March 28, 1889] 



NA TURE 



513 



well known that the production of fruit and seed " ex- 

 hausts" the plant : in the case of annuals it completely 

 drains their resources, and every apple-grower knows that 

 the trees need *' rest " after a good crop. In view of all 

 the facts, then, it is most probable ^ that the stores of starch 

 in the beech are put up in reserve for the enormous 

 drain which the " seed-year " will involve, and we shall 

 see that this idea is fully borne out by the chemical 

 analyses, which show that certain valuable minerals are 

 similarly stored for the seeds. 



But this does not fully explain why the stores diminish 

 inwards. Two causes are adduced for this. In the first 

 place, a seed-year having exhausted nearly all the supply 

 of starch, we have seen that succeeding deposits only 

 occur in the outermost rings of wood, and so there is no 

 restoration of the deposits deeper in the tree ; secondly, 

 some of the stored starch in the deeper layers gradu- 

 ally undergoes change into the drops of " wood-gum " 

 {Holsgiwuni)^ of which mention was made above. 



Some " practical results " of the above may now be 

 noted, the most important being that the difference in 

 weight between wood felled in summer and that felled 

 in winter is, in effect, nil, contradicting a wide-spread 

 assumption, and confirming a doubt which Nordlinger 

 had already put forward. It thus follows that the want 

 of durability in summer wood depends on other causes, 

 and Hartig considers it due to the fact that winter wood 

 has time to dry on the outside before the atmospheric 

 influences are favourable for the development of Fungi, 

 the spores of which are always about, but dormant in 

 the cold of winter. No doubt there are other factors to 

 be considered also, but the importance of the above has 

 been too much overlooked or under-estimated. 



Another interesting section of the work is that dealing 

 with the formation of the annual rings. By cutting disks at 

 equal distances apart on simultaneously-felled trees of 50, 

 100, and 150 years old, and measuring the breadth, &c., of 

 the rings at eight points round the disks, some further dis- 

 coveries were made. Generally put, it was found that (in 

 the case of beeches near Munich, at any rate) the annual 

 ring commences to form at about the end of May, the 

 tree being already in full leaf; by the middle of June the 

 ring is one-third its normal breadth, and is half finished 

 early in July, attaining its normal complete state before 

 the end of August. Hence the whole period of the acti- 

 vity of the cambium only amounts to about two months 

 and a half. 



As regards the parts of the tree, it is found that the 

 active division of the cambium commences first in the 

 twigs and small branches ; it is later in the trunk proper, 

 and begins at different parts, according to circumstances. 

 In the oldest trees (150 years) the cambium was found 

 in an active state at 3 to 4 feet up, while parts above and , 

 below were still dormant ; whereas in somewhat younger 

 trees the process of ring-formation began simultaneously 

 all up and down the trunk. In still younger trees the 

 cambium was found to awaken first in the higher parts 

 of the trunk. More investigation is still needed here, 

 however, before several dark points can be regarded as 

 explained. 



Some generalizations as regards the growth in thick- 

 ness of the beech deserve notice. In the crown, the 

 annual increment — i.e. the quantity of wood produced by 

 the cambium during one period of its activity — increases 

 more or less rapidly as we proceed from the tips of the 

 branches to their point of origin from the trunk ; but this 

 is by no means the case in the trunk itself, and several 

 cases have to be considered. 



In those trees which, owing to close crowding in the 

 forest, have developed only feeble crowns, the annual in- 

 crement is greatest Just beneath the crown, and diminishes 

 regularly downwards ; and in very closely crowded trees 



'Hartig has since proved that this explanation is the correct one 

 {J}otanisc/te Zeitung, December 28, i888, p. 837). 



the cambium in the lowermost parts of the stem may 

 even stop dividi?ig altogether : in such cases the ordinary 

 mode of ascertaining the age of the tree would yield false 

 results, for the number of annual rings at 3 to 4 feet high 

 is less than the number of years of the tree's life. The 

 physiological meaning of the above is, that the small 

 leaf-area does not supply sufficient food-material to pro- 

 vide for the needs of the whole sheet of cambium, and 

 the upper parts take all that is sent down, leaving none 

 for those below. 



In those trees which have well-developed leafy crowns, 

 more exposed to light and air, the annual increment 

 follows a rule exactly the converse of the last — the amount 

 of wood formed per annum is greater as we proceed from 

 the upper part of the stem to the lower. If we leave out of 

 account the lowermost 6 to 12 feet, every gradation can be 

 found, and in rare cases the breadth of the annual ring 

 may be constant from above downwards. 



Now comes in a remarkable discovery. If such trees 

 as the above are suddenly exposed to full light and air, 

 &c., by cutting down the neighbouring trees, the annual 

 rings in the lower parts of the stem suddenly become 

 much broader : no such stimulation of the increment 

 occurs in the upper parts. 



Now as to the explanation of these remarkable pheno- 

 mena. There can be no reasonable doubt that the pre- 

 cedence shown by the upper parts of crowded trees is due 

 to the rapid warming which they receive from the air in 

 the spring sunshine : the lower parts of such trees, how- 

 ever, have to wait until the water which they absorb from 

 the soil raises their temperature to the minimum cardinal 

 point, and by the time the water of the soil is sufficiently 

 warm for this, the cambium in the upper parts is far ahead, 

 and working under such favourable circumstances that 

 the rings maintain their greater breadth to the end. But 

 the chief factor in the process is that the upper cambium 

 gets the first supplies of food-substances, and in larger 

 quantities, because lower down the diminished supplies 

 have to spread over a larger area. 



In the case of trees exposed freely to the light and air, 

 the sun's rays warm the thinly covered soil (and its water) 

 around the roots, and so the cambium is enabled to re- 

 commence its annual work pretty nearly at the same time 

 over the whole stem : in this case thicker rings in the 

 upper parts of the stem must be due to the nutrition being 

 more abundant. All this still fails to explain the sudden 

 stimulus to the annual rings in the lowermost parts of 

 suddenly isolated trees, and Hartig suggests that the 

 probable cause is an increased supply of potassium salts 

 and phosphates, rendered available at the roots. This 

 of course implies the further assumption that such mine- 

 rals are employed directly, and however probable this 

 may be, it is by no means proved. 



The removal of branches from the tree leads to the 

 same results as crowding, i.e. the rings formed below are 

 thinner, because the supplies are not sufficient to feed the 

 sheet of cambium equally from above downwards. More- 

 over, the complementary case may occur : a tree in the 

 open may have too many leaves, as is proved by the fact 

 that it may be pruned without any loss of increment. 

 The leaf-area of a tree is by no means always proportional 

 to the supplies of food-materials from the soil : it may be 

 too large or too small to be working economically, or so 

 large that each leaf is sluggish — lazy, so to speak, and not 

 doing anything like the amount of work it is capable of. 

 Not only is this idea interesting and suggestive in itself,, 

 but it has important bearings on the question of the 

 thinning and treatment of forests generally. 



We must leave this topic, however, and pass to one of 

 a differe'nt nature, but no less scientifically important. 

 This is the weight of the wood. Although certain practical 

 ends can be roughly attained by merely weighing equal- 

 sized blocks of any particular kind of timber, at any time 

 or in any state, it is, nevertheless, easy to see that such 



