332 DISEASES OF PLANTS. 



on the hygrometric state of the atmosphere. The vessels and cells are 

 said to become charged with fluids, stagnation of the circulation takes 

 place, and thus disease and death are induced.* 



692. Gangrene in plants, is caused by alterations in the contents of the 

 cells, leading to death of a part. In succulent plants, as Cactuses, 

 this disease is apt to occur. Sometimes excision of the diseased part 

 checks the progress of the gangrene. Canker, which attacks Apple 

 and Pear trees, is a kind of gangrene. Some of the most important 

 diseases of corn and other agricultural crops, are owing to the pro- 

 duction of Fungi. These have been divided into 1. Those attacking 

 the grain, as Uredo fcetida, or pepper-brand. 2. Those attacking the 

 flower, as Uredo segetum, or smut. 3. Those attacking the leaves and 

 chaff", as Uredo Kubigo, or rust. 4. Those attacking the straw, as 

 Puccinia graminis, or corn mildew. 



693. Smut-balls, pepper-brand, or blight, is a powdery matter, occu- 

 pying the interior of the grain of wheat, &c. When examined under 

 the microscope, it consists of minute balls, four millions of which may 

 exist in a single grain, and each of these contains numerous excessively 

 minute sporules. It is caused by the attack of Uredo Caries, or 

 fcetida. In this disease the seed retains its form and appearance, 

 and the parasitic fungus has a peculiarly foetid odour, hence called 

 stinking rust. 



694. Smut or dust-brand is a sooty powder, having no odour, found 

 in Oats and Barley, and produced by Uredo segetum. The disease 

 shows itself conspicuously before the ripening of the crop. Bauer 

 says that in TWooo P art of a square inch he counted 49 spores of the 

 uredo. 



695. Rust is an orange powder, exuding from the inner chaff" scales, 

 and forming yellow or brown spots and blotches in various parts of 

 corn plants. It owes its presence to the attack of Uredo Rubigo. It 

 is sometimes caUed red gum, red robin, red rust, and red rag. Some 

 consider Mildew (Uredo linearis) as another state of the same disease. 



696. Those Fungi which are developed in the interior of plants, and 

 appear afterwards on the surface, are called entophytic (ivrog, within, 

 and QVTOV, a plant). Their minute sporules are either directly applied 

 to the plants entering by their stomata, or they are taken up from the 

 soil. Many other Fungi grow parasitically on plants, and either give 

 rise to disease ; or modify it in a peculiar way. Among them may be 

 mentioned species of Botrytis, Fusisporium, Depazia, Sclerotium, Fu- 

 sarium, and Erysiphe. Fusisporium solani is considered by Martius 

 as the cause of a certain disease in the Potato. In the recent potato 

 disease, the Botrytis infestans, a species of Fusarium and other Fungi, 

 committed great ravages, spreading their mycelium or spawn through 



* See remarks on this subject by Klotzsch, translated by Gregory, in the Appendix to Liebig's 

 work on the Motion of the Juices. London, 1848. 



