DEVELOPMENT OF PLANTS 221 



this order are largely saprophytic and often attain considerable 

 size. Forms of Morchella occasionally reaching the height of 

 a foot and some species of Gyromitra weigh over a pound. 



CLASS C. BASIDIOMYCETES OR BASIDIA-FORMING FUNGI 



93. 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 sap- 

 rophytes 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. 168, 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. 167; 170, B). There is no known 

 sexual reproduction, although it is supposed that these basidia- 

 producing structures may have been formed originally as the 

 result of fertilization. The more important orders are the 

 following : 



94. 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 speciesare 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. 

 $18,000,000 in the United States. They exhibit a degree of 

 variation not paralleled among other plants ; as many as five 

 different kinds of spores being formed by some species in their 

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

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

 fall) and the plants upon which the fungus grows may be a 

 contributing factor to the variation', for one and the same fungus 

 may grow upon different plants, producing one or more kinds 

 of spores on each. 



(a) The Life History of Pucdnia. Several species of this 

 genus infest wheat and illustrate the many forms that may ap- 



