AMERICAN 
JOURNAL OF BOTANY 
Vol. V February, 191 8 No. 2 
THE STRUCTURE OF THE UREDINIUM IN 
PUCCINIASTRUM AGRIMONIAE 
C. A. LuDWiG AND C. C. Rees 
A number of genera of the Melampsoraceae (Uredinaceae) are 
characterized in the uredinial stage by definite, punctate, usually 
small sori. Each sorus is surrounded by a peridium composed of 
cells which are more or less isodiametric when seen in face view and 
which form a membrane-like tissue. The genera Pucciniastrum, 
Melampsoridium, and Melampsorella, belonging to the subfamily 
Pucciniastratae, have uredinia of this type. Although the peridium 
is quite fragile in appearance, it is nevertheless true that the sorus 
maintains its shape remarkably well ; and the spores make their escape 
only by a central pore until the sorus is quite aged. The spores, 
whether borne in chains or on pedicels, are easily loosened from their 
attachment and so, quite early in the development of the sorus, lose 
their original arrangement and become packed in without any special 
order. 
The customary method of studying rust morphology with dried 
herbarium material (i. e., by scraping up the spores or by cutting free- 
hand sections) is not sufficient, in the case of many sori of this sort, 
to give results of reasonable certainty. Especially is this true when 
an attempt is made to learn the manner in which the spores are borne, 
although such technique is amply satisfactory for the study of coarser 
details in many other kinds of sori. It has been only rather recently, 
however, that a full realization of the limitations of the value of free- 
hand sections in this group has come about. 
The mature urediniospore (fig. 4) has an echinulate wall and 
usually an easily discernible hilum, both of which features are especially 
characteristic of pedicellate spores. When, therefore, such spores were 
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