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that Mr Babington named the plant H. Lapeyrousii. Probably, however, the former was the more mature judgment on Mr Babington's part; it agrees with that of Backhouse, who has written H. strictum upon several specimens from the same place collected by Dr Dewar. There is a specimen collected at Inverarnan by Professor Balfour in August 1847. It has been referred to H. denticulatum (?) by him, to H. strictum by Backhouse, and to H. rigidum by some one else. There is another specimen gathered by Professor Balfour at Killin in July 1848. The species was not given at first, but subsequently Dr Balfour wrote the name denticulatum. He further added, “Fries refers it now to H. strictum." There is also a specimen collected by Professor Balfour at Inversnaid in July 1846, and labelled by him H. rigidum.

XX. Notice of a New Species of Agaric (Agaricus (Clitocybe) Sadleri, Berkeley). By JOHN SADLER. (Plate IV.)

(Read January 1878).

In October 1877 upwards of a dozen specimens of a beautiful lemon-coloured Agaric appeared on the decaying inner surface of an oak tub in one of the stoves at the Royal Botanic Garden. Being unlike any species I had hitherto seen, I submitted fresh examples of the plant to the Rev. M. J. Berkeley, M.A., F.L.S., who pronounced it to be new to science. A second crop of the plant came up in the same tub in January 1878, but although the specimens were as bright in colour, they were not so large as those of the first crop.

The following are the characters of the species drawn up by Mr Berkeley, who has done me the honour of naming it after me:--

Agaricus (Clitocybe) Sadleri, Berk.

Caspitosus olidus; pileo plano-depresso vel umbilicato, flavo, centro fulvo, primum sericello demum versus centrum glabrescente; stipite deorsum incrassato luteo fulvo-fibrilloso glabrescente, lamellis citrinis tenuibus confertissimis decurrentibus, margine integerrimis.

Pileus 2-2 inches across; stem 3-4 inches high, about thick except at the base; gills pale lemon-coloured, very thin crowded decurrent.

Allied to A. illudens, Schwein., which is a far coarser species. The taste is intensely acrid, like that of A. fascicularis. Agaricus crocobaphus, B. and Br., a species from Ceylon, is also nearly allied, but differs in the prominent umbo, and gills at first rounded behind, and never so decurrent as in this species.

Specimens are preserved in the Museum and Herbarium at the Edinburgh Royal Botanic Garden; others have been sent to Kew.

XXI. On the Exact Measurement of Trees, and its Applications. By Sir ROBERT CHRISTISON, Bart., Hon. V.-Pt. R.S.E. &c.

(Read 11th July 1878.)

Having been accidentally interested not long ago in the history of the Fortingal Yew, I was referred for its reputed age to no less an authority than De Candolle. I found in his "Physiologie Végétale," ii. 974, that the basis of this estimate is the general rule, founded on the study of sections of old and middle-aged trees of various kinds,-the yew being one,―that, after a certain number of years of rapid and irregular growth, the diametral increment of their trunks is uniform, or very nearly so, to the extent of their lives, if the measure be taken by decades of years, and not annually, in order to avoid irregularities occasioned by differences in the geniality of the successive annual seasons of growth. This statement, in such a quarter, and based on such a profusion of laborious observations always consistent, filled me with surprise, as being directly at variance with what I thought I had myself invariably observed. On then subjecting the rule expressly to the practical test, I find that it is erroneous. The demonstration of this error is one of the objects of the present paper.

In order to apply to a standing tree this rule of De Candolle, or any other more exact rule of the same kind which future experience and observation may substitute, the first necessary condition is to ascertain the girth of the trunk of a tree. But here two grievous errors are generally committed.

In point of fact, of the numerous measurements to be found in books of travels, or on forestry, few are of any real value, either for this or any other practical purpose, because the conditions necessary for accuracy have either been expressly violated or have not been mentioned as having been kept in view. Another object, therefore, of this paper is to point out what these conditions are.

The accurate measurement of the girth of a tree is a point not merely of scientific interest, but likewise of great practical consequence,-firstly, to the wood-merchant, for estimating the cubic feet of timber in the tree; secondly, to the forester, for ascertaining by successive observations whether or not a tree has substantially ceased to grow, and consequently whether or not, if cultivated for profit, the time has arrived for felling it; and thirdly, also to the forester, for ascertaining by successive observations the influence of soil, elevation, thinning, manuring, pruning, shelter, and other points of treatment on the growth and vigour of trees.

It has been said, indeed, that human life is too short for comparative observations at successive periods of time. But this objection applies only to the rude methods of measurement usually practised, the errors of which can be met only by long intervals of time being allowed to elapse between the observations. For it is not difficult to show how measurements may be taken with such minuteness and accuracy as to indicate precisely even each annual incre

ment.

The first point to be attended to is to be provided with a 33 feet waxed tape, which should be accurately divided into tenths of an inch, as may be easily done with pen and ink. A short measure of any kind, so short as to require to be frequently removed and re-applied, is out of the question when real exactitude is desired. A tape so long as 33 feet is convenient; because, though more than half of it will seldom be needed, the inner half will serve to test from time to time whether the outer half has become stretched by frequent use.

The next consideration is the place of measurement. In this respect errors are often committed, and no well-understood rules exist. In measuring remarkable trees many observers take the girth at the ground; others choose one

foot above it; 3 feet seems the favourite point; 5 feet is far from being so; when the girth is given higher up still it is only for comparison with that stated at one or more lower levels; and lastly, very often the level of measurement is not mentioned at all. But on considering the mode of growth of the trunk of a tree of some maturity, it will be seen that the suitable place of measurement is far from being a matter of indifference.

When a tree begins to swell at the base with the progress of years, and the increase of weight to be supported by its trunk, the enlargement, whether by general dilatation or by fluted buttresses, is effected simply by increased width of the succeeding annual layers of wood; and this process extends upwards with advancing years, so as at length to reach the height of 3, 4, or even 5 feet from the ground, but seldom any higher, except in the case of trees of very great age and gigantic limbs and head. It is evident that the measurement of any part of this rapidly enlarging base cannot be a true measure of a trunk 20, 15, or even only 10 feet in length,-whether the object be a comparison with another tree which may have also an inflated base, but much shorter or much higher,—or above all the determination of its own rate of growth by the comparative girth gained in a term of years. In like manner, the woody layers expand at the upper part of the trunk, to support the gradually increasing weight of the first great branches; and the limit of this expansion extends downwards with the progress of years, so that this part too of the trunk cannot be the seat of a suitable measurement for any practical or scientific purpose.

As a general rule, the only suitable point is 5 feet above the ground, except in the case of young trees, and also, for a special reason, the yew. But in point of fact, every level is liable to fallacy unless chosen with due regard to the form of the trunk in each individual instance. Measurements at the ground serve no purpose except to enhance the wonder.

See a subsequent paper on the Fortingal Yew. The yew has two forms. A rare one has a tall stem and lofty head, quite like the Pine tribe; and this, when aged, has the usual expanded base. The common form has a short trunk with a low, widely-extended head; and in general its trunk is narrowest at the very level of the ground, and immediately swells towards its first great branches.

of the reader, and ought to be given up as utterly untrustworthy, unless in exceptional cases. The only correct rule is to choose a point above the conoidal base, or the abutments, which support the greater number of old trees, and also low enough to keep clear of the swelling of the trunk which in all trees supports the lowest great limbs.

Thus in the adjoining diagram of the trunk of a fine standing oak, 175 years old, in the park of Craigie Hall, the seat of Mr Hope-Vere, near Cramond Bridge, the circumference at the ground is 22 feet; at 1 foot above, 14 feet 11 inches; at 3 feet, 11 feet 07 inches; at 5 feet, 10 feet 04 inches; above which the girth continues the

8 feet

5 feet

3feet

foot

same till, about 8 feet from the ground, the trunk swells to support the first large branch. It is evident, from a simple inspection of this diagram, that an exaggerated idea would be conveyed of the magnitude of the trunk by giving the girth so low as at the favourite level of 3 feet above the ground; that the upper end of the conoidal basement of the trunk extends even above 4 feet;

and that 5 feet is the lowest point above which the trunk puts on its proper gently-tapering form. Numberless instances of such exaggerations have appeared in print. For example, the larger of the two famous larches at Dunkeld is usually spoken of as being 17 feet in girth; and on visiting these trees a few weeks ago (June 29, 1878), I found it described on a tablet as being 17 feet 2 inches in 1872 at 3 feet from the ground. But the conoidal basement, forming an unusually large angle with the trunk, evidently reaches a good deal higher; 5 feet is the level. at which the tall trunk first puts on its gently-tapering, almost cylindrical, form; and when I measured it at that height with the aid of the Duke of Athole's forester, Mr Macgregor, we found its girth to be exactly 14 feet 9.6.

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