CITRUS-ROOT NEMATODE 
By N. A. Cobb, 
Technologist in Charge , Agricultural Technology, Cotton Standardization, Fiber and 
Paper Plant Investigations, Bureau of Plant Industry 
INTRODUCTION 
Our ignorance concerning nematodes in general and soil-inhabiting 
nematodes in particular is well illustrated by the history of the Citrus- 
root parasite Tylenchulus semipenetrans , which, within a few months 
of its discovery in California, has been located in such widely separated 
places as Florida, Spain, Malta, Palestine, and Australia. This series 
of events is, the writer believes, simply illustrative of the surprises in 
store whenever the soil-inhabiting nematodes receive at the hands of 
agricultural scientists the attention they merit. 
Nematodes are distributed far and wide in inconceivable numbers 1 
and without doubt constitute a group in the animal kingdom comparable 
with insects both in number of species and economic importance. 
THE) CITRUS-ROOT NEMATODE TYLENCHUS SEMIPENETRANS 
The anatomical features of Tylenchulus semipenetrans 2 are so well set 
forth in the accompanying illustrations that it is unnecessary to describe 
them further. It is well to add that the drawings are so carefully made 3 
that many features set forth in them can not be seen in the natural ob¬ 
ject, except with the aid of the best immersion lenses skillfully used 
under favorable conditions. 
Tylenchulus semipenetrans (fig. i) was first discovered in California on 
Citrus roots by J. R. Hodges, Horticultural Inspector for Covina County, 
Cal., and was first carefully studied by E. E. Thomas, of the Agricultural 
Station of the University of California, at Berkeley. 4 
1 Cobb, N. A. Antarctic Marine Free-Giving Nematodes of the Shackleton Expedition. Contributions 
to a Science erf Nematology.—I. 33 p. t illus. Baltimore, 1914. 
- North American freshwater Nematodes. Contributions to a Science of Nematology.—II. 65 
p., 80 fig., 7 pi. 1914* Also published in Frans. Amer. Micros. Soc., 1914. 
*Cobb, N. A. Notes on Monnochus and Tylenchulus. In Jour. Wash. Acad. Sci., v. 3,no. 10, p. 288,1913. 
This article contains the writer’s diagnosis of the new genus Tylenchulus and its type species, T* semi - 
penetrans , Cobb. 
8 The drawings were prepared under the author’s direction by Mr. W. E. Chambers, of the Bureau of 
Plant Industry. 
4 Thomas, E. E-, A preliminary report of a nematode observed on Citrus roots and its possible relation 
with the mottled appearance of Citrus trees. Cal. Agr. Exp. v Sta. Circ. 85, 14 p., 8 fig., 1913 (J. R. Hodges 
mentioned). 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C 
217 
Vol. II, No. 3 
June 15,1914 
G— 23 
