i go NATURAL SCIENCE. March, 



merit of the woody rings of growth. The cells round about the 

 parasites die, turn brown, and become disorganised ; thus small open 

 spaces are formed in the living cortex (Plate, Fig. V.). Many of 

 the cells become filled with a dark-brown resinous substance. 



Knowing the extreme similarity of many of these nematodes, 

 especially in the larval state, and the consequent difficulty of 

 accurate identification of the species, I asked Dr. de Man, of Ierseke, 

 and Dr. J. Ritzema Bos, Wageningen, Netherlands, to examine 

 them. Both gentlemen kindly did so, and pronounced the Tylenchus 

 to be T. devastatvi.x. Dr. Ritzema Bos, who has made a special study 

 of this particular species, was at first inclined to believe that the 

 hop-root Tylenchus, although very nearly allied to T. devastatrix, was 

 not identical with it. On more careful examination, however, he 

 informs me that he can find no constant difference between the two 

 species, except that the hop-root variety is smaller than any he has 

 seen in other plants. 



It is worthy of note that males and larvae are much more abun- 

 dant than females in all the cases I have examined. 



The Heterodem females I first discovered loose in the cortex of 

 the thicker roots, along with Tylenchus-like larvae, which no doubt be- 

 longed to H. schachtii, but which at the time I was unable to identify 

 with certainty. There was, however, no difficulty in recognising the 

 motionless citron-shaped females as of this species. Subsequent 

 examination showed that these had become detached from finer 

 rootlets, which, in a large number of instances, are found to grow and 

 permeate the decaying bark of the root, instead of making their way 

 into the soil. 



An examination of the finer rootlets of well-marked " nettle- 

 headed " plants revealed the presence of females of the Heterodem in 

 thousands on all the younger fibrils, even among the root-hairs. 

 They are readily seen with a pocket lens or with the naked eye as 

 whitish points about the size of a very small pin's head, seated on the 

 outside of the rootlet (Plate, Fig. VI.), and even distributed in the 

 soil close by. In older stages they are a rich chestnut-brown, con- 

 siderably darker than the root epidermis. Developing beneath the 

 epidermis, they swell up and ultimately break through it, and become 

 exposed on the outside of the root (Plate, Fig. VII.). The delicate 

 youngest roots are seriously injured in this way, and with an ordinary 

 lens the older fine root-fibrils are seen to be covered with minute holes 

 and slits, the work of former generations of the nematode. With a 

 slightly higher magnifying power all the various stages of growth of 

 the female may be observed on the root. The rootlets are slightly 

 thicker and more stumpy in character than usual, and not so copiously 

 branched as in healthy plants. Nothing, however, like the galls of 

 H. radicicola is produced, nor are the parasites so far embedded in the 

 tissues of the plant, as is usual with the latter species. Dr. de Man 

 found dead females in pieces of roots which I forwarded to him ; and 



