434 



BOTANY OF THE LIVING PLANT 



and the aecidium-stage on the common Nettle, causing contorted 

 swellings upon its stem and leaves. Thus the Rust of Wheat is an 

 example of a life-history that is not uncommon. 



Fig. 330. 



Part of shoot of Barberry with leaves attacked by Puccinia graminis which forms yellow 



cushions, or cluster -cups, on the leaf -blades and stalks. (After Marshall Ward.) 



Sections through a diseased leaf of wheat in summer reveal the 

 branched and septate hyphae closely packed in the intercellular 

 spaces, and investing the green cells. They accumulate below the 

 epidermis, forming a dense mass or sorus, the end of individual hyphae 

 swelling into the uredospores, which, increasing in bulk and number, 

 burst the epidermis, and are shed (Fig. 331). Each spore is covered 



^i^^giii^^^ OT^r 



Fig. 331. 



Longitudinal section of a leaf of Wheat, showing a tuft of uredospore bursting 

 through the epidermis. Highly magnified. (After Marshall Ward.) 



(It will be realised from this illustration that uredospores are really conidia : 

 for historic reasons, however, it is convenient to retain the term uredospore.) 



by a thick wall, containing dense protoplasm with oily globules, and 

 two nuclei. Their walls are marked by thin spots round the equator. 

 It is from these spots that the germ- tubes emerge when grown in w r ater 

 (Fig. 33 2 )- K this germination takes place on a wet leaf of wheat, the 

 tube growing over the surface finds entry by a stoma (Fig. S33), an d 



