142 



SYSTEMATIC BOTANY. 



successive perio3s two or more distinct forms, so uulike 

 each other generally that the different stages were formerly- 

 considered as distinct plants. Nothing is yet known as to 

 their sexual organs. The mycelium penetrates between 

 the cells in the tissue of leaves, causing abnormal growth 

 and more or less distortion. There then appear, beneath 

 the epidermis, globular masses (Fig. 257), having within 

 at their base a compact layer of upright hyphae, each of 

 which produces a chaiu of conidial spores. The epidermis 

 is ruptured by the growing mass ; and the thin layer of cells, 



or periderm (Fig. 

 257,^), enclosing the 

 spores, breaks open; 

 the yellow spores, 

 hitherto many-sided 

 from mutual pressure, 

 become round and es- 

 cape. This stage for- 

 merly received the generic name of JEcidium (Fig. 257). 

 The spores may, therefore, be called the ceddiospores. 

 There are often present also smaller flask-shaped repro- 

 ductive bodies, called spermogonia (Fig. 257, «p), contain- 

 ing hair-like filaments, which break up into exceedingly 

 small bodies, called the spermatia. The exact function of 

 these is not accurately known. The secidiospores, when 

 they fall on the proper host-plants, germinate, penetrate 

 through the stomates, and form a dense mycelium in the 

 parenchyma of the leaf; from this mycelium grow pedi- 

 celled spores, called uredo-spores, and form, when they 

 burst through the epidermis, orange-colored spots. This 



Fig. 257. Yellow Cluster-cup, or .^Icidium (aec) of Ptcccinia graminis , and sper- 

 mogonia (j/) on ihe Barberry leaf; e, epidermis; ^, peridium; sj>, spermogonia. 



