2G2 STRUCTURAL BOTANY 



few hours of their discharge, if sufficient moisture be 

 present. They do not, however, infect the host on 

 which they were produced, but are only able to form a 

 mycelium if conveyed by the wind or rain on to the 

 leaves of some member of the Graminese, such as the 

 Wheat or Eye. In this case a hypha is sent out through 

 one or more of the germ-pores. The hypha receives 

 the protoplasm from the spore and goes on growing, 

 bending first in one direction and then in another until 

 its tip lights on a stoma. Then the hypha turns in 

 through the pore of the stoma, and so makes its way 

 into the intercellular spaces of the host, where it 

 develops a mycelium from which uredospores are soon 

 produced. Thus the cycle of the parasite's existence 

 is completed. 



There remains, however, yet another form of repro- 

 ductive structure to be considered before we proceed 

 to sum up the life-history. Accompanying the dEcidium 

 on the Burberry, but usually on the upper surface of 

 the leaf, are minute bodies called the spermogonia, which 

 are visible to the naked eye merely as minute black 

 specks. They make their appearance before the cups 

 on the opposite side of the leaf are ripe. Each of these 

 spermogonia, when observed in a section vertical to the 

 surface of the leaf (see Fig. 106, B, sp), is found to be a 

 little flask-shaped body, consisting of a sheath of slender 

 converging hyphae, leaving a cavity in the middle. 

 The spermogonium arises from the mycelium below the 

 epidermis of the host, but ultimately breaks through it, 

 so that the neck of the flask reaches the surface (Fig. 

 106, B). The hyphse which project into the cavity form 

 minute cells at their ends, a little row of such cells 

 being formed in each filament. These minute cells the 



