THE FUNGI WHICH CAUSE PLANT DISEASE 391 



of pine and continues to extend for years causing swellings of 

 twigs. Pycnia are either subcuticular or subepidermal and the 

 pycniospores often issue in a sweetish liquid. iEcia occur as 

 wrinkled sacs emerging from the bark of the swollen places and 

 bear spores pereimially. 

 A key to some thirty species is given by Arthur & Kern."* 

 So far as it relates to the distribution of the Peridermiums to 

 their telial genera it is as follows: 



Key to Species of Peridennium 



Pycnia subcuticular 



jEcia cylindrical Pucciniastrum. 



.^cia tongue-shaped Melampsorella, Melamp- 



soiidium. 

 Pycnia subepidermal 

 ^cial peridia one cell thick 



On Pinus Coleosporium. 



On Picea Melampsoropsis. 



On Abies Uredinopsis. 



Pycnia subcorticular 

 JEcial peridia more than one cell thick. . . Cronartium. 



Such forms as are of economic interest and of which the telial 

 stage is known are discussed under Coleosporium, Cronartium, 

 Melampsorella, Melampsoropsis and Pucciniastrum. 



Several other forms are found on pine, spruce and Tsuga. 



RoBstelia Rebentisch (p. 389) 



0. Pycnia spherical or cup- 

 formed. 



1. .^cia with strongly de- 

 veloped thick-walled peri- 



dium, flask-shaped or cylin- Fio. 281.— R. pyrata, cups showing peridial 



dric; spores globose, 1-celled, '^^- After King, 



brown to yellow, catenulate, with several evident germ pores. 

 The forms are the secial stages of Gymnosporangiums and 

 occur mostly on Rosaceous hosts. The economic forms will be 

 found under Gymnosporangium. 



