AECIA 319 



spores are loosely compacted, the structure is called a caeoma. 

 If the peridium is strongly developed and extends prominently 

 above the chains of aeciospores, the structure is spoken of as a 

 roestelium or a peridermium. In general, the term roestelia is 

 limited to the aecial stage of species of Gymnosporangium, 

 whereas peridermium applies to the aecial stage of Coleosporium, 

 Cronartium, Melampsora, and various other genera having non- 

 pedicellate teliospores. 



Another very peculiar type of aecium occurs in Dasyspora 

 faveolata. A much-branched hyphal mass protrudes from the 

 host stomata. Each branch bears at its apex a verrucose aecio- 

 spore that is abstricted by a narrow stalk cell. 



The aeciospores, it has been pointed out, arise in series from 

 a binucleate mother cell. First the mother cell elongates, and 

 conjugate division occurs; then by a transverse septum a terminal 

 cell containing a pair of nuclei is cut off. This terminal cell again 

 divides transversely. Usually the upper cell so formed is the 

 larger and becomes the aeciospore; the smaller cell is a buffer or 

 intercalary cell. Sometimes the first cell formed by the mother 

 cell becomes a peridial cell. At any rate the basal mother cell 

 repeats the process of conjugate nuclear division and delimitation 

 of the upper portion, and eventually chains of aeciospores and 

 intercalary cells are formed. The intercalary cells serve as dis- 

 junctors and may early become disorganized. 



The aeciospores are always unicellular, and their wall is 

 hyaline and two-layered. Sometimes the outer membrane is 

 echinulate. Mutual pressure causes the aeciospores to be polygo- 

 nal. Their content is orange-colored. The germ tubes usually 

 emerge through one or the other of the thin places in the spore 

 wall. 



Fromme (1914) studied the development of the aecia of Piic- 

 ciiiia claytoniata. He observed that the primordium is at first a 

 tangled globular mass of hyphae. Near the basal portion a layer 

 of mother cells is differentiated. Sporulation begins first with 

 the mother cells near the center of the hymenium and extends 

 outward in a centrifugal manner. The first cells abstricted ad- 

 here laterally and constitute the upper and lateral walls of the 

 peridium. In some instances the young peridial cells divide to 

 form intercalary cells, but such a procedure is exceptional. Since 

 the mother cells near the center of the hymenium sporulate first. 



