DEVELOPMENT OF PLANTS 251 



Class C. Basidiomycetes or Basidia-Forming Fungi 



94. General Features. — This class contains two important 

 groups of fungi; one of which includes very destructive para- 

 sites, and the other comprises those conspicuous and familiar 

 saprophytes known as mushrooms, bracket fungi, puff balls, 

 etc. The striking feature of the class is the formation of spores 

 on club-like hyphae called basidia (sing, basidium) (Fig. 171, D). 

 These organs are often developed in rather complex outgrowths 

 of the mycelium which may be fleshy or woody, as in the mush- 

 room and bracket fungi (Figs. 170; 173, B). There is no known 

 sexual reproduction, save possibly in the first order mentioned 

 below where a vegetative union of cells appears to have been 

 substituted for a fertilization process which was originally like 

 that seen in the red algae. The more important orders are the 

 following : 



95. Order a. Uredinales or Rusts. — These parasites are well 

 known by the streaks and blotches of yellow or black rust which 

 they produce on the leaves and stems of a great variety of plants. 

 About 2,000 species are known in the United States. They are 

 among the most destructive parasites, causing great damage to 

 wheat, oats, apples, quinces, roses, carnations, etc. The yearly 

 loss from grain rust alone is estimated at considerably over 

 $i8,OQO,ooo in the United States, They exhibit a degree of 

 variation not paralleled among other plants, more than five dif- 

 ferent kinds of spores being formed by some species in their 

 life history. This is due, doubtless, to the influence of the 

 climate, the spores varying with the season (spring, summer and 

 fall) and the plants upon which the fungus grows, for one and the 

 same fungus may grow upon different plants, producing one or 

 more kinds of spores on each. Forms infesting different species 

 of plants are said to be heteroecious and those living upon but one 

 species are termed autoecious. 



(a) The Life History of Puccinia. — Several species of this 

 genus infest wheat and illustrate the many forms that may 

 appear in the life history of a rust. One phase of the life of this 

 parasite appears upon the leaves of the barberry. During May 

 and June the mycelium growing in the leaves forms roundish 



