There is much confusion among the different species of poplar, but it is clear that in these verses Tennyson had in view our native abele or grey poplar (Populus canescens), a native of Great Britain, often mistaken for the white poplar (P. alba), which nearly resembles the grey, and has been planted in this country, but is probably an exotic. The poet's epithet "silver green" admirably describes the foliage of the grey poplar, for some of the shoots bear green leaves, others white ones, others again green leaves on the lower part and white on the upper.

Of all known species of poplar, thirty or so in number, the abele produces the choicest timber, much in request by carriage-builders, who sometimes pay as much as 2s. 6d. a cubic foot for well-grown logs. It is excellent timber for flooring bedrooms, being less inflammable than any other British-grown wood except larch. It is, therefore, characteristic of British neglect of woodland resources that this tree is hardly ever planted, though it is most easily propagated from suckers or cuttings, and attains an immense size long before an oak could reach maturity.

The abele is more common in Scotland than in England, and many large trees might be mentioned in the North. It would be difficult, however, to find any to surpass two growing at Mauldslie Castle, in Clydesdale, one of which in 1911 measured 100 feet high and 21 feet 3 inches in girth, the other 117 feet by 16 feet 5 inches. It should be noted that the girth of both was taken at between 2 and 3 feet from the ground, instead of 5 feet, which is the proper height for measurement.

Next in economic importance to the grey poplar stands what is popularly known in this country as the black Italian poplar (P. serotina), which is not Italian in any sense, but a hybrid originating in France (where it is called peuplier suisse) between an American species and the true black poplar (P. nigra). This confusion of names is all the more perplexing because the upright variety of the true black poplar goes by the name of Lombardy poplar. However, one must use the names most generally recognised among woodmen, and the black Italian poplar is well worthy of more attention than it has hitherto received in this country, for it produces valuable timber in greater bulk in a short term of years than any other British-grown tree. Mr. Elwes has recorded how thirty poplars of this variety, planted on cold clay in Gloucestershire, not worth 5s. an acre, were sold for £3 apiece at forty-eight years of age. He lays stress on the importance of giving this tree plenty of room at all stages of growth, planting them at 15 to 20 feet apart, for the timber is little worth unless the tree gets enough light to enable it to produce wood rapidly. This precept applies to every species of poplar.

The tallest black Italian poplar in Great Britain is probably one growing on the banks of the Tillingbourne, in Albury Park, Surrey, which in 1912 measured 150 feet high, with a girth of 15 feet 3 inches. There are many fine specimens in Scotland, notably one at Scone Palace, which in 1904 was 132 feet high, with a girth of 15 feet 4 inches. Another at Monzie, in Perthshire, measured at the same time, stood 125 feet high.

Turning now to the true black poplar (Populus nigra), we find that this species, a native of Midland England, but probably not of Scotland, has become established in the eastern United States, having been introduced there by British colonists. It has often been confused with the black Italian variety, but may easily be distinguished in this country by the large burrs on the trunk, by its earlier leafing, and by the young foliage being green, instead of reddish, as in the black Italian. The true black poplar also sheds its leaves much earlier in autumn than does the other. It is not a tree commonly planted in Scotland, but there are specimens ranging from 90 to 100 feet high at Dalzell, Ross, and Cambusnethan, in Lanarkshire; at Auchentorlie, on the Clyde; and at Smeaton-Hepburn, in East Lothian.

The variety of this tree so well known as the Lombardy poplar forms a notable feature in the landscapes of Southern England, Central and Southern Europe, and a great part of Asia. As it can only be propagated by cuttings, it is believed that all the millions of Lombardy poplars spread over the continents of Europe and Asia originated with a single "sport" growing on the bank of the river Po early in the eighteenth century. Probably the first of its race was brought to England about 1750 by the third Duke of Argyll, and planted by him at Whitton, near Hounslow. This tree, which has now disappeared, was measured by Loudon before 1838 as 115 feet high.

One peculiarity of the Lombardy poplar I do not remember to have seen noticed by any writer on forestry. Other poplars of all sorts, including the black poplar whereof this is only a variety, mingle branches freely with their neighbours; but the Lombardy poplar is a regular Sainte-Nitouche, and will not suffer contact with any other tree, even one of its own race. A curious example of this may be seen in London. When the Buckingham Palace Hotel was built, somewhere about 1860, Queen Victoria desired that a screen of trees should be planted within the Palace enclosure to shut the hotel out of view. The Office of Works chose the Lombardy poplar, calculating that it would form a lofty, thick hedge. Not a bit of it! The trees died rather than touch each other; they have been replaced times without number; but the Office of Works has never discerned the secret of their temperament, and continue their task of Sisyphus year after year, filling the gaps caused by death with trees of the same kind. Had a row of true planes been set there at first, the privacy of the Palace would have been secured long before this.

Despite this constant characteristic of the Lombardy poplar, which anybody may verify for himself by examining the fine groups of them near Maidenhead and Windsor, Selby committed himself to the extraordinary statement that this tree, "planted so as to form a hedge, and being cut even at a certain height and regularly trimmed, becomes a thick and verdant hedge."[11]

The asp (Populus tremula) is now generally spoken of by the adjectival form "aspen." Its ceaseless movement earned it the name of "quick-beam" in Anglo-Saxon, and the Lowland Scots name, "quakin' asp" (corrupted into "quakin' ash") has, so far, survived the operations of School Boards. Long may it do so! The same characteristic in this tree gave it the Gaelic name of crithean (creean) or criothach (creeagh), "the trembler," which may be recognised in such place-names as Creechan in Dumfriesshire and, perhaps, Crieff, in Perthshire. Although in bulk and stature one of the humbler members of the poplar family, the asp exhibits in an extreme form a peculiarity common to all the genus—namely, that of hanging the leaves vertically, instead of holding them horizontally. The leaves are glandular on both surfaces, which may be either the effect of or the reason for their assuming a position protecting both surfaces from the direct rays of the sun. To secure this position, the petiole, or foot stalk, of each leaf, being cylindrical in most of its length, is suddenly flattened midway between the leaf and the twig, as if it had been pinched while soft. This causes the leaf to hang as described, and to quiver with the slightest breath of air.

The asp is a hardy mountaineer; its graceful foliage and eau-de-Nile bark saves many a Highland hillside from dreariness, but it has long ceased to have the economic importance it once had. By an Act of the English Parliament (4 Henry V. c. 3), a penalty of 100 shillings was imposed upon anyone who put aspen wood to any other purpose than the making of arrows. Mrs. Hemans has woven into verse the mediÆval myth which taught men to reckon this pretty tree accursed:

Oh! a cause more deep,
More solemn far, the rustic doth assign
To the strange restlessness of those wan leaves.
The cross, he deems, the blessed cross whereon
The meek Redeemer bowed His head to death,
Was formed of aspen wood; and since that hour
Through all its race the pale tree hath sent down
A thrilling consciousness, a secret awe,
Making them tremulous, when not a breeze
Disturbs the airy thistle-down, or shakes
The light lines of the shining gossamer.

Gerard, writing in the sixteenth century, says, with scant gallantry, that the asp "may also be called tremble after the French name, considering it is the matter whereof women's toongs were made, which seldom cease wagging."

Professor Sargent enumerates eleven species of poplar as indigenous to North America, some of which, such as the Balsam poplar (P. balsamifera), the Ontario poplar (P. candicans), and the Carolina poplar (P. angulata), have risen to large dimensions in British woodland; but to follow out these, and their constantly recurring hybrids, would far exceed the limits of this paper.

There are many Asiatic species also, one of which (P. euphratica) we are now taught to recognise as the "arabim" whereon the captive Jews hung their harps (Psalm cxxxvii. 2). The weeping willow, named by LinnÆus babylonica, is not found in the valley of the Euphrates.

It is time that British planters should recognise the importance of the more vigorous species of poplar as rapid timber-producers, thriving in cold, wet ground where no other crop could be raised so successfully. A useful example is set in this matter by French cultivators, who plant more poplars than any other tree. Moreover, all the species are most easily propagated and handled in planting. They should be grown from cuttings; it is futile to attempt raising them from seed—a most uncertain process, and unsatisfactory when it succeeds, cutting-grown plants being far more vigorous than seedlings.