Mew South Wales. Ill 



III. 



The Disease and its Remedies. 



WuAT is the exact nature of the disease caused by a root-inhabiting 

 Tylenchus? How docs it spread from one plant to another, and from 

 infected to non-infected lands ? Is there any cure or preveutive ? These 

 are the questions to bo answered in the present section. In the two previous 

 sections the subject has been dealt with rather after the manner of a scientific 

 treatise. Such a course needs, however, no apology, for it will now be seen 

 that all the remedies for, and precautions against, this root disease depend 

 completely upon a scientiiic Itnowledge of the microscopic worm which is its 

 sole cause. 



The nature of the disease caused by Tyhmchus arenarins will become clear 

 as soon as the diseased tissues are carefully examined. Fig. 7 represents a 

 portion of a diseased parsnip rootlet, considerably mag- . 



nified. The two swellings have been caused by, and ',1 



contain, the gall-worms. Between the swellings the )^ 



rootlet retains its normal size and structure, except that \1^ r, 



some modification may occur through sympathy with the y ( 



diseased part. If a thin section of the undiseased part \ I 



be examined, it will bo found to present the usual struc- 

 ture. Beneath the epidermis of the rootlet lie cells of a EootIet'of PiRsmp. 

 constituting the hypoderm, and in the midst of these showing two gaiis due 

 is found a single largo central vascular bundle surrounded to attack of gaii-worms ; 

 by pericambium. The tissue of the central vascular '""^'i' 

 bundle is made up of three portions, — the woody portion or xyloui, having 

 in the section the contour of an hour-glass or dumb-bell ; the sieve tissue 

 or phloem, appearing iu the section as two narrow crescent-shaped areas, 

 lying between the two parts of the xylom and the surrounding pericam- 

 bium ; the cambium occupying the remaining space, i.e., the two angles 

 where the two xylom portions come into contact near the centre of the root- 

 let. If now a section of the diseased part of the same rootlet be examined, 

 it will be found that the additional size is caused by an increase in the 

 amount of each tissue, but particularly of those constituting the vascular 

 bundle. The epidermis and hypoderm remain comparatively unaltered in 

 structure, but have increased somewhat in amount. The central vascular 

 tissue, on the other hand, is much altered. It is increased in quantity, and 

 the vessels have become much distorted. Instead of continuing parallel to the 

 axis of the rootlet, as they would normally do, tlie vessels have become twisted 

 about, and are often found turned to one side or the other, passing some- 

 times in a radial direction, and even in some cases turning backward. 

 Whatever portion of the tissue has been actually invaded by the worm is 

 easily recognized by its yellow colour. In the majority of cases, according to 

 my observations, it is the cambium of the rootlet that suffers the greatest 

 destruction. 



The rootlets are the most fundamental organs of a land-plant. Upon 

 them depends its 8up])ly of water and earthy material. Taking this fact into 

 consideration, we shall no longer wonder after noting the changes wrought 

 by the gall-worm, why so small an assailant can do so great injury. The 

 plant is attacked at its weaicest point. The tissues of one of its most 



