636 Journal of Agricultural Research voi. xv, No. iz 



comes at a stoma, sometimes extending from one stoma to another; but 

 the break in the peridium, when it finally occurs, is confined more or less 

 definitely to the top of the dome. Usually the peridial cells around the 

 break are irregularly thickened (PI. 55, C). The orange-yellow uredinio- 

 spores work their way out through the opening, sticking more or less 

 closely to each other to form a spore crown on the top of the peridium. 

 The basal cells continue to cut off urediniospore initials by a process sim- 

 ilar to that involved in the formation of the primary initials, which has 

 been described above. The secondary urediniospore initial may be 

 formed alongside the base of the stalk of the first-formed urediniospore 

 before the latter has reached maturity (PI. 55, C, c; 59, N). The second- 

 ary initial then divides to form a urediniospore and stalk cell, in the same 

 way as the primary initials. Meanwhile the stalk cell of the primary 

 urediniospore elongates, withers, and goes to pieces. While no more 

 than two spores have been found in connection with a single basal cell, 

 each basal cell must frequently give rise to several spores, to judge from 

 the number produced in a single sorus. The size of the mature spore 

 (PI. 59, P) is 10 to 20 by 19 to 45 ix. 



The spore-bearing basal cells are confined to the middle part of the 

 sorus. In figure C of Plate 55 it will be noted that on either side of the 

 group of spores there is a group of parenchyma-like cells {g) made up of 

 units which are arranged in more or less vertical rows. The cells of any 

 individual row of four cells may be homologized with the cells lettered 

 a, b, c, and d in figure B of Plate 55. This parenchyma-like tissue forms 

 an encircling bank of cells which completely surrounds the mature uredi- 

 nium. In this tissue the row arrangement of the cells shown in figure B 

 persists, although the cells themselves lose their contents and become 

 practically dead by the time the basal cells in the middle of the sorus 

 are actively forming urediniospores. 



The uredinium seems to be limited more definitely than either the 

 pycnium or secium in its ability to extend in a centrifugal direction. Its 

 extent is predetermined, much more exactly than in the case of the two 

 other sori, by the amount of massed myceHum from which it arises. 

 Both pycnia and aecia originate in tissue which is comparatively full of 

 myceHum before they start to develop. In the leaf of species of Ribes 

 there is rarely any massing of the hyphae to form the packed mycelium so 

 common in the pine host, and the leaf cells are only occasionally dis- 

 torted except in the immediate vicinity of the uredinia and telia. 



GERMINATION OF THE UREDINIOSPORES 



Urediniospores exhibit the same irregularity in germinating in arti- 

 ficial cultures as do the geciospores. Even when they are dusted over the 

 surface of young, fresh, moistened leaves and placed in a damp chamber 

 they may or may not germinate in large numbers. On a water film they 

 absorb water and swell considerably. The cytoplasm becomxcs vacuo- 



