562 TEXTBOOK OF BOTANY 



hot dry summers and so do not cause infection of the young wheat plants 

 there in the following fall. In early spring the red spores from southern 

 wheat are blown northward and successively lead to infection of the 

 growing wheat of higher and higher latitudes. This cycle of infection by 

 means of red spores alone, makes possible widespread epidemics of 

 wheat rust in the absence of barberry plants. Such a condition can be 

 controlled only by procuring immune races of wheat. These general 

 epidemics, however, occur more rarely than the local epidemics in 

 regions in which the common barberry is growing. 



The problem of crossing and selecting varieties of wheat immune to 

 the stem rust is further comphcated by the fact that there are about 180 

 races of the fungus which pass one stage of their life cycle in barberry. 

 A variety of wheat may be immune to most of these races, but a few 

 races may infect and injure it so severely that it is unprofitable to culti- 

 vate except in the limited areas where these particular races do not 

 occur. 



Other interesting examples of two-host rusts are the white pine blister 

 rust ( white pine stem, and currant or gooseberry leaves ) , and apple rust 

 (apple leaves and fruits and red cedar shoots). However, not all rusts 

 involve two host plants, and many of them do not infect crop plants. 



Slime molds. A rather curious group of non-green organisms are the 

 slime molds. They are frequently classified as plants, sometimes as ani- 

 mals, and may be discussed in connection with the fungi. The vegetative 

 part of the plant is a multinucleate mass of naked protoplasm with about 

 the consistency of mayonnaise, called a plasmodiwn. The plasmodium 

 lives in moist places and streams through and over the substrate and 

 finally comes to rest on the surface of plants and other objects. The 

 different species of slime molds have characteristically colored plasmodia 

 and variously shaped sporangia bearing a large number of spores. 



Lichens. Certain fungi and certain algae live together, forming the 

 compound plant structures called lichens (Figs. 251-254, and Plate 3). 

 These plants grow in a great diversity of habitats, such as on exposed 

 rocks, on tree trunks, and on the ground. They grow and survive in the 

 most extreme habitats on the earth, from the mountains of the antarctic 

 where the temperatures are rarely above freezing, to the deserts of 

 southern California where midsummer temperatures of rocks on which 

 they grow mav be as high as 175° F. Some kinds of lichens appear almost 

 structureless, forming a thin coating on the rock surface and on soil 

 (crustose); others are somewhat leaf -like in appearance (foliose); and 



