been gradually extended until it is now possible to present a treat- 

 ment of all the species that have been described from any part of 

 the world up to the present time, so far as they are known to the 



PART I 



BIOLOGY OF THE GENUS GYMNOSPORANGIUM 

 1. Introduction 



Life-History 

 The genus Gymnosporangium is composed of a group of species 

 of strictly parasitic fungi having pleomorphic spore-forms. It 

 is a member of the order Uredinales Diet, and is one of the several 

 genera in that order in which an alternation of phases and heteroe- 

 cism is conspicuously shown. The mycelium of one phase pro- 

 duces two spore-forms, pycnia (spermogonia) and aecia (aecidia), 

 and inhabits, with three known exceptions, dicotyledonous angio- 

 sperms of the order Rosales, family Malaceae (Pomaceae). The 

 mycelium of the other phase produces only one spore-form', telia 

 (teleutosori) and inhabits, without any known exceptions, gymno- 

 sperms of the order Pinales, family Juniperaceae (Cupressineae). 

 Gymnosporangium differs from all the other rust-genera which are 

 heteroecious, except Calyptospora, by the lack of a repeating spore- 

 form,* uredinial-stage (uredo-stage), in its life-cycle. It is a 

 general rule that all species of the order Uredinales which are 

 heteroecious possess four spore-forms, pycnia and aecia on one set 

 of hosts and uredinia and telia on the other set, and it is a notable 

 fact that only in the genera Gymnosporangium and Calyptospora are 

 found species which are heteroecious and yet have one of these 

 spore-forms omitted. In these two genera urediniospores are 

 lacking in all of the present known species. 



General Characters 

 Gymnosporangium as a genus is characterized, with a few ex- 

 ceptions, by a dingy-white, membranous peridium (called also 

 pseudoperidium), which elongates into a tubular form and tends 

 to rupture along the sides; by large, loosely joined, peridial cells 

 usually conspicuously sculptured on the inner and side walls; 

 by aeciospores having colored, mostly brownish, walls, usually 

 with evident germ-pores; and by teliospores having hyaline pedicels, 



and over indefinitely. 



