186 



The Potato 



molds. A fungus is itself a plant, but without leaves or 

 green color.. It is not able to prepare its own food as 

 higher plants do, but must live upon the product of other 

 plants. Many fungi and bacteria and most slime-molds 

 are saprophytes, that is, they obtain food from the prod- 

 ucts of plants or animals that are dead. Others are 

 parasites — obtaining their food from the products of 

 living plants by attacking the living tissues, injuring or 

 killing them and thus causing a diseased condition in the 

 plant or the death of the entire plant. 



A fungus differs from a higher plant in that it does not 

 have roots, stems or leaves in the sense in which we 

 generally regard these organs, but in its vegetative stage 



it consists of tiny thread- 

 like tubes or strands 

 known collectively as 

 mycelia. The mycelium 

 penetrates the food ma- 

 terial and obtains its nu- 

 trition from it. After a 

 period of growth the 

 mycelium produces its 

 fruiting body which va- 

 ries greatly in size, shape 

 and condition in the dif- 

 ferent fungi. The fruit- 

 ing body bears the re- 

 productive bodies known 

 as spores (Fig. 15), of which enormous numbers are 

 usually produced. These are disseminated by water, air 

 or animals, depending on the kind of fungus producing 

 them. When spores of parasitic fungi fall upon their 

 host plant, that is, the plant from which they are able 



Pio. 16. 



(After U. S. Dept. Agr.) 

 Below from right to left 

 are shown four stages in the formation 

 of swarm spores. The way in which 

 the swarm spores germinate is shown 

 above. 



