514 



NATURE 



{March 28, 1889 



weighings are of little or no scientific value: only the 

 weight of the fresh timber immediately it is felled, and 

 the absolute dry weight (after exposure to 105° C. long 

 enough to drive off all moisture) yield results of really 

 scientific value. 



If v/e regard i cubic metre as the unit of volume, 

 we may obtain some useful factors by ascertaining the 

 weight of dry woody substance in such a volume, from 

 different parts of the tree, and from trees grown under 

 different conditions, &c. The amount of water driven 

 oft", i.e. the difference between the fresh weight and the 

 absolute dry weight, is found to vary much, and Hartig 

 some time ago obtained most valuable results, bearing on 

 the difficult question of the ascent of water in tall trees, by 

 comparisons of this kind. Moreover, the real test of 

 quality of wood — its value as fuel, and other technical 

 properties— is given in the absolute dry weight. 



Passing over the methods, and other details, it may 

 next be pointed out that the weight of a given volume of 

 wood depends chiefly on the sizes and distribution of the 

 histological elements — vessels, tracheides, fibres, &c. — 

 and in the case of beech-wood, it is especially the sizes 

 and numbers of the vessels that have to be taken into 

 account, and as these stand in direct relation with the 

 magnitude of transpiration, it is clear that the quality of 

 the timber as estimated by its weight depends on the 

 quantity of leaves. 



Neglecting the roots, we may regard the tree as con- 

 sisting of three parts : the stock, the shaft, and the crown. 

 Now, the root-stock and the crown contain wood of the 

 best quality, and some curious results come out on 

 examining why this is. 



As is well known, the base of the tree widens at the 

 origins of the main roots, and here the annual rings are 

 broadest : if we bear in mind that the number of vessels 

 in each annual ring remains constant, it is easy to under- 

 stand why the wood is better — it is simply that the vessels 

 are dispersed over a larger sectional area, and are separ- 

 ated by more numerous fibres, the elements which give 

 solidity to the wood. 



We have seen that in the trunk of a tree with a large 

 crown of leaves, the mass increment increases from above 

 downwards : this means that the same number of vessels 

 (per annual ring) are distributed over a smaller sectional 

 area above. In a given case, on i square millimetre of area, 

 there were 115 vessels at a height of r3 metre, but at 107 

 metres height there were 175 vessels on the same area ; 

 hence the latter was lighter and worse wood. 



By thus counting the number of vessels per square 

 millimetre, and taking the average size of the main 

 vessels, it was possible to get an expression of the rela- 

 tive area occupied by the lumina, and that of the rest of 

 the annual ring ; of course this is only approximate. 



It comes out that, in trees with large crowns, while the 

 number of vessels is the same at all heights in the stem, 

 the number of v^%s&\% per square millimeire is much fewer 

 below than above. 



In the crown of the tree, however, things are very 

 different ; the number of vessels in each annual ring 

 rapidly diminishes, because at each branching a number 

 are given off. Thus, where 200,000 vessels were found in 

 an annual ring in the stem, the same in the crown gave 

 only 57,750. This alone would explain the better quality 

 of the wood, but the number of vessels per square milli- 

 metre is also found to increase in the crown, and this 

 means corresponding depreciation. But the most im- 

 portant factor in explaining the superior hardness, &c., 

 of the wood in the branches is that the average size of the 

 vessels is less, and therefore the area of lumina in the 

 cross-section is reduced. 



Physiologically, the reduction in the lumina of the 

 vessels is in relation with the decrease in the volume of 

 water- current as we ascend, and several facts point to the 

 constancy of this relation. It is well known that, if the 



soil around a tree is suddenly deprived of much of its 

 water, the tips of the tree die off first : " stag-headed " 

 trees are often produced by over-exposure. This is 

 because the average size of the vessels has been adapted 

 for a richer supply of water than comes to them under the 

 new conditions. Hartig says that the average size of the 

 vessels throughout is reduced if the land is deprived of 

 cover, and the tree exposed too much. 



As has been seen, the wood of trees below 60 years 

 of age contains fewer vessels, and these with smaller 

 lumina, than afterwards. It is also known that the as- 

 cending water-current is confined to the younger outer 

 wood, or alburnum ; and if we neglect younger trees, it 

 seems that in the beech it is only the 20 or 30 outer annual 

 rings which conduct the water. 



Now the authors of the book referred to find an un- 

 expected relation between the amount of wood produced 

 annually, and the current of water passing up the stem. 

 By an ingenious series of measurements and calculations, 

 it results that much more room is provided for the water- 

 flow in early years than in old age. Thus, a given 

 amount of water, which has for its passage in a tree 

 30 years old an area of wood expressed by the number 

 4'04, has only an area equal to i 64 at 140 years of age. 

 Hence, in order to conduct the larger quantities of water 

 which must pass to the larger crown, the smaller area of 

 wood, in the older tree, has to increase the member and 

 size of its vessels, and so the wood is lighter and poorer. 



It is impossible here to enter into the bearing of these 

 matters on questions of forest management ; it is only a 

 particular caiie of the dependence of technical forestry 

 throughout on the teachings of science, the principles of 

 which it applies. 



An interesting experiment may be quoted. Two 

 beeches 150 years old were felled and examined; they 

 had been completely freed from neighbouring trees 7 

 years previously. The effect of the sudden exposure to 

 free light, air, &c., was that the mass increment rose to 

 2-4 times greater than previously, and the weight of the 

 wood formed during the 7 years of exposure was 700 

 kilogrammes per unit volume, as against 600 kilogrammes 

 previously, i.e. 167 per cent, more wood-substance was 

 formed. On going into details, it was found that Jive 

 times as much wood-substance was formed each year, and 

 twice as many vessels were developed in each annual ring. 

 But since these twice as many vessels were distributed 

 over five times the quantity of wood, the wood was still 

 heavier than that of 7 years previously. On the square 

 millimetre there were 63 vessels, as contrasted with 140. 



The reason that letting in the light and air around the 

 tree has such enormous effects is obvious enough to the 

 physiological botanist, but it should also be clear that the 

 knowledge thus obtained is the best guide to such forest 

 practices as thinning and freeing timber : into these 

 matters, however, we do not propose to enter further 

 here, but must pass to other matters. In the section on 

 the course of growth of the beech, an interesting discus- 

 sion on the limits of height of trees occurs : Hartig regards 

 the chief limiting cause to be the gradual disappearance 

 of the difference in tension between the air-bubbles in 

 contiguous elements : the osmotic forces remain constant 

 throughout, but the lifting power diminishes with age 

 and height, until it ceases to suffice for movement. The 

 influences of etiolation, and judicious crowding, and other 

 devices for timber-growing, are then discussed in the light 

 of what has been already said, and with the aid of numerous 

 tables of close-set and well-classified figures, sufficient 

 illustrations are given to satisfy the most stiff-necked 

 critic of the value of these results. 



The chemist's results, however dry they may appear 

 from the tables and curve-diagrams, allow of summary in 

 a way that endows them with an interest to the general 

 reader, no less real than that which attaches to other 

 parts of the work. Meihods may be passed over here. 



