ATMOSPHERE IN EELATION TO HUMAN LIFE AND HEALTH. 305 



the barberry plant for one of its stages of development. The spores 

 of mildews and microscopic fungi are generally ejected in great num- 

 bers and with some force into the air, and are carried from plant to 

 plant, or field to field, by the air, as, for instance, the potato disease, 

 Peronospora infestans, and the mildew of the coffee plant. Heat and 

 moisture, dew and gentle rain, are favorable to the growth and spread 

 of most diseases of plants. The fungus of dry rot grows in damp, 

 unventilated places on badly seasoned wood, and when about to 

 produce spores, seeks the light; its sporangia dry up and discharge 

 innumerable spores. The common ferment of grape juice, the Saccha- 

 romyces elUpsoideus, grows on the surface of the grape, and when it 

 gains access to the fermenting vats develops enormously by budding 

 and division; when its development is hindered, as by drying up of the 

 liquid, spores are formed, which are capable of resisting dryness, high 

 temperature, and various conditions without losiug their power of ger- 

 mination. They may thus be carried alive to a new habitat. This 

 action is characteristic of a great number of ferments, of minute fungi, 

 and of microbes generally, and explains the transmission of many dis- 

 eases both of plants and animals. The globular spore case of mold, 

 such as appears on fruit, jam, bread, etc., scatters its spores in all direc- 

 tions, each spore being about one three-thousandth of an inch in diam- 

 eter. These float in the air in great numbers. The spores of oidium, 

 again, a vine disease, escape into the air as fine dust, and spread with 

 extreme facility. The sudden appearance of potato disease in a field 

 is due to the field having been sprinkled with the spores of the perono- 

 spora in dry weather, and to the quick development of the zoospores 

 when favored by damp, either rain or dew. The smut of corn produces 

 extremely light spores, about one five-thousandth of an inch in diame- 

 ter; these float in the air, and have so strong a resistent power that 

 they will germinate in water after having been kept for years in a dry 

 place. The peziza of the lily disease fires off ascospores which are 

 carried by the wind to rich soil where they germinate, produce hyphre, 

 bore into the tissues of the plant, and shed millions of spores around. 

 A disease of the pine is associated also with the groundsel, on which 

 the fungus spends a portion of its existence. The hop mildew is borne 

 by the wind, and has been found to be to some extent averted from 

 threatened fields by thick woods or large hedge rows. 



A great deal of disease in plants and forests is produced through 

 wounds, to which the air conveys fungi which accelerate decay. The 

 decomposed organic matter becomes a suitable soil for the development 

 of fungi, which are not parasitic on living parts, and spores from these 

 are very abundant. The hyphse of the disease fungus follow up the 

 poisonous action of the juices of the mold fungus and spread into the 

 contiguous wood. True wound parasites also alight on the damp sur- 

 face of a cut or broken branch and extend their mycelium into the 

 living tissues, gradually bringing about the death of the tree. 

 SM 95 20 



