CANKER OF THE APPLE TREE. 573

Structure and Origin of Canker of the AppleTree.

James E. Blomflcld, M.A., M.D.(Oxun.)-

With Plato 32.

THE object of the present communication is to give anaccount of the structure and origin of the tumours producedon the apple by the woolly aphis, Sch izoneura lan igera .

These tumours are familiar objects in many orchards, andare well known to gardeners who call the disease " canker."

My reason for investigating these tumours and their modeof origin arose from the circumstance that in examining atransverse section stained differentially in carmine and methylgreen there appeared to be a transition of the wood cells intothe tumour cells suggestive of a malignant process. Theliterature of vegetable pathology, as far as l could gain accessto it, gave me no help in discovering the meaning of thisappearance. Prillieux had described the structure of thetumour in 'Bull, de la Soc. Bot. de France/ 1875. His re-searches are quoted by Kiister in his ' Pathologische FflanzeuAnatomie,' but this authority notes that the subject ofwood-galls requires more exact investigation. This book,published in 1903, gives exhaustive references to pi-ev.iousobservations in vegetable pathology, and I think that I amjustified in concluding that there is no satisfactory account ofhow these tumours originate.

574 JAMES E. BLOMFIELD.

The insect which produces this disease is an aphis of apurplish-red colour. Crushed between the fingers it leavesa blood-red stain, whence it derives its German name ofBlood-louse, bub its chief characteristic which makes its pres-ence easily noted is a fluffy covering of a white or greyishcolour, sticky and resinous in consistence which exudes fromtubercles on the back of the insect, and which, from the factthat it is insoluble in water, serves to protect the insect fromwet aud damp, and to make difficult its eradication from anorchard when once it is seriously invaded by the aphis. Thematerial is soluble in alcohol, and I imagine, though I canspeak from no practical experience, that the reason that in-secticides are so useless is owing to the fact that this resinousmaterial is not removed before they are applied.

The general structure of the Schizoueurais the same as thatof other aphides except that the cornicles are atrophied. Therostrum in the immature forms is as long as, or longer than,the body. The rostrum or haustellum is as in other Herni-ptera an extension of the labium. It consists of three jointswhich are grooved on their upper surface to receive the setaeor lancing organs which prick the juicy parts of plants tocause a flow of sap on which the insects feed. The sette arethree in number, and represent the mandibles and maxillte ofother insects. During the act of sucking the rostrum isclosely applied to the plant surface and secured there bycoarse hairs at its tip, the setse are run along their grooveinto the soft plant tissue, which they lance and stab to ensurea flow of sap which can be sucked up partly by capillaryattraction and partly by a pumping action on the part of theinsect.

The question whether salivary glands are present in allaphides appears to be yet unsolved. They were found byBuckston (' Brit. Aphides,' Ray Society) in some specimens,the name of which he does not give, but in connection withthe present subject the matter would seem to have some im-portance because it is to the secretion of some such glandthat we must look as a cause for the peculiar action of the

CANKER OP THE APPLE TREE. 575

gall-produciug Aphides, Lachnus, and Schizoneura. Thecommon aphides of the ivy and the rose prick and suck thesoft young stems, but there is no specific reaction on the partof the plant such as we see in the case under consideration.

The natural history of the Schizoneura is as follows. Dur-ing the winter months the mature insects fiud shelter in thecavities and crannies of the nodosities, and in the early daysof spring their presence is noticeable from the patches ofwhite fluff. These increase iu size owing to growth in numberof the insects, and as the summer advances masses of stickyfluff envelope the branches in which are found crowds ofaphides. Some of these become " nurses " and produce livingyoung parthenogenetically. A new generation is said to beproduced every fourteen days, and as the young twigs of thetree grow new colonies are founded. The place where thecolony is started on the twig of the year does not appear tobe a matter of chance but rather of selection. At the timethat the twigs are invaded the young leaves are well formed,the distal end of the stalk is green, but towards the parenttree the green passes into a reddish-brown colour indicatingthe formation of a periderm, which, in the case of the apple,is derived from the epidermis. The place of selection is notthe green portion of the twig at the extremity, but nearer tothe old wood at a point where the wood has definitely formedand the formation of the periderm commenced. Some of the"nurses" descend to the roots as in the Phylloxera andestablish colonies there producing deformities similar to thoseon the stem.

If a tumour is selected for study on which the aphides areactively feeding, and after fixing, hardening, etc., sectionsare prepared and stained in a manner to differentiate thetissue, such as by iron haematoxylin method and fuchsiu, theappearance represented by fig. 3 will be seen.

On the outside is a layer of cells two or three in thickness,which is the periderm, beneath this is the cortex with strandsof sclerenchyma. If we follow the cortex over the tumourwe shall see that it has undergone slight alteration only, a

576 JAMES E. BLOMPIBLD.

stretching and thinning from the growth that is taking placebeneath it. The sclerenchyma bundles are, however, lessnumerous and less defined. Beneath the cortex is a layerconsisting of bast and cambium, the separation of which ismarked in all parts except over the tumour, where no distinc-tion can be made. In the centre of the section is the pith,surrounded on all sides by the wood, the continuity of whosering is interrupted by the tumour which dips into the woodring in a wedge-like manner. As a rule it does not reach thepith unless the section has passed near a leaf-shoot, whichmay happen, because a favourite place for the young colonyis just above a leaf-shoot which serves to shelter and pi'otect itfrom wind and rain, both of which are very disastrous for thepropagation of the species.

In such a section as I have described all lignified cells arestained pink with the fuchsin, and it is easy to distinguishthe soft cellular cells of the tumour from the lignified cells ofthe wood. If examination is now made with a higher powerit will be found that no hard and fast line separates thetumour from the wood, but that the tumour cells seem toarise by alteration of the wood parenchyma cells. Amongthe tumour cells, however, will be discovered pink-stained,large, irregular cells, which evidently are altered, pitted, andscalariform wood-vessels, enlarged in size, and irregular inshape. These must have arisen from altered cambium cells.The general arrangement of the cells of the tumour is in aradial direction. Large oblong nuclei can be seen in eachcell, and in some cases there are several in each cell, a factwhich was pointed out by Prillieux. The soft walls of thecells are composed of cellulose, as shown by Schultz reagent.There is hardly any starch present, as revealed by iodine,but there is sugar in relatively large quantities. If a sectionis made of a fresh tumour, and tested with Fehling's solution,this fact comes out plainly by the reduction that takes placein the tumour. A rough quantitative estimation showed1 per cent, of sugar in. the tumour, while small fragmentsfrom the same stem hardly yielded any reaction at all. A

OANKBE OF THE APPLE TREE. 577

small quantity of coagulated plasma may be found in eachcell (fig. 4).

If the cambial region of the tumour is examined with ahigh power it is evident that this tissue is in a state of greatactivity. In appropriate sections the tracks left by the setasof the insect may be traced through the periderm and cortextill they terminate in the cambium, and it is around theseterminations that the greatest activity is taking place. Thedistinction between bast and cambium is made out withdifficulty, as both kinds of cells are enlarged, and containlarge, well-defined nuclei. In places the division of thenuclei exceeds in rapidity that of the cells, with the resultthat a multinucleated mass is produced. This cell divisionno doubt takes place by mitosis, evidence of which I obtained,but the material is difficult to cut with sufficient accuracy fora study of this process. After the cells are produced by thecambium further division is undergone, or multiplication ofnuclei may take place without corresponding cell divisiou.The protoplasm exhibits vacuoles, which increase in size tillthe whole cell consists of a wall of cellulose, a small quantityof plasma, with a nucleus and a large quantity of cell sap,consisting chiefly of sugar.

That these changes are produced by the aphides is shownby the fact that, if they are swept away by wind or rain, thecambium resumes its normal activity, and gives rise to cells,which pursue their destiuy of lignification in a normalmanner, enclosing a portion of the tumour, which itselfundergoes lignification, but, from the displacement, increasein number (hyperplasia) and in size (hypertrophy), theelements are abnormally arranged, and produce a conditionwhich is known as wound wood (fig. 6).

If the aphides linger on their tumour and the weather isdry the soft pai'enchymatous tissue may split and allow agenus of destructive fungi such as Nectria to enter, pro-ducing necrosis and ulceration of the tissue. This the planttries to counteract by its powers of healing, and new cambialtissue is produced, to be quickly utilised by the aphides for

578 JAMES E. BI.OMFIELD.

their nourishment, till large gnarled nodosities are producedconsisting of dead, necrosed wood, hypertrophied tumours,and wound wood, which may attain the size of a man's fist.It is not necessary that the tumour be split for ISTectria togain entrance. In comparatively young tumours the fungusand its necrosing action may be seen, though there is nobreach of the surface except the punctures made by theinsect, and it is, no doubt, by these that the germ gainsentrance.

To shortly resume the origin of these tumours, we haveseen that they are produced by the pricks of the aphides.That during this process some influence is brought to bearon the active cambial cells which leads to their enlargementand increase. That the cells are arrested in their normaldevelopment and destiny, and that as long as this influencelasts they serve the purposes of their parasitic victors, that,when these retire, they are able again to pursue their deve-lopment and destiny, but in such a way that the traces oftheir experiences are not obliterated.

I have sketched out this view in the diagrams (fig. 7).What is the agent of this influence which the Schizoneura

is able to exercise on the cambium of the young twig ? As Inoted above, mechanical irritation we must dismiss as a cause,and we can only fall back on the hypothesis of a ferment,such as Beyernick suggested under the term growth enzyme.This may come fi'om the salivary glands of the Aphis.

I have tried to test this question by acting on a suggestionmade by Prof. Farmer of inserting a glycerine and waterextract of the insects by means of capillary tubes-as near thecambium as possible in such a way that the liquid wouldconstantly bathe the cells. No success has followed theseattempts. The slight reaction visible at the point of insertiondid not amount to more than that produced by a fine wireinserted in a similar way.

CANKER OF THE APPLE TREE. 579

EXPLANATION OP PLATE 32,

Illustrating Dr. James E. Blomfield's paper on " Structureand Origin of Canker of the Apple Tree."

FIG. 1 is a photograph of branclilet of au apple representing the growth.ofthree years. Eacli year's growth, numbered successively 1, 2, and 3, isinfected by Schizoneura, and shows the characteristic boil-like swelling.

FIG. 2 is a photograph of two sections, one through a normal stem, andthe other through a canker. At * there is a recent swelling caused bySchizoneura.

FIG. 3 is a drawing of a slightly magnified section through a young tumour.A is periderm commencing in the epidermis. B is the cortex. C is asclerenchyma strand. D is the bast. E is the cambium. F is the wood.G is the tumour with a few large vessels cut across.

FIG. 4 is a portion of a longitudinal section of tumour, x 700, showing thepointed cells (W) which would have been wood-cells, and the square-shapedcells (M) which represent modified medullary ray cells.

FIG. 5 represents a portion of the tumour in the region of the cambium.A are cortical cells. B are modified bast cells. C, modified cambial cells.D, modified wood cells. E, the track of a seta with proliferating cells in itsneighbourhood, x 700.

FIG. 6 shows a tumour passing into a condition of wound wood. Theenlarged wood cells are twisted, dislocated, and separated by collections ofcells, which represent the hypertrophied medullary rays.

FIG. 7 represents in a diagrammatic manner the changes undergone by acambium cell (A) in becoming wood, and by an affected cambial cell.

The series a to f represents the normal course; «' t o y the course underthe influence of the Schizoneura, with its cessation after a time

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