FUNGI ON LIVING PLANTS. 221 



dead, inert matter, are called " saprophytes," from a Greek 

 word meaning rotten. Of course, none of the fungi of this 

 class are the source of disease in fruit-trees. Other fungi 

 grow upon living plants and animals, and produce many 

 serious diseases. Of the fungi which grow upon living 

 plants, the greater part of them are limited in their range, and 

 do not grow indifferently upon any plant, hut either upon a 

 particular species, or on several species which are botanically 

 nearly related. The reason for this selection of plants upon 

 which to prey is probably that different chemical substances 

 are necessary to the existence of the different species of fungi ; 

 and consequently they can only flourish on the plants which 

 produce those substances. This is a matter of inference 

 rather than observation ; for we do not yet know, for example, 

 what the chemical substance is wliich enables the funG:iis 

 known as Splioeria morhosa to grow on plums, and some varie- 

 ties of cherries, and produce on them the excrescence known 

 as " black knot," while it will not grow on apples or pears. 

 The limitation of different species of fungi to a single plant, or 

 at least to a comparatively few plants, is a great protection 

 to the farmer ; for, were this not the case, a disease once 

 attacking a single crop would cause a general devastation. 



Not only are fungi destitute of the green coloring-matter 

 found in the greater part of other plants, but their structure 

 is much more simple than that of the ordinary plants with 

 which you are most familiar. Instead of being composed of 

 a more or less solid mass of cells packed together, as are the 

 bricks in a house, for a great part of their existence fungi 

 are composed only of delicate, colorless threads. When 

 we say delicate, we must understand that the word refers 

 only to the general appearance of the threads, and not 

 to a lack of resisting power. If we submit the cells of the 

 higher plants and the threads of fungi to the action of strong 

 chemical re-agents, as caustic potash or acids, we shall find 

 that the threads are less quickly destroyed than the cells : 

 consequently, if we have a leaf in which a fungus is 

 growing, we are able, on boiling it in caustic potash, to 

 cause a separation and disintegration of the leaf-cells, while the 

 threads of the fungus remain comparatively unaffected. The 

 name given to the tlureads of the fungus taken collectively is 

 mycelium ; and the separate threads are called hyphce, ovflocci. 



