THE PAGE OF NATURE. 



260 



has painful knowledge of the disease called mildew (Puc- 

 cinia Graminis, Fig. 37). It attacks the leaves and 

 culms of corn, as well as many of the grasses employed in 

 hay-making, and proves most injurious when developed 

 to a great extent, as is often the case when severe frost 

 immediately succeeds copious and continuous rain in 

 autumn. It appears on the diseased leaves in pale whit- 

 ish spots, which speedily diffuse themselves, and become 

 confluent, until the whole plant is covered. These whit- 

 ish spots, under the microscope, are found to consist of a 

 number of filaments aggregated together, on each of which 

 are situated two or three small cells, at first green, 

 then black ; the upper one being filled with a large 

 quantity of minute spores. Another parasitic fungus, to 

 which the name of mildew 

 has been sometimes applied, 

 is frequently developed in 

 the same situations, but it 

 is not nearly so injurious 

 as the true mildew. It is 

 known as rust (Uredo rubi- 

 go, Fig. 38), and consists 

 of yellow oval spots scat- 

 tered on the parts affected. 

 The spores, which are of a red-brown colour, and exceed- 

 ingly numerous and minute, are very easily dispersed. 



One of the most remarkable diseases affecting the 

 cereals is ergot. Though found in various kinds of 

 grasses, such as Agrostis, Festuca, Elymus, and Dactylis, 

 this disease is most frequently produced in rye, and hence 

 it is commonly known as ergot of rye. It is not very 



FIG. 38. UREDO RUBIGO. 



(a) Diseased chaff-scale, (b) Spores. 



