256 INTRODUCTION TO BOTANY 



a period of dormancy, usually lasting through the winter, 

 these spores germinate. From each cell of the winter spores 

 in the spring there grows a small hypha (fig. 198, 7?). Each 

 cell of this hypha forms one thin-walled spore (sporidium). 



This rust (Pucdnia graminis) has another stage in its life 

 cycle. The sporidia, when alighting upon leaves of a shrubby 

 plant known as the barberry, grows and produces within the 

 leaf an extensive growth of mycelium. When this mycelium 

 produces spores, they appear in a peculiar cup on the under- 

 side of the barberry leaf (fig. 198, F^). These spores, being 

 different from any of the three described, and being formed 

 in a cup, are called cup spores. Cup spores reproduce the rust 

 plant upon wheat and oats. Summer spores persist through 

 the winter, and it is thought that they also reproduce the rust 

 upon oats and wheat in the following spring. No satisfactory 

 preventive for this fungus has been discovered. Some progress 

 has been made by learning which varieties of wheat and oats 

 are most resistant to attacks by the parasite. 1 



243. Mushrooms. In this sub-division of stalk fungi those 

 members that are good to eat have been popularly called 

 mushrooms, while those not edible were called toadstools. 

 Scientifically there is no accepted distinction of any kind, and 

 the name mushroom is now being used for the whole group. 

 In a given genus s'ome species may be edible, others not. 

 Also, some species are edible while young, but not so when 

 older. Some of the more common edible species are easily 

 learned and not readily confused with poisonous forms. In 

 the United States over one thousand edible forms are known, 

 but some of them are very rare. 



244. The general character of mushrooms. The mycelium 

 of mushrooms lives entirely within the material which fur- 

 nishes its nourishment, and from this mycelium it may send 

 up into the air the spore-bearing structure commonly regarded 

 as the entire mushroom. The mycelium becomes very exten- 

 sive and forms moldy or cobweb-like threads within the rich 



1 " Rusts of Cereals," Bulletin 109, S.Dak. Agr. Exp. Sta., 1908. 



