DISEASES 'OF PLANTS. 399 



with the epochs of maximum sun-spots. When we arrive at an ex- 

 planation of the sun-spots we may be able to prove some connection. 



Gangrene in plants is caused by alterations in the contents of the 

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

 this disease is apt to occur. It is capable of extension by contact of 

 the diseased cells. 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 com and other agricultu- 

 ral crops are owing to the production of Fungi. These have been 

 divided into 1. Those attacking the grain, as Uredo fostida, pepper- 

 brand. 2. Those attacking the flower, as tlredo segetum, smut. 3. 

 Those attacking the leaves and chaff, as Uredo Eubigo, rust. 4. 

 Those attacking the straw, as Puccinia graminis, com mildew. 



Bunt, smut-balls, pepper-brand, or blight, is a powdery matter, occu- 

 pying the interior of the grain of wheat, etc. 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 

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

 the parasitic fungus has a peculiarly fetid odour, hence called stinking 

 rust. 



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 ■jao'oaa part of a square inch he, counted 49 spores of the 

 uredo. 



Rust is an orange powder, exuding from the inner chaff scales, and 

 forming yeUow or brown spots and blotches in various parts of corn 

 plants. It owes its presence to the attack of Uredo Eubigo. It is 

 sometimes called red gum, red robin, red rust, and red raff. Some con- 

 sider Uredo linearis as another state of the same disease. 



Mildew is a disease caused by a Pungus denominated Puccinia 

 graminis. The ripe spore-cases of this plant are small dark-brown 

 club-shaped bodies, their thicker end being divided into two chambers, 

 each filled with minute spores, and their lower end tapering into a 

 fine stalk. The sori or clusters of spore-cases burst through the 

 epidermis, sometimes in vast numbers. The minute spores seem to 

 enter the plant by the stomata. Some think that they, as well as 

 other minute spores, are absorbed by the roots. The disease attacks 

 Wheat. Spring Wheat is less liable to this disease than winter 

 Wheat, and heavy soUs are less subject to it than light ones. Many 

 have supposed that the Barberry is in some way connected with the 

 production of Mildew. It has been ascertained that there is a curious 

 connection beween them. It has been shown that the Pungus called 



