1890.] | Botany. 177 
BOTANY.! 
Peridial Cell Characters in the Classification of the Ure- 
dinez.—In the genus Roestelia peridial cell characters are frequently 
given considerable prominence, and surely frequently add to the cer- 
tainty of our identification. e question then naturally arises, why 
are not such characters valuable in the related genus Æcidium, where if 
anywhere in the Uredinee we need all possible characters for certainty 
in identification. 
The characters most frequently used are position and size of zcidia, 
size and outward appearance of spores, and most important of all, on 
what host plant. All are very variable, even the latter and most important 
one, many rusts sometimes occurring on ~~ pene Rost, and pore tly je 
same rust on many hosts. The position 
epiphyllous—is Saisie determined, I think, largely by the. hire 
of the leaf, as I have shown elsewhere. The size of the æcidia varies 
also with a change of host-plant and immediate conditions of moisture 
and heat, as do also the æcidiospores. 
What are the peridial cells? They are very likely, as usually sup- 
posed, slightly modified chains of æcidiospores loosely attached into a 
surrounding pseudo tissue layer for protection. This is readily believed 
when we observe that the peridial cells usually partake, more or less of 
the character of the æcidiospores, in shape, thickness of wall, rough- 
ness or smoothness, etc., and from their breaking apart readily into 
chains appearing much as the æcidisopores. Believing thus that the 
peridial cells are developed from the æcidisopores what would seem 
more natural than that we should examine and describe them as we do 
the æcidiospores. 
In the examination of Uredineæ I have noticed that while the peri- 
dial cells are usually very similar in shape and size (yet no more so than 
the æcidiospores), they are frequently quite characteristic. 
In Æcidium pentstemonis for instance, the peridial cells are angular, 
subrotund-elliptical, thick walled, smooth, 19—22 by 22-31 ø., while in 
the Æcidium Puccinia tanaceti D.C., on Artemisia cana and A .canadensis 
they are subrotund-angular, 15-21 by 19-26 „., being very similar to 
the last, but distinguished by size, being in general smaller. In ei- 
dium composttarum Mart. var. lygodesmie@ Webber, they are angular- 
elliptical and usually strongly tuberculate, distinguished from the pre- 
1 Edited by Prof. C. E. Bessey, Lincoln, Neb. 

