194 NATURAL SCIENCE. March, 



but there is danger in the fact that many growers have allowed the 

 plants to grow as long as any hops were produced, and thus the roots 

 and ground have become thoroughly permeated with the parasites 

 before steps have been taken to eradicate the evil. This is a bad 

 policy, not only because the plant becomes a centre of infection for 

 the immediate neighbourhood, but also because there is some evidence 

 that by such unconscious rearing of successive generations of these 

 nematodes upon one kind of plant, a new " race," so to speak, is 

 raised, which becomes more virulent in its action and more completely 

 adapted to its surroundings. 



It is perhaps necessary to point out that the soil of a hop-garden 

 teems with living organisms on account of the excessively large amount 

 of humus present, due to the use of heterogeneous organic substances 

 as manures (sprats, star-fish, leather, wool, fur waste, etc.), and 

 enormous numbers of free-living non-parasitic nematodes are met 

 with. Such fungi as live on decaying vegetable matter, especially 

 those imperfect conidial forms belonging to the group Hyphomyceies, 

 are also very abundant, both in the soil and upon every dying and 

 dead leaf throughout the gardens. 



In order that credit may be given to whom credit is due, I take 

 the opportunity of claiming priority for an early English observer 

 whose work on the subject of nematodes in root-galls has escaped the 

 notice of continental writers. I refer to the late Rev. M. J. Berkeley. 



Schacht, in 1859, while investigating diseased sugar-beet plants, 

 became acquainted with peculiar white points upon the outside of the 

 finer rootlets, and ultimately found that these were " sacs " filled 

 with eggs and larva? of an eel worm. In 1862 he discovered free male 

 nematodes in the soil, which from their close similarity to the embryo 

 worms obtained from the " sacs," he concluded belonged to the same 

 species. Although he was informed by Lieberkiihn and Wagener 

 that it was a new species, he gave no name to the worm, and little 

 notice was taken of his discovery until 1871, when A. Schmidt worked 

 at the parasite. A complete study of the development was made, and 

 although his observations were faulty in one or two minor zoological 

 points, Schmidt nevertheless established the remarkable dimorphism 

 which exists between male and female. As has previously been 

 pointed out, the sexes differ so much in size and shape that were the 

 development unknown the relationship of the two forms would readily 

 be overlooked. Schmidt gave a name to the parasite, calling it 

 Heterodera schaclitii — a new species of a new genus. Kiihn (1881), 

 Strubell (1886), and others, have since cleared up the obscure points 

 in its life-history. 



Between the time of Schacht's last paper in 1862 and Schmidt's 

 work in 1871, another species of eel worm had been seen by Greeff in 

 1864 in root-galls on certain grasses ; and again, in 1870, by Magnus, 

 in excrescences upon the roots of Dodavtia ovicntalis. Greeff looked 



