304 



PART III. THE CLASSIFICATION OF PLANTS. 



to their host-plants at a certain stage of tbeir life-history, when they bear at the 

 surface a great number of orange-coloured gonidia. 



The life-history of these plants presents, in many cases, a clear alternation 

 of generations, together with polymorphism of the gametophyte which has an 

 asexual form ; moreover the different forms of the gametophyte are sometimes 

 hetercecious, that is, they inhabit different hosts. 



FIG. 214. Puccinia Graminis. I Transverse section of a leaf of Barberry, with secidia 

 (a) ; p the wall of the secidium ; u, lower, o upper surface of the leaf, which has become 

 thickened at u, y, in consequence of the presence of the parasite ; on the upper surface are 

 spermogonia (sp). A A young eecidium which has not yet opened. II Sorus of teleuto- 

 gonidia (t) on the leaf of Triticum repens ; e its epidermis. Ill Part of a sorus of uredo- 

 gonidia on the same plant; ur the uredogonidia ; t a teleutogonidium. (After Sachs.) 



Puccinia Graminis affords an example of the most complex life-history with 

 heteroecism. It inhabits Wheat, Rye, and other Grasses, and develop|s its 

 mycelium in the tissues of the young plants. During the summer it produces 

 groups of simple gonidiophores, at the apex of each of which a single oval 



