242 



PEACTICAL BOTANY 



Corn smut ( Ustilago Zece') infests many of the corn plants 

 in an ordinary field, and when abrasions of the plants occur 

 the unattractive smut masses frequently appear. They most 



often appear in the 

 tassel or ear, and 

 may completely or 

 partially destroy 

 both (Fig. 197).i . 

 232. The rusts. 

 The intricate struc- 

 tures and habits of 

 living of the rusts 

 are objects of great 

 interest to bota- 

 nists. Their effect 

 upon useful and 

 decorative plants 

 that serve as their 

 hosts gives them 

 great economic im- 

 portance. A given 

 species of rust may 

 live for a time upon 

 one kind of plant 

 and later upon host 

 plants that belong 

 to other groups. 

 In each of these 

 stages the parasite 

 has distinctly dif- 

 ferent structures and produces quite different effects upon 

 its host. So unlike are these stages that formerly they were 

 named as distinctly different plants, and it is only recently 

 that enough has been learned about them to enable us to 

 know some of the different appearances they may assume. 

 1 " Corn Smut," Ind. Agr. Kxp. Sta., 1000. 



Fig. 197. An ear of corn within and upon wliicli 

 corn smut ( Ustilago Zeae) has grown 



The bracts which inclose the ear (E) have peculiar 



leaf-lilie extensions of their tips. Masses of the spores 



of the smut {sm) have grown and extruded at the tip 



of the ear 



