HOW PLANTS OBTAIN FOOD. 



87 



rounded on the lower angle and are not attached to the stem. 

 The longer ones extend from near the stem to the margin of 

 the pileus, and the V-shaped spaces between them are occupied 

 by successively shorter ones. Around the stem a little below 

 the gills is a collar, termed the ring or annulus. 



Fig. 62. 

 Agaricus campestris. View of under side showing stem, annulus, gills, and margin of pileus. 



159. Nutrition of parasitic fungi. Certain of the fungi 

 grow on or within the higher plants and derive their food 

 materials from them and at their expense. Such a fungus is 

 called a parasite, and there are a large number of these plants, 

 which are known as parasitic fungi. The plant at whose 

 expense they grow is called the ' ' host. ' ' 



One of these parasitic fungi, which it is quite easy to obtain 

 in greenhouses or conservatories during the autumn and winter, 

 is the carnation rust (Uromyces caryophyllinus], since it breaks 

 out in rusty dark brown patches on the leaves and stems of the 

 carnation (see fig. 63). If we make thin cross-sections through 

 one of these spots on a leaf, and place them for a few minutes 

 in a solution of chloral hydrate, portions of the tissues of the 



