Dec. 23. 1918 Parasitism of Cronartium rihicola 653 



which the urediniospores arise, but his description of the maimer in which 

 the urediniospores in Pucciniastrum are borne is correct. Fischer's {13) 

 figure of the uredinial peridium of C. ascelpiadeum (Willd.) Fr. evidently 

 represents the same type of peridium as is present in the uredinium of C. 

 rihicola. The peridial cells surrounding the break in the top of the peri- 

 dium in the latter fungus are thickened irregularly, but they do not often 

 appear as conspicuous as the large cells figured by Fischer {13) in the case 

 of the uredinial peridium in species of Pucciniastrum or by Ludwig and 

 Rees {28) for P. agrinwniae. 



In the discussion of the telium it has been shown that it originates in a 

 way which makes it impossible to tell the young telium from the young 

 uredinium. Under these circumstances it is most natural that a peridium 

 should be found over the telium, as in the case of the young uredinium. 

 The same holds true for the parenchyma-like bank of cells which encircle 

 the base of the column. Both the peridium and the bank of encircling 

 cells come into being before the sorus becomes differentiated into either 

 uredinium or telium. With these facts clearly in view it may be reasona- 

 bly safe to predict that a telial peridium, a structure which has apparently 

 not been reported previously, wall be found to be present in other species 

 of the genus Cronartium. The manner of the germination of the telio- 

 spores, previously discussed, adds to our knowledge of the morphology 

 and behavior of the spores in the telial column, at the same time confirm- 

 ing the observations of Tulasne {61) and Sappin-Trouffy (51) on other 

 species. 



This paper is offered as a contribution to our knowledge of the para- 

 sitism, morphology, and cytology of the rusts, and especially of the genus 

 Cronartium. It is hoped that the data presented may prove valuable in 

 stimulating further research on the interrelations of rust parasites and 

 their hosts, which will, of course, involve more accurate study of the anat- 

 omy of the hosts and the modifications in the normal structure of the 

 host tissues under the action of the parasites. The need for a compara- 

 tive paper on the haustoria of the rusts scarcely requires emphasis. 

 It will be interesting to compare the results of investigations on the cytol- 

 ogy of other deep seated caulicolous aecia with those herein presented, 

 especially with reference to the process of the formation of basal cells, 

 the phenomena of polarization of the nuclei, and the centrosomes and 

 chromosomes. 



SUMMARY 



(i) In the foregoing paper hitherto unpublished data on the mor- 

 phology and cytology of Cronartium rihicola Fischer and the inter- 

 relations of the parasite and its hosts, Pinus strohus and Ribes spp., are 

 presented and fully illustrated. 



(2) The mycelium is more abundant in the Pinus strohus than in 

 species .of Ribes. In the former the hypse force the cortex and phloem 



