis, 

 OapiUaria, Polyactis. Berkeley supports the Fungus-theory of Potato- 

 disease, while SoUy thinks that the development of parasites is a 

 secondary step in the morbid process. The prevalence of hot or cold 

 weather, the amount of light and moisture, changes in the atmosphere, 

 and electrical conditions of the air and earth, are in all probability 

 connected with epidemic diseases. Some, with Liebig, attribute the 

 late potato-disease to suppressed evaporation and transpiration, depend- 

 ing on the hygrometric state of the atmosphere. The vessels and 

 cells are said to become charged with fluids, stagnation of the circular 

 tion takes place, and thus disease and death are induced. 



Balfour Stewart remarks that the researches ^of BaxendeU, Mel- 

 drum, Smyth, and others, go to show that the convection-currents 

 of the earth are influenced by the state of the solar surface. Any- 

 thing that influences the motions of our atmosphere may readily be 

 supposed to influence the distribution and activity of those disease- 

 germs that are now believed to be present in the atmosphere. 

 Some kinds of blight seem to be associated with the prevalence of 

 certain winds. The maximum of sun-spots may be connected also with 

 the state of the atmosphere ; and great auroral outbursts are connected 



