UREDINALES 



561 



(Poirault, 1913, 1915) and of Puccinia Pruni-spinosae (Kursanov, 1914), 

 a layer of palisade cells is normally formed at the base of the aecia. This 

 layer occasionally may cut off a few sterile cells. Although these are 

 still uninucleate, they proceed without apparent cause to differentiate 

 aeciospore mother cells which appear entirely similar to the binucleate 

 ones and, like these, divide into aeciospores and intercalary spores. 

 The aeciospores are naturally uninucleate, i.e., the whole course 

 of development proceeds parthenogenetically. 

 The reverse condition has been demonstrated 

 for another strain of Endophyllum Euphorbia- 

 silvaticae (Sapin-Trouffy, 1896; M. and Mme. 

 Moreau, 1918, 1919). In it the whole mycelium 

 is binucleate; consequently plasmogamy at the 

 base of the aecia is omitted and development 

 proceeds apogamously, as will be described later. 



The developmental history of the aecidium 

 agrees with that of the caeoma only in so far as 

 plasmogamy generally precedes the formation of 

 the initial cells. 



The aecidium is distinguished from the 

 caeoma by its pseudoperidium, which arises as 

 follows: The uppermost aeciospores of a chain 

 occasionally lose their spore character, as is the 

 case in the buffer cells of Albugo; they increase 

 in size in all directions, and adhere laterally to 

 similarly deformed spores to form a peridium 

 of a single layer of cells (Fig. 376, 2). As the 

 maturation of the aecidia takes place from the 

 center to the margin, new cells are always added 

 laterally to this cover. When this transforma- 

 tion has reached the periphery, it proceeds basipetally in the outer chains. 

 In these outermost cells, not only the uppermost cells are deformed as 

 peridial cells but the whole of the outermost chains is used for vegetative 

 purposes. The basal cells of these chains proceed with the cutting off 

 the initial cells, thus adding new cells to the pseudoperidium from beneath 

 and enabling it to keep up with the growth of the central spore chains. 

 The pseudoperidium here retains a firm texture as a result of the 

 peculiar development of the intercalary cells. In these outermost chains, 

 therefore, they are not separated directly beneath the aeciospores but 

 obliquely toward the outer side. In this manner, they no longer 

 function as disjunctors and the metamorphosed aeciospores adhere to 

 each other as in the middle of a dome. In most forms, these peripheral 

 intercalary cells degenerate early. In Pvccinia graminis and 

 Gymnosporangium juniperinum, on the other hand, they increase in 



Fig. 377. — Puccinia gram- 

 inis. Peripheral portion of 

 aecidium. Section of the 

 basal cell, B.z., and the lower 

 portion of the peridium, 

 Per., with intercalary cells, 

 Zw.Z. (X 500; after Kur- 

 sanov, 1914.) 



