2o6 



NATURE 



[Dec. 29, 1887 



more than is commonly supposed, and there are certain 

 details to be considered in employing weight as a 

 criterion which have not always been sufficiently kept in 

 mind. 



A cubic foot of "seasoned" timber of the Indian tree 

 Hardwickia binata weighs about 80 lbs. to 84 lbs., while 

 a cubic foot of Bombax fnalabaricum may weigh less 

 than 20 lbs., and all gradations are possible with 

 various timbers between these or even greater extremes. 

 If we keep in mind the structure of wood, it is evident 

 that the weights of equal volumes of merely seasoned 

 timber will yield only approximate results. For even if 

 the seasoning, weighing, &c., are effected in a constant 

 atmosphere, woods which differ in " porosity " and other 

 properties will differ in the extent to which they absorb 

 moisture from damp air or give it up to dry air. 



In our climate, timber which is felled in April or May, 

 generally speaking, contains much more water than if 

 felled in July and August : it is, in fact, no uncommon 

 event to find that about half the weight, or even more, 

 of a piece of recently felled timber is due to the 

 water it contains. If this water is driven off by heat, and 

 the piece of wood thoroughly dried, the latter will be 

 found to weigh so much less, but it will increase in weight 

 gradually as it imbibes moisture again. 



Now it happens that the weight of a piece of timber, 

 compared with that of an equal volume of some standard 

 substance — in other words, the specific weight — is of very 

 great importance, because several other properties of 

 wood stand in relation with it, e.g. the hardness, dura- 

 bility, value as fuel, tendency to shrink, &c. Fresh-cut 

 tiinber in very many cases contains on an average about 

 45 to 50 per cent, of its weight of water, and if " seasoned " 

 in the ordinary way this is reduced to about 15 to 20 per 

 cent.; but the fresh timber also contains air, as may easily 

 be shown by warming one end at the fire or in hot water 

 and watching the bubbles driven out, and the seasoned 

 timber contains less water and more air in proportion, so 

 that we see how many sources of error are possible in 

 the usual weighings of timber. At the same time, many 

 comparative weighings of equal volumes of well-seasoned 

 timber do yield results which are of rough practical 

 use. 



The fact is that the so-called " specific weight " of 

 timber, as usually given, is not the specific gravity of the 

 wood-substance, but of \!a.'\t plus entangled air and water. 

 It is interesting to note that, although we associate the 

 property of floating with wood, timber deprived of its 

 air will sink rapidly, being about half as heavy again as 

 water, volume for volume. 



The point just now, however, is not to discuss these 

 matters in detail, but rather to indicate that, other things 

 equal, the density of a piece of timber will be greater, the 

 more of that closely-packed, thick-walled autumn wood 

 it contains ; while tha timber will be specifically lighter 

 and contain more air when dry, the greater the proportion 

 of the looser, thin-walled spring wood in its "annual 

 rings." In other words, if we could induce the cambium 

 to form more autumn wood and less spring wood in each 

 annual ring, we could improve the quality of the timber ; 

 and, in view of the statement which has been made, to 

 the effect that large quantities of timber of poor quality 

 reach the Continental wood-yards every year, this is 

 obviously an important question, or at any rate may be- 

 come one. The remainder of this article must be devoted 

 to this question alone, though it should be mentioned 

 that several other questions of scientific and practical 

 importance are connected with it. 



The first point to notice is that the cambium-cells, like all 

 other living cells which grow and divide, are sensitive to 

 the action of the environment. If the temperature is too 

 high or too low, their activity is affected and may even be 

 brought to an end ; if the supply of oxygen is too small, 

 their life must cease, since they need oxygen for respira- 



tion just as do other living cells ; if they are deprived of 

 water, they cannot grow — and if they cease to grow they 

 cannot divide, and any shortcomings in the matter of 

 water-supply will have for effect a diminution of activity 

 on the part of the cambium. The same is true of the 

 supply of food-substances : certain mineral salts brought 

 up from the soil through the roots, and certain organic 

 substances (especially proteids and carbo-hydrates) pre- 

 pared in the leaves, are as necessary to the life of a 

 cambium-cell as they are to the life of other cells in the 

 plant. Now, since the manufacture of these organic 

 substances depends on the exposure of the green leaves 

 to the light, in an atmosphere containing small quantities 

 of carbon-dioxide, and since the quantities manufactured 

 are in direct relation to the area of the leaf-surface — the 

 size and numbers of the leaves — it is obvious that the 

 proper nourishment of the cambium is directly dependent 

 on the development of the crown of foliage in a tree. 

 Again, since the amount of water (and mineral salts dis- 

 solved in it) will vary with the larger or smaller area of 

 the rootlets and absorbing root-hairs (other things equal), 

 this also becomes a factor directly affecting our problem. 

 Of the interdependencies of other kinds between these 

 various factors we cannot here speak, since they would 

 carry the argument too far for the space at command ; 

 some of them are obvious, but there are correlations of a 

 subtle and complex nature also. 



First as to temperature. The dormant condition of 

 the cambium in our European winter is directly depen- 

 dent on the low temperature : as the sun's rays warm the 

 environment, the cambial cells begin to grow and divide 

 again. The solar heat acts in two ways : it warms the 

 soil and air, and it warms the plant. Wood, however, is 

 a bad conductor of heat, and the trunk of a tree is 

 covered by the thick corky bark, also an extremely bad 

 conductor, and it would probably need the greater part of 

 the early summer to raise the temperature of the cambium 

 sufficiently for activity in the lower parts of a tree by 

 direct solar heat : the small twigs, on the contrary, which 

 are covered by a thin layer of cortex, and epidermis, are no 

 doubt thus warmedfairly rapidly, and their early awakening 

 is to be referred to this cause. The cambium in the trunk, 

 however, is not raised to the requisite temperature until 

 the water passing up through the wood from the roots is 

 sufficiently warm to transmit some of the heat brought 

 with it from the soil to the cells of the cambium. This 

 also is a somewhat slow process, for it takes some time 

 for the sun's rays to raise the temperature of the soil 

 while the days are short and the nights cold. Hartig has 

 shown that the cambium in the lower part of the trunk of 

 a tree may be still dormant three weeks or a month after 

 it has begun to act in the twigs and small branches ; and 

 it has also been pointed out that trees standing in open 

 sunny situations begin to renew their growth earlier than 

 trees of the same species growing in shady or crowded 

 plantations, where the moss and leaf-mould, &c., prevent 

 the sun from warming the soil and roots so quickly. These 

 observations have also a direct bearing on the later re- 

 newal of cambial activity in trees growing on mountains 

 or in high latitudes. Moreover, though I cannot here open 

 up this interesting subject in detail, these facts have their 

 connection with the dying off of temperate trees in the 

 tropics, as well as with the killing of trees by frost in 

 climates like our own. One important practical point in 

 this connection may be adverted to. Growers of conifers 

 are well aware that certain species cannot be safely grown 

 in this country (or only in favoured spots) because the 

 sun's rays rouse them to activity at a time when spring 

 frosts are still common at night, and their young tissues 

 are destroyed by the frosts. Prof R. Hartig has pointed 

 out a very instructive case. The larch is an Alpine plant, 

 growing naturally at elevations where the temperature of 

 the soil is not high enough to communicate the necessary 

 stimulus to the cambium until the end of May or June. 



