SAPROPHYTES AND PARASITES 



91 



days it will be mouldy. The spores were in the air, or per- 

 haps they had already fallen on the bread but had not had 

 opportunity to grow. 



193. Saprophytes break down or decompose organic 

 „ substances. Chief of these saprophytes are the 

 microscopic organisms known as bacteria (Fig. 

 136). These innumerable bodies are immersed in 

 water or in animal and plant 

 juices, and absorb food over 

 their entire surface. By 

 breaking down organic com- 

 binations, they produce de- 

 cay. Largely through their 

 agency, and that of many 

 true but microscopic fungi, 

 all things pass into soil and 



gas. Thus are the bodies Of 133 - A mushroom, example 

 , i-i , of a saprophytic plant. 



plants and animals removed 

 and the continuing round of life is maintained. 

 194. A plant that secures its nutrition di- 

 rectly from a living plant or animal is a parasite, 

 and the plant or animal on which it lives is the 

 host. The dodder is a true parasite. So are the 

 rusts and mildews that attack leaves and shoots 

 and injure them. The threads of the parasitic 

 fungus usually creep through the intercellular 

 spaces in the leaf or stem and send suckers (or 

 haustoria) into the cells. (Fig. 137.) 

 In some forms these threads (or 

 lyphse) penetrate the cells. The 

 hyphse clog the air-spaces of 'the leaf 

 and often plug the stomata, and they 

 also appropriate and disorganize the 

 cell fluids: thus they injure or kill 

 showing the mycorhizas. ' their host. The mass of hyphse of a 



