298 PART IV. CLASSIFICATION. 



Sub-Class V. ^ECIDIOMYCETES. This sub-class includes a con- 

 siderable number of parasitic plants known as Rusts and Smuts. 

 They are characterised by their remarkably complex life-history, 

 due to polymorphism, presenting two or more spore-bearing forms : 

 and by the fact that the spores are not developed in the interior 

 of a sporangium, but are formed by abstriction. 



FIG. 178. Puccmta Graminis. I Transverse section of a leaf of Barberry, with secidia 

 (a) ; p the wall of the aeculiuin ; 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 secidium which has not yet opened. II Sorus of teleuto- 

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

 spores on the same plant ; ur the uredospores; t a teleutospore. (After Sachs.) 



The sub-class is divisible into two orders : 



Order 1. Uredineae : have an secidium-form, as a rule. 



Order 2. Ustilaginese : never have an secidium-form. 



