DISEASES OF PLANTS. 331 



can be completed. It is, however, of great importance, whether we 

 regard its bearing on the productions of the garden or the field. Some 

 have divided the diseases of plants into general, or those affecting the 

 whole plant, and local, or those affecting a part only. A better 

 arrangement seems to be founded on their apparent causes, and in 

 this way they have been divided by Lankester into four groups. 1. 

 Diseases produced by changes in the external conditions of life ; as 

 by redundancy or deficiency of the ingredients of the soil, of light, 

 heat, air, and moisture. 2. Diseases produced by poisonous agents, 

 as by injurious gases, or miasmata in the atmosphere, or poisonous 

 matter in the soil. 3. Diseases arising from the growth of parasitic 

 plants, as Fungi, Dodder, &c. 4. Diseases arising from mechanical 

 injuries, as wounds and attacks of insects. 



690. Plants are often rendered liable to the attacks of disease by 

 the state of their growth. Thus, cultivated plants, especially such as 

 become succulent by the increase of cellular tissue, appear to be 

 more predisposed to certain diseases than others. Concerning the 

 first two causes of disease very little is known. Absence of light 

 causes blanching, which may be looked upon as a diseased state of the 

 tissues. Excess of light may cause disease in plants whose natural 

 habitat is shady places. Excess of heat is sometimes the occasion 

 of a barren or diseased state of some of the organs of the flowers, and 

 frost acts prejudicially on the leaves, stem, and flowers. By excess 

 of moisture, a dropsical state of the tissue is induced. 



691. Concerning the influence of atmospheric changes on plants, 

 very little has been determined. Many extensive epidemics seem to 

 depend on this cause. Thus, the late potato disease must be traced, 

 apparently, to some 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 disorganization of the textures, alteration 

 in the contents of the cells and vessels, and the production of Fungi, 

 &c. In the early stage of the disease, 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 trans- 

 parency, and the latter becoming agglomerated in masses. Subse- 

 quently to this, parasitic organisms of various kinds make their appear- 

 ance, cavities were formed, and rapid decay took place. Among the 

 vegetable parasites, were detected species of Fusisporium, Oidium, 

 Botrytis, Capillaria, Polyactis, &c. 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. By some, the late potato disease 

 is attributed to suppressed evaporation and transpiration, depending 



