Letters on the Diseases of JPlants. 



37 



Fig. 49.— Portion of a rootlet 

 of parsnip showing two galls, 

 due to the attack of gall- 

 worms. Magnified from Pig. 

 48. 



already so badly infested as to be almost unable to grow a decent crop of any 

 sort will, I fear, be the only ones who will realise the full force of my words. 



The nature of the disease caused by the gall-worm will become clear 

 as soon as the diseased tissues are carefully examined. Fig. 49 represents a 

 portion of a diseased parsnip rootlet, considerably magnified. The two swell- 

 ings have been caused by, and contain, the gall- 

 worms. Between the swellings the rootlet retains 

 its normal size and structure, except that some 

 modification may occur through sympathy with the 

 diseased part. If a thin section of the undiseased 

 part be examined, it will be found to present the 

 usual structure. Beneath the epidermis of the root- 

 let lie cells constituting the hypoderm, and in the 

 midst of these is found a single large central vas- 

 cular bundle surrounded by pericambium. The tissue 

 of the central vascular bundle is made up of three 

 portions, — the woody portion or xylom, having 

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

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

 lying between the two parts of the xylom and the surrounding pericambium; 

 the cambium occupying the remaining space, i.e., the two angles where the 

 two xylom portions come into contact near the centre of the rootlet. 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, the vessels have become twisted about, 

 and are often found turned to one side or the other, passing sometimes 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 recognised 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 supply 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. Tlae plant 

 is attacked at its weakest point. The tissues of one 

 of its most essential sets of organs, the rootlets, be- 

 come aborted. The absolutely essential food due 

 the plant from the soil is cut off, and unable to live 

 upon air alone it dies. 



Let us now turn to the disease as manifested in 

 the potato. Fig. 50 represents more or less spherical 

 growths which appear in the substance of attacked 

 potatoes. These growths seem always to be con- 

 nected with distorted vascular tissue. They are 

 found to vary much in size, and there seems little 

 doubt that the "knobs," characteristic of the disease as it appears on the 

 potato, are the result of these small beginnings. Each such body is composed 

 of a thick outer wall, and an inner granular mass. 



Fig. 50. — Tissue of potato 

 magnified 40 times, o, cell 

 containing starch grains ; h, 

 spherical growths seen in 

 potato attacked by gall- 

 worms ; c, vesicular tissues. 



