620 Journal of Agricultural Research voi. xv, no. 12 



LIFE HISTORY AND HOSTS 



The life history of Cronartium rihicola has been so well reported by 

 Tubeuf {59, 60), Klebahn (25), Spaulding (54, 55), and others that it is 

 only necessary to repeat it in outline by way of introduction to what 

 follows. Sporidia from the teliospores produced on Ribes spp. infect 

 young stems and branches of Pinus strobus and other 5-needled pines. 

 The pycnial and secial stages subsequently develop on the pines. Mcio- 

 spores from the pine infect leaves of Ribes spp., on which the uredinia 

 are shortly formed. These sori are produced in successive generations 

 throughout the summer. Telia develop from old uredinia, or as separate 

 entities, in the form of compact columns. The teliospores germinate 

 in situ, each one producing a promycelium which gives rise to four 

 sporida. A diagram of the life cycle is presented in text figure i } 



In the United States and Canada the pine most frequently attacked is 

 Pinus strobus, although P. flexilis James and P. parviflora Sieb. and Zucc. 

 have been found infected. Practically every known species of Ribes is 

 susceptible to infection to a greater or less degree, and therefore the dis- 

 covery of an immune variety is much to be desired. The results of 

 inoculations on Ribes spp. in America have been reported by Spaulding 

 and Gravatt {36), and further work is being conducted with all the species 

 and varieties of Ribes obtainable. 



EXPERIMENTAL METHODS 



The method used for rapid examination of specimens of pine suspected 

 of being infected has been described in detail in a previous paper (7). 

 This procedure in brief is as follows : Sections from fresh pine bark are cut 

 on an ether freezing microtome, rinsed in water, stained in safranin and 

 Lichigriin, cleared in clove oil followed by xylol, and mounted in balsam. 

 They are then examined, preferably with an oil-immersion lens, to deter- 

 mine the presence or absence of the characteristic mycelium and the 

 striking haustoria of the rust, the latter being especially important from 

 a diagnostic standpoint. This method yields transparent sections which 

 for general morphological study have not been surpassed by following any 

 of the more complicated methods given below. Very little shrinkage 

 occurs in the mycelium, and as the method is usually employed before 

 pycnia and aecia appear, the preservation of the hyphae in as near their 

 natural shape as possible is practically all that is necessary. Moreover, 

 the host tissue shrinks so sUghtly that the distortion is entirely negligible. 



Two killing agents were used in the preparation of material for paraffin 

 or celloidin embedding : (i) formalin-alcohol, made by adding 6 cc. of full 

 strength commercial formalin (U. S. P. VIII) to 94 cc. of 70 per cent 



1 The drawings for the text figures were made by the aid of a camera lucida and a special projection appa- 

 ratus; for the drawings on the plates a camera lucida was used. The photographs and photomicrographs 

 are all original. The author is indebted to Miss Minnie W. Taylor, Assistant in Forest Pathology, for the 

 preparation of the drawing for text figure C of plate 56. 



