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



the peridium (Pi. 56, B) ; the smaller ones go to pieces. At the periphery of 

 the sorus several adjacent basal cells cut oflf units which never become 

 functional aeciospores but which always form peridium. The small 

 cells between these peridial cells indicate that the potential aeciospore 

 initial, although destined to become a peridial cell, regularly divides to 

 form two cells homologous to an aeciospore and an intercalary cell (Pi. 

 56, A). Thus, the multiple-layered peridium of Cronartium ribicola is 

 formed in the same general manner as reported by Fromme (75) for 

 the peridia of other deep-seated aecia. When first formed, the individual 

 cells are subspherical to elliptical, and smooth-walled. As the sorus 

 matures, their walls thicken and their outline changes according to their 

 position in the peridium. All tend to become more or less polyhedral. 

 The cells at the top of the sorus are usually more rounded than those at 

 the sides, since the latter are elongated by the pull exerted on the sides 

 as the developing spore chains force the central part of the peridium out. 

 Therefore, the size of the cells varies widely (18 to 40 by 12 to 42 fx). 

 The mature wall is 3 to 9 ^i thick. The walls of the outermost cells are 

 smooth or slightly granular, while the inner cell walls are studded with 

 short tubercles v/hich sometimes appear to mesh with those of the adjacent 

 cells, perhaps contributing thereby to the strength of the peridium as a 

 whole (Pi. 56, B). The cell contents of the peridial cells slowly degen- 

 erates until they become empty shells. 



After the cells which form the peridium are abstricted from the basal 

 cells, the aeciospore initials are cut off. Each aeciospore initial cell under- 

 goes division into a larger upper cell, the aeciospore, and a smaller lower 

 cell, the intercalary cell (PI. 58, W, M, b, X, Y, c, d). By the repeated 

 divisions of the basal cell a row of alternating spores and intercalary 

 cells is formed which constitutes the aeciospore chain (PI. 50, B; PI. 58, 

 Y). After the division of the aeciospore initial cell, both resulting cells, 

 the aeciospore and intercalary cell, grow rapidly. The aeciospore reaches 

 its normal broadly elliptical shape when about three or four spores dis- 

 tant from the basal cell. The intercalary cells elongate, eventually 

 becoming mere thin connecting elements between the aeciospores in the 

 chain, and finally disintegrate entirely. The spore w^all thickens greatly 

 when the spore has attained its full size. A thin space in the wall, sug- 

 gesting the germ pore of other rust spores, is evident at the point of 

 attachment of the aeciospore and the intercalary cell below it (PI. 58, 

 Y, Z). This thinner place in the v^^all may persist even in the completely 

 matured spore. It is comparable to similar phenomena in other rusts 

 and does not normally function as a germ pore. The mature spore 

 measures 18 to 21 by 20 to 26 /x- The aeciospore wall seems to be made 

 up of two parts, an endospore overlain by a somewhat thicker exospore. 

 The latter is distinctly characteristic of aeciospores of Peridermium spp. 

 Part of it is cracked up into tubercles or warts, which makes it decid- 

 edly verrucose, and the rest of it is smooth (PI. 58, Z). The smooth 



