페이지 이미지
PDF
ePub

in the Dean forest; and Ringdon, in Swindon, Wilts, though disafforested, can yet show noble trees of this form. Indeed throughout England it is difficult to meet with many examples of any other form, except in Wyre forest, Worcestershire, where the tree next to be described is perhaps the more general, and it would also appear that in the New Forest the Q. sessiliflora is also frequently met with.

Quercus Robur sessiliflora may be generally described as of a more upright and formal habit. Limbs straighter and less gnarled. Bark usually smoother than the former. The leaf has many sinuosities, and is set on a comparatively long leafstalk (petiole)-(Plate II. fig. a).

The fruit on the contrary is so nearly sessile that it may be said to have little more than the indication of a peduncle (fig. 6).

We have already stated our opinion that the sessile-fruited oak does not usually attain the huge dimensions of the pedunculate form ; but on the other hand we incline to the belief that it grows more rapidly, and is best adapted for a lighter soil than the latter. There are conditions which might to a greater or less extent affect the quality of its timber, but we do not think that there is much difference in this respect. We believe that their wood has been used indifferently, and the quality is influenced by surrounding circumstances. Selby, in his “History of Forest 'Trees,” states on this head, “ The result, perhaps, of some original constitutional defect, or arising from the nature of the soil, situation, or other local peculiarities of the ground upon which the timber has been raised, such at least is the result of our own experience, as we have met with oak of the peluncled kind, its timber possessing all the inferior qualities attributed to, and supposed to be possessed exclusively by Q. sessiliflora.The longer, straighter spars of the Sessiliflora, in days when oak was so uniformly used for roofs, seem to have pointed out this variety for roof-timbering, and hence some of the finest ancient timbered roofs of this country have been ascertained to have been formed from its wood. With respect to these the opinion long prevailed that they were formed of the wood of the Spanish chestnut. This, however, is but a poor timber tree, as long before it could afford so large a scantling as would be required by the roof of the Parliament House at Edinburgh or of Westminster Abbey (both of which were supposed to be of chestnut), the chestnut would begin to decay at the heart; in fact just at the period when the heart-wood of oak begins to harden, that of the chestnut would appear to deteriorate.

Quercus Robur intermedia, having a petiole intermediate in length between the other two varieties described, and a peduncle

varying from a quarter to one inch in length, may with propriety be deemed a variety intermediate between “Sessiliflora ” and “Pedunculata," and a comparison of the three will substantiate its claim to this title.

As a tree it is impossible to make out any specific character from its mode of growth, and, indeed, without the fruit, it is extremely difficult even to distinguish it as a variety.

It occurs-only occasionally—in the Cotteswold district, and we suppose the same elsewhere. One meets with it here and there in the hedgerows, and in Oakley Park, the seat of Earl Bathurst, we can point out a few specimens.

Passing from the subject of the varieties of our British oak, it now remains to mention a most formidable enemy by which it has of late years been attacked, and so exclusively, that in plantations where may be found the American oaks, the Ilex oak, and Turkey oak trees, it has been the only one subjected to the operations of the new gall pest. It has long been known that our native oaks were subject to excrescences of different forms and sizes, such, for example, as oak apples, oakleaf galls, oak span

gles, &c., all of which were ascertained to be caused by several species of cynips; but lately, we have to lament the introduction of a new species of the same insect, forming a new kind of gall, which, instead of attacking the backs of the leaves, as does the oakleaf gall, occupies the stem that belongs to the leaf; in fact, the attacked leaves seem to be converted into bunches of galls, as represented in the adjoining figure, which presents an illustration of the new pest. They are hard galls, more or less like the “nut-gall” from Aleppo, of which ink is made, and it will be seen that the little twig supports no less than

five galls, in the interior of Galls of the Cynips Quercus-petioluta. each of which may be found (Natural size).

the maggot or larva of an insect; and, as this is effected at the expense of the buds and leaves, the mode of injury must be obvious, as the new twigs which would have been formed, had there been no galls, would

[graphic]

in their turn have produced branches and leaves. Trees thus infested are crippled as though they had been subjected to constant pruning.

As much of the natural history of the cynips, by which these gall-nuts are formed, as is necessary for our purpose, may be gathered from a paper by Mr. Parfitt, who seems to have well studied the gall insect in Devon, its head quarters. We quote it from the Journal of the Bath and West of England Agricultural Society for 1861 :

The eggs deposited by the females in the oak buds in September remain there in a state of apparent quiescence till the following spring; then as soon as the sap begins to flow, the irritant injected into the wound at the same time the egg was deposited, or possibly the combined action of the egg and irritant, causes the sap to diverge ; that portion of the bud which should have formed a young shoot, is converted into a spherical ball; the outer scales of the bud fall away, and it is the woody secretion which entirely forms the gall. The cells in the gall are not elongated and regular, as in the young shoot, but confused and irregular; and in the centre of each gall lies a young grub of the cynips, forming a living nucleus, around which is deposited a thin hard woody envelope, much more compact in substance than the sponge-like tissue which fills up the interstice between it and the shining outer coat of the gall. This compactness of structure is a necessary and all-wise provision of nature for protecting the delicate insect which lies within from destruction ; for if the gall were composed entirely of large spongy cells, the rapid flow of sap in the early spring would be more than the creature could consume, and it would consequently be drowned. I am aware that some naturalists incline to the opinion that the larvce of the cynips feed on the gall. From this view, however, I venture to dissent; for not only is it inconsistent with the structure of the creature's mouth, and the position in which the young larve are invariably found, with the head tucked under the apex of the abdomen, but if they fed on the substance or crude material of the gall, the undigested parts would certainly be found in the interior of its cell : in other words, the excrement would be there, for there is no outlet, and the lacteals or absorbent vessels of the gall could not take it up. I therefore think that the creature feeds entirely on the sap of the tree-an elaborate food fit for it without the need of mastication. This explains how it happens that the galls of commerce, with the insects in them, are so much better and dearer than those from which the cynips has escaped ; in a word, the tannic acid is more abundant.

It has been before observed, that there are two broods of the insect in a season; thus, those which do not emerge from the gall in September, remain on till the following April or May. This is a wise provision of nature for continuing the species, should anything befall the autumn brood ; and it is the more deserving of notice, because the gall-producing cynips has a deadly enemy which accompanies or follows it in its flight from bud to bud, and deposits an pod wherever it finds the egg of the cynips. Here, as soon as the cynips larva is hatched, the larva of the parasite is hatched also ; forthwith the latter proceeds to eat a hole in the skin of the rightful occupant of the nidus, and the two larvæ go on growing together till the cynips is ready to assume the pupa state ; then the parasite cuts the vital thread of the cymips and uses its skin for a pupal envelope for itself, and thus, instead of the gall-fly emerging into day, a beautiful green insect makes its appearance on the stage of life. I had the pleasure of first discovering this parasite while engaged in studying the cynips ; it belongs to the genus Callimome, and from the fact of having discovered it in Devonshire, I gave it the name of Callimome Deronicnsis. It is one of the handsomest of our British insects ; its costume a brilliant green, shot with gold ; the abdominal segments green, gold, and purple ; legs yellow; tarsi reddish ; and it has four beautiful transparent and irridescent wings.

It has been stated that oak-galls are produced at the expense of acorns. From this view, my experience leads me to dissent. In exceptional instances, it may have been the case ; but, as a rule, the comips confines its attacks to young trees and young growths in hedges, within a range of ten or twelve feet from the ground, and the nearer the ground the more numerous the galls. Young trees which have not attained a greater height than that I have indicated, suffer so much, that many of them can scarcely make headway against their foe ; and in several nurseries I have visited, where it might be expected that greater care would be paid than in the case of ordinary plantations, the young stock of oaks has been rendered quite unsaleable by the pest. This year I have noticed the progress of the insect on two groups of young English and Turkey oaks growing side by side ; and although there are hundreds of galls on the English oaks, there are none on the Turkey oaks. From this I am led to infer that the species of cynips now under notice is confined in its depredations to the English oak; and as it invariably selects trees of younger or restricted growth-probably because the temperature at a higher elevation than ten or twelve feet from the earth is unfavourable to it-it would seem that children might be advantageously employed in young plantations in collecting the galls by means of cutting hooks, such as are used for thistles. The galls, when once collected, might either be crushed for tanning purposes, or consumed by fire, and if the process were repeated for two or three seasons, it is more than probable that the plantation would be altogether freed from the pest.

These able remarks not only well describe the nature of the attack, but also point to a cure, a matter to which we would direct the most serious attention of the planter; for we may state that, in 1853, we saw some very small oak trees, in the neighbourhood of Dawlish, Devon, from which some hundreds of these galls might have been gathered. This was the first time we had noticed this pest, though it appears that it had been under Mr. Parfitt's notice as long as a dozen years. Since then (1853) we have traced it in its progress as follows:

Having observed the galls in Devon in 1853, we were yearly on the look-out in the Midland and Eastern counties for its appearance, and the following dates will show that its spread, though gradual, was sufficiently rapid :

« 이전계속 »