102 



DOMESTIC BOTANY. 



walls and under floors. It penetrates the wood in all direc- 

 tions, in many cases doing irreparable mischief before being 

 observed. The perfect plant is only occasionally developed 

 through crevices. The following species are active dry-rot 

 operators : — Polypoms hyhridus^ Thelephora puteana, Meru- 

 lius lacrymans, and M. vastata. I^eziza ceruginosa penetrates 

 and imparts a fine green colour like verdigris to wood of 

 which ladies' work-boxes, toys, &c., are made. 



Mildew. The microscope reveals many kinds of mildew, 

 their structure consisting of cells (gonidia), reproducing their 

 like, and forming chain-like, branched filaments, which rapidly 

 spread over vegetable substances like a thin web, and although 

 infinitesimally small, act an important part in the economy of 

 nature. They, with their allies the dry-rots, are powerful agents 

 in reducing vegetable substances to their original and invisible 

 elements. Their effects are generally limited, but when whole 

 fields are attacked they become more serious. The most 

 important instance is that which took place in 1846—7, when 

 the potato plant throughout Great Britain and Ireland was 

 suddenly smitten with a disease hitherto unknown, and the 

 failure of the crop caused a famine in Ireland. This new 

 disease rapidly spread itself over Europe and distant coun- 

 tries, and now periodically shows itself with more or less 

 virulence in different parts of the country. As might reason- 

 ably be expected, the origin and effects of this terrible scourge 

 led to much research and investigation, and now it is gene- 

 rally admitted to be caused by a mildew which has received 

 the name of Botrytis infestans. Shortly after the advent of 

 the potato disease, the grape-vine became attacked by a mil- 

 dew, which showed itself in vineries until it covered the 

 leaves and grapes to that extent that black grapes became 

 white with the web. In a few years it was found ravaging 

 the vineyards of Europe, causing great deterioration in the 

 produce of grapes and quality of wine. The disease reached 

 Madeira, producing ruin and famine in that island. Like the 

 potato disease it has become endemic, and is known by the 

 name of Oidium Tuckeri. Other field plants are also occa- 



