l8 9 5 . AN EEL WORM DISEASE OF HOPS. 189 



takes place leaves exhibit this peculiarity, and although all which 

 subsequently curl and die do not show it, I have never seen a case of 

 true "nettle-headed" hop without the leaves about half-way up the 

 bine being altered in this manner. Moreover, these changes take 

 place in regular order after their first appearance, being always met 

 with in leaves belonging to successive nodes and never at random on 

 the stem. A cross section of the leaf at these altered points shows an 

 abnormal production of " palisade " parenchyma, extending about 

 half-way round on each side of the fibro-vascular bundle, and giving 

 a darker green appearance (Plate III., Fig. II.). In some cases 

 increased growth of soft tissue goes on until a very minute leaf-blade 

 is produced on the side of the fibro-vascular bundle (Plate III., Fig. 

 III.). In the lighter parts between the veins, the tissue shows no 

 distinct differentiation into " palisade " and " spongy " parenchyma. 

 This peculiarity may be brought about by strains in the older parts, 

 caused by the subsequent growth of new tissue, but I have not had 

 time to follow the development completely. 



It was the discovery of the shortening of the fibro-vascular 

 system in the young shoots and leaves, and of the concomitant 

 increased cell-division in the leaf, leading to the abnormal production 

 of parenchyma along the sides of the circulatory system there, that 

 led me at once to suspect that nematodes, or eelworms, were at work 

 in the root. Although such features are not invariably met with in 

 all attacks by nematodes, in a large number of instances I have 

 found such to occur. Complete diseased plants were kindly supplied 

 to me by several growers, and an examination of the roots led to the 

 discovery of large numbers of eelworms in different parts of the 

 cortex and young root-fibrils. The parasites belong to the family of 

 Anguillulidae, and are representatives of two distinct genera, the 

 respective species being Tylenchus devastatrix (Kiihn), known as the 

 " stem eelworm," and Heterodera schachtii, A. Schm, the much-dreaded 

 " sugar-beet eelworm." 



So far as I am aware, no species of eelworm has previously been 

 recorded as attacking the hop in any part of the world ; nor has 

 Tylenchus devastatrix ever been met with in the roots of any kind of 

 plant. It is, I believe, also the first appearance of the " beet " eel- 

 worm in the British Isles, although its ally, the " root-knot " eelworm, 

 Heterodera radicicola (Greeff) Miiller, is a well-known pest of cu- 

 cumbers, tomatoes, and various other plants. 



The Tylenchi occur in the thicker parts of the roots and live 

 within the cortex, destroying it, and making their way close up to 

 the living bast. They are not uniformly distributed all over the root, 

 but only in isolated patches, and contrary to their usual behaviour 

 when met with in stems and leaves of plants, produce no excessive 

 growth of tissue in their immediate neighbourhood. Where they are 

 present the cortex loses its bright orange colour, decay sets in, and 

 the root at that point is generally found to have an eccentric develop- 



