Ostenfeldiella, a New Genus of Plasmodiophoraceae, 
BY 
C. FERDINANDSEN 
AND 
O. WINGE. 
With Plate XLV and four Figures in the Text, 
URING a stay in the Danish West Indies in the winter 1913-14 
-L/ Dr. C. H. Ostenfeld was studying Diplanthera Wrigktii, Aschers., 
a Potamogetonacea, growing in shallow water on muddy soil on the coast of 
the island of St. Croix. This plant has a horizontal monopodial rhizome 
with partly stretched, partly shortened internodes, some of the latter being 
barely half a millimetre in length. From the rooting nodes branches 
are sent out in horizontal and upward directions. Dr. Ostenfeld noticed 
that sometimes the shortened internodes of the ascending branches, which 
are clothed with the sheaths of the linear alternating leaves, were con- 
spicuously thickened, their diameter being about two to three times that of 
the normal ones, and the whole branch thereby bearing a certain resemblance 
to a string of pearls (Text-fig. 1). Dr. Ostenfeld found the swellings to be 
due to a Plasmodiophoracea, and after his return from the West Indies he 
placed his material at our disposal for closer investigation. 
We regret not to have been able entirely to elucidate the cytology of 
the fungus, the material being fixed in alcohol only. On the other hand, 
the preservation was good enough to allow a thorough investigation of the 
ordinary biology of the parasite, and its relation to the host plant. 
The Anatomy of the Normal Diplanthera Stem. 
In order to determine the anatomical changes called forth by the fungus, 
we first studied sections of the normal stem. Text-fig. 2 shows a part of 
a transverse section through one of the shortened internodes of an ascending 
normal axis. In the middle of the stem is seen the central cylinder sur- 
rounded by the endodermis. The relations are diagrammatically represented 
[Annals of Botany, Vol. XXVIil. No. CXII. October, jgi4«] 
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