38 



Letters on the Diseases of Plants. 



These appearances iu the parsnip and potato lead me to suspect that the 

 abnormal growths caused by the gall-worm are probably to be compared 

 to the galls produced on leaves by various insects. It is well known that 

 leaf-galls are caused in the first place by a disturbance of the vascular tissues. 

 The gall-producing insect commonly pierces a vein of the leaf with its 

 ovipositor when depositing its egg. It is a common belief that the gall 

 appears in consequence of a fluid injected by the insect at the time of laying 

 the egg. I do not know whether this belief is supported by any good 

 evidence. Possibly the mere irritation of such a foreign body as the egg of 

 an insect or the wriggling larva hatched from it may be sufficient to account 

 for the growth of the gall. 



If I am right in comparing the swellings produced by this nematode 

 to the leaf -galls produced by insects, then the former should be called root- 

 galls, and the nematode itself may appropiately bear the name of the gall- 

 worm. Dr. Neal has called the disease, as it appears in the United States, 

 the root-knot disease. His name can have no referance to knots, the places 

 where branches originate, otherwise it would be entirely inappropiate, but 

 it refers to the characteristic appearance produced by the disease on rootlets 

 which has been compared to the appearence of a thread with knots tied in 

 it. The German name for the disease caused by Tylenchus Schachtii is 

 Eiibenmlidigkeit — that is, turnip-lassitute or beetroot-lassitute, referring to 

 the tardy growth of the diseased plants. I believe both these names will be 

 supplanted by the simple term " root-gall " (Wurzelgalle), which may be 

 thus defined — abnormal growths on roots and rootlets, caused by the attacks 



of gall-worms. 



Historical. 



It is not until within recent years that we have arrived at an accurate 

 knowledge of the habits of the gall-worm, although the disease root-gall has 

 been known for a very long time. How long root-gall has been recognised 

 as a distinct disease of the sugar-beet of Europe I am unable to say, but 

 that it is very many years is certain. The root-gall of the peach has, 



according to Dr. Neal's exceedingly useful 

 pamphlet, been known to the white people 

 of the South Atlantic and Grulf States of 

 America since the earliest settlement of 

 the country ; and, according to the same 

 authority, reliable agriculturists state that 

 the disease is indigenous there, they having 

 seen it in places where neither trees nor 

 plants had ever been introduced from 

 other sections. The disease is now known 

 to occur in North America on a belt of 

 land 150 miles wide, extending from Texas 

 along the Gulf of Mexico and Atlantic 

 coast northward to the January isothermal 

 of 50° Fah. According to the best testi- 

 mony yet obtained , the peach-tree formerly 

 grew on this area with no other disease 

 than the borer, except in damj) localities ; 

 while now in many places, owing to the 

 prevalence of root-gall, the trees that do well are the exception. This fact 

 shows how the disease has spread, or at least increased, and should serve as 

 a warning to Australians. 



Fig. 51. — Gall-worms, taken in different 

 stages, from the interior of a potato. 

 I, yonng worm ; ii, female becoming 

 gravid ; iii, full-grown fertile female ; 

 IV, eggs in two iirst stages of seg- 

 mentation ; V, vulva. I, II, and ill 

 are magnified 25 times; iv is mag- 

 nified 100 times. 



