284 



Minnesota Plant Diseases. 



The leaf underneath the spots is abnormally increased in size 

 and distorted in shape. The pycnidia usually accompany 

 the cluster cups and come from the same mycelium 1 , but are gen- 

 erally to be found on the upper surface of the leaf. They are 

 probably male-cell receptacles which have lost their fertilizing- 

 power and are now functionless. They illustrate a persistence of 

 a habit after its usefulness has passed, a by no means uncommon 

 phenomenon in nature. 



FIG. 140. Stem rust of wheat (Puccinia graminis). A section of such a stem as is shown in 

 Fig. 139, highly magnified. Clusters of winter spores have broken through the skin 

 cells of the wheat stem. The skin cells of the wheat are seen as erect chains of cells, 

 which have been thrown back by the growing out of the winter spores. Such wounds 

 allow the water in the stems to escape since the skin cells of the wheat, which normally 

 prevent the escape of water, are broken. Thus the wheat plants are dried up as well 

 as starved by the drain of the parasite. Each winter spore of the fungus is seen to be 

 two-celled. Highly magnified. Microphotograph by E. W. D. Holway. 



The cluster-cup is composed of a thin wall, enclosing an in- 

 ternal mass of orange red spores. The wall splits at the summit 

 and opens out often in star-shaped fashion. The spores are 

 formed in chains from the floor of the cup. The cluster-cup 

 spores are scattered, when mature, by the wind and alight on 

 some grass plant, where they germinate into a tube, which pene- 

 trates into the interior of the host through an air-pore, and forms 



