648 



THE TmPtCAL AGmcVtfVmBf. [APRtL i, 2887, 



twelve species of Rholodendrons, in Manipur alone. 

 The Rhododeodion F<ilconeri foun I in the Naga Hills 

 by Sir Joseph Hooker is nowhere met with in the 

 immense tract bptween the Naga His and Sikkim. 

 This and the epiphytic R. Da!housiaD, which throws on 

 a hill thirty miles north of Darjeeling, Dr. Watt 

 found in the Xaga Hills at an altitule of 6000 to 8000 

 feet, but these Rhododendrons never occur in Sikkim 

 below 10,000 to 13,000 feet. There were many instances 

 of plants falling in their altitude as the traveller passed 

 to the ea.st and south-east from Sikkim, until at 

 Moulmein a Rhododendron was found growing near the 

 sea — a circumstance which was not met with in any 

 other part of Asia. — Gardeners' Chroniclt'. 



Formation of Dew.— From a paper read before the 

 American Association for the Advancement of Science, 

 in August, 1886, by Prof. H. E. Alvord, on the 

 " Formation of Dew," we learn that the facts established 

 by repeated observations are, that the lower stratum of 

 air rapidly increases in cold as we descend towards the 

 ground, until checked by the radiation of heat stored up 

 there during the day. Ihe height of this critical 

 stratum, where the two temperatures just counteract 

 each other, varies under different conditions. On bare 

 soil it is usually very near the surface, and even 

 may be slightly below it. Grass or other thick 

 growths of vegetntioa behavps like the soil in retaining 

 a considerable des^ree of heat in the air entangled among 

 its foliage, so that its presence restrains the cold 

 stratum of air at a still greater height. There is, 

 therefore, on the occasions when dew is formed most 

 copious'y a stratum of air of markedly low temperature 

 lymg close down 1 1 the surface of the ground, with its 

 lower limit sharply defined, but above gradually 

 increasing in heat. Above this stratum is the com- 

 paratively warm atmosphere, bea'ing watery vapour; 

 below it a thin layer of air supphed with heat and 

 moisture from the soil. The result is the formation 

 of dew by condensation of watery vapour within the 

 cold stratum. JMost of the moisrure seen upon many 

 plants in the ni Tning, in the course of a season, is 

 transpired by the plants themselves, and this trans- 

 piration is sufficient in quantity to supply an appreci- 

 able portion of the atmospheric vanour subsequently 

 deposited as dew. — Gardeners' Chronicle. 



An Easy Method of Mbas' ring the Height of a 

 Growing Tree. — Stveral methods of metsuring grow- 

 ing trees have appeared in your columns at various 

 times, but all requiring a more or less complicated 

 calculation, and consequently can only be worked out 

 by those who have a fairly good education, and in some 

 case.«, probably, expensive instruments. An accurate 

 measurement may be quickly taken without either, as 

 any one may make all necessary apparatus in a very 

 short time, and at a cost of not more than sixpence. 

 Take a straight piece of common slater's lath, say, for 

 convenience in measuring, 10 feet long, plane smooth, 

 and mark it in feet and inches ; also a small piece of 

 board, half an inch thick and 6 inches square will do 

 very well (it must be perfectly square) ; draw a line 

 diagonally across one side of the board, and on the 

 other side screw two square staples sufficiently large to 

 admit the la' h to slide closely through ; these must be 

 exactly opposite each other. At from 5 feet to 6 i iches 

 above bore holes in the lath to admit a peg to hold the 

 board in position, to suit the height of different persons 

 (a stout wire nail makes a good pe;;); ajardof fine 

 string, with a loop at one end and small weight at the 

 other for a plumb-line, and that is all th it is required. 

 To_ measure the tree put the peg in a hole to suit your 

 height, slide the board on to the lath to the peg, and 

 hang the plumb-line on the peg, then walk to a 

 distance from the tree, so that when holding the 

 lath perfectly upright, which the plumb-line will 

 show on looking up the diagonal line across the 

 board it is exactly straight to the top of the tree, 

 the corner of the board nearest you will be exactly the 

 same dist ince from the tree that the tree is high from 

 that level, which can at once be measured by the 10 

 feet lath ; add to that the height the board was from 

 the ground, and you have the exact height of the tree. 

 It is simply two sides of a square which must be equal 

 This can be done by any one, and certainly will not take 

 $Vf wiuutes.— Thos. WooDroRD, The Nurserjesj AtUer- 



stone. (This method is accurate enough if the ground be 

 level, but it sometimes happens that it is not so, or the 

 b.ase line is inaccessible, owing to water or other trees 

 or objects of some kind. Ed. Gardeners' Chroidcle.] 



Fruit trees Cankering (An Old Gar deiier).— While 

 it is not wise to be prejudiced against what you call 

 " new notions," we think you are right in considering 

 that the truth of the "insect theory" is not yet es- 

 tablished beyond dispute. But this is not such a " new 

 notion " as you appear to imagine. We have heard 

 of it years ago, and seen the insects alluded to after 

 the canker appeared, not before. Insects cause ulcer 

 atioii, as in the case of the Aphis lanigera, or American 

 blight, but that is not canker as the term is generally 

 understool. The following remarks cited from the 

 " Cottage Gardener's Dictionary " are pertinent to your 

 inquiry, and we have not a doubt there is a consider- 

 able amount of truth in them: — "This disease is 

 accompanied by different symptoms, according to the 

 species of the tree which it infects. In some of those 

 whose true sap contains a considerable quantity of free 

 acid, as in the genus Pyrus, it is rarely accompanied 

 by any discharge. To "this dry form of the disease 

 it would be well to confine the term canker. In other 

 trees, with sap abounding in astringent or gummy 

 constituents, it is usually attended by a discharge. In 

 such instances it might strictly be designated ulcer. This 

 disease has a considerable resemblance to the tendency 

 to ossification, which appears in most aged animals, 

 arising from their marked tendency to secrete the 

 calcareous saline compounds that chiefly constitute 

 their skeletons. Tlie consequence is an enlargement 

 of the joints and ossification of the circulatory vessels 

 and other parts — phenomena very analogous to those 

 attending the cankering of trees. As in animals, this 

 tendency is general throughout their system ; but, as 

 is observed by Mr. Knight, 'like the mortification in 

 the limbs of elderly people,' it may be determined, 

 as to its point of attack, by the irritability of that 

 part of the system. This disease commences with an 

 enlargement cf the vessels of the bark of a branch or 

 of the stem. This swelling invariably attends the 

 disease when it attacks the Apple tree. In the Pear, 

 the enlargement is less, yet is always present. In the 

 Elm and the Oak sometimes no swelling occurs ; and 

 in the Peach we do not recollect to have seen any. 

 The swelling is soon communicated to the wood, which, 

 if laid open to view on its first appearance by the 

 removal of the bark, exhibits no marks of disease 

 beyond the mere unnatural enlargement. In the 

 course of a few years, less in number in propoi-tion to 

 the advanced age of the tree and the unfavourable 

 circumstances under which it is vegetat'ng, the swel- 

 ling is greatly increased in size, and Ihe alburnum 

 has become extensively dead ; the bark above it cracks, 

 rises in discoloured scales, and decays even more 

 rapidly than the wood beneath. If the canker is 

 upon at moderately sized branch, the decay soon com- 

 pletely encircles it, extending through the whole 

 alburnum and bark. The circulation of the sap being 

 thus entirely prevented, all the parts above the 

 disease perish. Trees injudiciously pruned, or growing 

 upon an ungenial soil, are more frequently attacked 

 than those which are advancing under contrary circum- 

 stances. The soil has a very considerable influence 

 in inducing the disease. If the subsoil be an irony 

 giavel, or if it is not well drained, the canker is almost 

 certain to make its appearance amongst the trees they 

 sustain, however young and vigorous t'ley were when 

 first planted. All the.'c facts before ns unite in assur- 

 ing us that the canker arises from the tree's weak- 

 ness, from a deficiency in its vital energy, and con- 

 sequent inability to imbibe and elaborate the nourish- 

 ment necessary to sustain its frame in vigour, and 

 much less to supply the healthy development of new 

 I)arts. It is (juite true that over -luxuriant trees arc 

 particularly liable to this disease ; but over-luxuriance 

 is really a demonstration that the tree does not digest 

 and secrete its juices healthily." We do not consider 

 that canker arises from either weakness or exuberance 

 only, but both are contributory causes, and when they 

 exist remedial measures should be adopted accordingly. 

 _ Journal of Horticidtare. 



