Journal Royal Horticultural Society 5: 105-111 (1850)
XIII.—Will Tubers Grow after the Destruction of the Leaves of a Plant?
By John Lindley, Ph.D., F.R.S.

IN the summer of 1849 the Belgian Government communicated to H. M. Minister at the Court of Brussels a statement by M. Tombelle Lomba, of Namur, that he had saved his crop of Potatoes from disease every year by cutting off the stems after flowering, and whilst yet fresh and green, and then covering the ground with earth to the depth of about 1 1/2 inch; the top dressing thus applied not being disturbed till the potatoes were ripe.

Lord Howard de Walden having caused further inquiries to be addressed to M. Lomba, an extract from his reply was given in the 'Gardener's Chronicle,' July 7, 1849, in the following words:— "I can state in the most formal manner, that when the potato stems are cut off with a sickle properly sharpened, the tubers are not at all interrupted in their growth; that they remain attached to the stem until they are ripe, just as if the haulm had not been removed; and that they acquire as large a relative size as potatoes which have not undergone the operation. I have so often observed this continuation of growth, that I can speak positively to its going on without the slightest interruption, and that the treatment which I have recommended is not attended by any loss whatever of size or quality. It is only necessary to take great care that the instrument employed in cutting off the haulm shall be so sharp that the stems may be separated without disturbing the roots. It is also proper that the stems should be removed from the ground immediately after being cut off; and especially that no time should be lost in covering the surface of the ground with a layer of earth at least half an inch thick."

With a view to testing the constancy of this fact Mr. Thompson was directed to repeat M. Lomba's experiment; and he reports as follows:— "In the beginning of July, rows of Jersey Blues were cut close by the ground when in flower, with a sharp knife; and the ground immediately covered over with soil as recommended. Besides these, the north half of a row was cut over and covered in the same way, whilst the south half was left; and another row had the south half cut over, its other half not. In short, due precautions were taken to insure a fair result. On the 30th of August the rows cut over were dug up, and the produce weighed and compared with that from adjoining rows of Jersey Blues of which the stems had been allowed to remain. The rows were 2 1/2 feet apart, and 24 feet in length. The average sound produce of rows cut over was 4 lbs. 5.4 ozs., which is at the rate of 1 ton 8 cwt. 19 lbs. per acre. The average sound produce of rows not cut over was 28 lbs. 9 1/2 ozs., which is at the rate of 9 tons 7 cwt. per acre. Hence, the difference in favour of stems not cut down amounts to 7 tons 18 cwt. 93 lbs. per acre. With regard to diseased tubers, there were none in the rows cut down. In the rows not cut down the diseased portion averaged 13 ozs. per row, being at the rate of cwt. 29 lbs. per acre.

"It may be proper to observe that the potato plants experimented upon had not been earthed up, so that they were cut over by the ground-level. On taking them up, it was found that the portion of stem left under ground was quite dead. In some cases a fresh shoot had pushed; and such shoots were found to be making fresh roots, and commencing to form runners for tubers; of course these would be too late for attaining either size or maturity of any importance. Their tops were fresh; but had no living connexion with the tubers that had been formed from the original stems. These first-formed tubers were small, but firm and sound. Their growth must have been almost entirely arrested by the cutting down of the stems. The soil was dry. In former years, in moister soil, I have observed, since the disease commenced, that the tubers in many cases increased considerably in size after the total, but premature, decay of the tops.

"The results of the experiment are unfavourable to the proposed method of cutting off the stems; for although the tubers from the plants deprived of their foliage were sound, yet they were obtained at the loss of more than five-sixths of the crop which would otherwise have been produced.

"On referring to vol. iii. p. 180 of this Journal, it will be seen that some experiments were tried in the Society's Garden in 1846, with reference to cutting off the stems as soon as disease manifested itself upon them. It was found to have a somewhat beneficial effect where the tubers had previously acquired considerable size, but was found to have a diminishing effect on the amount of produce where the crop was not so far advanced."

From this it would appear that nothing is gained by the operation, and that the rate of growth was so impeded that, although the crop was saved from disease, the amount of loss in the crop that was untouched was trifling compared with that sustained by the removal of the stems. The experiment, moreover, does not show whether the tubers increased in weight or not after the operation.

But an experiment by Mr. Dooville, of Alphington, near Exeter, recorded in the 'Gardener's Chronicle,' does not confirm Mr. Thompson's conclusions. That gentleman says:—

"In the beginning of last November I planted the Early Frame Potato, a later white sort, and the Queen's Noble, a still later Potato. Fresh slaked lime was spread over the ground and turned in upon the sets as each row was planted, to prevent the ravage of slugs during the winter. No other manure was used.

"On the 14th of July, the Potatoes being still in flower, I cut off the stems of two rows of the later white sort, and earthed them over about 2 inches, leaving two rows in their natural state. Adjoining were several rows of the Queen's Noble. I cut down three rows of these and earthed them over. Early in July I perceived symptoms of disease (black spots) upon some of the leaves; it spread more after some showers which fell about the 24th. The haulms of the Early Frame had then assumed an appearance of natural decay. I cut them all down. They were taken up on the 21st of August, all sound, and a fair average crop.

"Finding the disease was spreading, and that the stems as well as the leaves of the Queen's Noble had become much affected, I, on the 14th of August, cut down the remainder, and earthed them over. On the 4th of September I caused three rows of the Queen's Noble, cut down on the 14th of July, and three rows cut down on the 14th of August, to be taken up. Those cut down July 14th produced—

1 row, 55 feet in length 15 1/4 lbs., tubers all sound.
2 do. do. 18 1/4 do. all sound.
3 do. do. 16 1/4 do. all sound

"The tubers small; the largest size weighed 3 oz. Those cut down on the 14th of August produced—

1 row 55 feet in length 44 3/4 lbs., 32 tubers diseased.
2 do. do. 42 3/4 do. 27 do. diseased.
3 do. do. 39 do. 8 do. diseased

"The tubers generally of good average size; the largest weighed 8 and 9 oz. I was, I must confess, disappointed with the result in the first case. I then proceeded to take up two rows of the White Potato, cut down on the 14th of July, and two rows which had been left untouched. Those cut down on the 14th of July produced—

1 row, 55 feet in length 41 lbs., tubers all sound.
2 do. do. 42 do. all sound.

"The tubers generally of a good size; what would, in fact, be called a fair sample: some of the largest weighed 5 and 6 oz. Those rows which had been left untouched produced—

1 row, 55 feet in length 60 1/4 lbs., 7 tubers diseased.
2 do. do. 69 do. 10 do. diseased.

"The tubers generally much larger, and many weighed 8 oz. The result in this instance is more favourable, and I think it may be accounted for in this way:—The White Potato is an earlier sort than the Queen's Noble, and, although both planted at the same time and under the same circumstances as to locality, soil, &c., it came into flower earlier, and I had, as is my usual practice, picked off the first flowers a week or ten days before the stems of both sorts were cut off. The tubers, therefore, were, in all probability, in a more advanced state, and in a better condition to draw nourishment, by their own vitality, from the soil.

"There is here, I opine, strong presumptive evidence that the tubers do, as affirmed by M. Lomba, grow unassisted by the stem and leaves, as it cannot be supposed they would attain a size to weigh 6 oz. whilst the plants are yet in flower. The difference in produce may probably arise from my having cut down the stems too soon. I think, indeed, the result in both cases, but more particularly in the Queen's Noble, clearly proves this to be where I have erred. The error, however, is instructive. It would also appear that the disease is communicated by the leaves and stem to the tubers; for in no instance where the stems were cut off before attacked by the disease are the tubers diseased, whereas in both of the other cases many of the tubers are diseased.

"The result of these experiments will, I think, justify the conclusion, that by autumn or early spring planting there is a better chance of a healthy crop, as the plants would, under favourable circumstances as to weather, &c., put forth blossom before the time the disease usually makes its appearance, and by adopting M. Tombelle Lomba's plan there would be a reasonable hope of securing an average crop."

I have also from Mr. Alexander Burnett, gardener at Roby Hall, near Liverpool, a further statement upon the same subject:—

One drill, "Pink-eye Kemps." Dug up Aug. 20.
Same length as the other four.
Four drills," Pink-eye Kemps." Tops cutoff August 20. Dug up September 21.
Tops not cut off. Tops cut, not earthed. Cut off, earthed, and trod. Pulled up, earthed, and trod.
Good Bad Good Bad Good Bad Good Bad Good Bad
lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs.
70 1/2 20 88 49 1/2 62 1/2 21 1/2 83 13 74 10 1/4

"The above were selected in the middle of a plot facing south, 105 feet long.

One drill, Robyhall Seedlings, round Potato. Taken up Aug. 4. Three drills. A Seedling raised here from Seeds of 1846. Tops cut Aug. 4. Taken up Sept. 21.
Tops not cut. Cut off, but not earthed. Cut off and earthed.
Good Bad Good Bad Good Bad Good Bad
lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs.
19 1/2 " 21 1/2 3 1/4 17 1/2 22 1/2

"The above were selected, side by side, from a plot upon a west border, each drill 16 feet in length. As soon as they were dug, I washed them clean and spread them out for an hour or two to dry previous to weighing them; and in order that there might be no mistake in the different operations, all was performed by myself."

In this case it would appear as if the removal of the leaves actually diminished the crop at the instant of removal, which is incredible. A space which produced 90 lbs. on the 20th of August, the tops being untouched, produced only 84 lbs., in two instances, a month later; one such space, however, yielded 96 lbs., having gained 5+ lbs. after the removal of the tops. The second experiment was of the same nature. And although both are of very small value, they seem to show that M. Lomba's statement is not confirmed by English experience.

On the other hand, Mr. Gwatkin, of Pare Behan, near Tregony, entirely confirms the Belgian account. In the 'Gardeners' Chronicle' of August 25, 1849, he says

"After reading the 'Belgian Farmer's' recommendation on the treatment of Potatoes at the time of flowering, I had my crop of 'Red Roughs' watched, and as soon as the flowering had ceased, I caused the haulms to be cut off within an inch of the surface, and the earth drawn over the plants an inch thick. This was done to two rows, leaving double the quantity of the same crop in its natural state. Yesterday the Potatoes of both were drawn: in the rows left untouched, two-thirds of the crop were found to be more or less affected; but in the rows on which the experiment was tried, every Potato was found perfect, and in size and quantity the same as the others. I beg to add that the same result followed a similar experiment, made in a garden by one of my workmen, upon a small crop of 'Snowballs.' "

Here then is a result diametrically opposite to that obtained by others, and in accordance with the statement of M. Tombelle Lomba. The experiment must, therefore, be repeated. It is probable that the discrepancy in the results arises from the plants under experiment not being in the same stage of growth. It appears to me essential to success that the flowering shall have been completed, and that the fruit shall have begun to form. At that time the office of supplying the tubers with matter from which they may organize their starch is nearly over, and the leaves are chiefly occupied in the nutrition of the growing fruit; so that the loss sustained by the tubers may be little or nothing.

That tubers and roots will grow for many years, although their tops are removed, is an undoubted fact, be the explanation of the process what it may. It is now seventeen years since M. Dutrochet brought to the notice of physiologists the unexpected fact that in the Jura may be found the roots of fir trees still alive and growing, at the end of forty-five years after the trunks were felled. A similar example is recorded by the Rev. Mr. Berkeley in the case of an Ash Tree which had been sawn over level with the ground. (See 'Gardener's Chronicle,' 1850, p. 99.) Gardeners know very well that the tuberous Tropaeolums, the stems of which have been accidentally broken off, will continue to grow for a long time afterwards: as also will tuberous Bindweeds. These are notorious facts, though they have never been recorded with the exactness which is desirable in scientific questions. And it so happens that I have now before me a very illustrative case, which places the matter beyond all further question.

It chanced that in the Conservatory of Chiswick House a plant of Sello's Ipomoea was, in November, 1840, destroyed to the ground by frost, since which period it has neither made buds nor leaves. Nevertheless, its roots have continued increasing rapidly in size. In fact, it has been frequently repotted as its increase in size demanded it; for in 1840, at the time of the accident, it was but a small root. During this long period it has been subjected to a high temperature. At this moment the root forms a coil, not unlike a boa constrictor, a foot across, 6 inches deep, and weighing 7 1/4 lbs. Although we have no record of its weight at the time when the stem perished, yet, as it has continued to grow for nine years and a half, and was originally in a small pot, it is not unreasonable to assume that it has acquired at least seven times its original weight. Although no leaves have been formed, yet many attempts at the production of stems are visible upon the specimen, in the form of short stunted tubercles or incipient branches; and the root is now so full of vitality that I entertain no doubt of the possibility of compelling it, by artificial means, to resume its growth.

Here then is a very striking proof that plants have an inherent power of growth without leaves. It is probable that in this case the bark, of which a large surface has been exposed to light, has acted as a substitute for foliage, perspiring, and assimilating food, as all green parts do, whether leaves or not. It is also probable that the surface of the root which rested upon the earth, and which still is colourless, has constantly attracted from the soil the food which the bark is assumed, in this case, to have assimilated. But if such a power can be recognized in an Ipomoea, we must also admit its existence in the tuber of a Potato, even although that tuber is not exposed to light; and the vital force of the latter must be allowed to be capable not only of converting into starch the gum which was supplied by the leaves, but of absorbing gaseous and fluid matters from the soil, and, by their assimilation, of continuing to grow, although perhaps for only a limited time.

It would indeed be an experiment worth trying, whether, by some artificial means, the Potato itself might not be made to go through the same kind of leafless enlargement as that now recorded in the Ipomoea.