196 AGRICULTUEAL EXPERIMENT STATION. [Jan. 



in the very nature of the fungi themselves. It is not easy 

 to comprehend that organisms so small and so inconspicuous 

 can possess such power for harm ; and it is not easy for the 

 layman to understand that, in spite of their minuteness, 

 they pass through life-cycles as constant and as definite as 

 those of the plants On which they grow. It seems, there- 

 fore, worth while to attempt a general sketch of the growth 

 and classification of those organisms of a vegetable nature, 

 which attack and cause diseases of plants cultivated for 

 useful products or for ornament. This account may serve 

 as an introduction to the present as well as to future 

 publications of this station on the subject of plant dis- 

 eases, and to familiarize the reader, once for all, wath the 

 use of certain technical terms which are essential to exact- 

 ness of statement. For the use of such terms no apology is 

 needed. Their seeming difficulty lies simply in their 

 unfomiliarity, which, as with all new words, soon wears 

 away through use ; while their advantage over words already 

 familiar is that they convey precise ideas, unmodified by 

 preconceptions, and so greatly aid in clearness and definite- 

 ness of thought. The words printed in small capitals on 

 the following pages may serve, also, as a general reference- 

 list of technical or semi-technical terms, whose use is 

 essential in treating of plant diseases, and whose meaning, 

 here explained, will be assumed for the future to be under- 

 stood by the readers of the publications of this station. 



Any plant consists of one or more of the elementary 

 plant-units, known as cells. A cell consists essentially of 

 a mass of the semi-fluid living substance wdiicli is the basis 

 of all life, usually surrounded by a firm membrane, known 

 as the cell-wall. The simplest plants consist of a single 

 cell each ; Init the higher plants, on the other hand, are 

 made up of immense numbers of cells, intimately united. 

 Every living plant requires, for the renewal of worn-out 

 parts and the growth of new parts, a supply of the materials 

 necessary to such renewal and growth. Since both the 

 living matter and the wall of the cell consist of compounds 

 of a highly complex chemical constitution, the plant must 

 be furnished with substances which contain the necessary 

 chemical elements, in such form as to be readily convertible 



