512 



NATURE 



{March 28, 1889 



Theodore Hartig, Sanio, and others, the length and 

 breadth of the various elements differ in different ^arts 

 of certain trees. Prof. R. Hartig has now worked out 

 this subject in the beech for the first time, giving long 

 lists of measurements at various heights, ages, &c., as 

 before. 



The recent vessel-segments, tracheides, and fibres in a 

 five-year-old beech-tree are only half as long as those in a 

 tree 120 years old, and this occurs in what at first sight 

 appears a very curious and inexplicable manner. 



The length of these organs at first rapidly increases, 

 until the tree is about 60 years old ; then they either no 

 longer show increase in length, or do so very slowly, till 

 the tree is about 120 years old. They then exhibit their 

 maximum length. Henceforward the elements formed 

 are shorter each year, and much so if the tree is growing 

 free in the open. 



Moreover, in the same tree, the longest elements occur 

 at the base of the trunk, and shorter and shorter ones 

 occur up to a height of about 5*5 metres ; then the 

 tracheides and vessel-segments are found to be longer 

 again, until the height of 15 "9 metres is attained. The 

 lengths are much less in the crown. The libriform fibres 

 decrease regularly in length all the way up. 



Hence, put generally, the elements are short in young 

 trees and in the upper {i.e. youngest) parts of older ones ; 

 their lengths increase afterwards year by year, but after 

 120 years only shorter and shorter elements are again 

 found. 



The lamina of the vessels also vary with the age of 

 the tree and with the height of the part. Taking, for 

 example, the vessels at a height of i -3 metres, the average 

 diameter is 005 millimetre during the first 30 years, but 

 between 30 and 60 years they are larger (o"o64 millimetre), 

 and maintain this average afterwards to the end of the 

 life of the tree. . Still more striking are the changes at 

 different heights in the tree : both in very young and in 

 very old trees the vessels in the crown may be very 

 narrow indeed compared with those elsewhere. 



As facts of great importance in its bearing on the 

 question of the specific gravity of wood, and the futility of 

 comparing rough weighings, we may select the following. 

 The three elements — vessels, tracheides, and libriform 

 fibres— are distributed very differently on the transverse 

 section of the annual rings according to the age of 

 the tree and the level of the section. The rule is that, at 

 the same level, the number of vessels per square milli- 

 metre increases as the tree ages. When it is shown that 

 the numbers may range between about 60 to 80 at 30 

 years, and 200 to 220 at 100 years or more, the conviction 

 arises that the question of specific gravity may be 

 complicated by many factors. 



As regards the level of the section examined, the rule 

 is that the number of vessels per square millimetre 

 increases as we go upwards. But it is found that the 

 number of vessels in any one annual ring remains about 

 the same : it is differences in the breadth of the rings 

 which cause the close packing or otherwise, and the 

 general tendency of the rings to be narrower upwards 

 explains the above. 



With respect to tracheides and fibres, it may be said 

 generally that young trees form few tracheides (and 

 chiefly near the vessels) but more are formed later ; but 

 again, in old age, in the open, the tracheides are replaced 

 by fibres. 



Some interesting observations follow on the micro- 

 chemistry of the wood : vanillin and coniferin occur in 

 the walls of the wood elements, and it is somewhat 

 remarkable that they should show a cellulose reaction 

 quite late. Relatively small quantities of tannin are 

 found in the cells, and drops of "wood-gum" are abundant. 

 It is interesting to note the infiltration of the walls with 

 tannin, and this gives the deeper colour to wood exposed 

 to air, owing to oxidation. 



The dark (false) heart of the beech is not due to the 

 presence of much tannin, and Hartig again insists that 

 this wood cannot be divided into heart-wood proper as 

 distinguished from sap-wood. The false heart is a 

 pathological production, and nearly always contains 

 Fungi. 



But perhaps the most interesting facts in this connec- 

 tion are those bearing on the starch-grains and their 

 movements. 



In an old beech-tree, the quantity of starch diminishes 

 from the periphery to the centre : little or none is found 

 within the last 50 annual rings. In the winter the outer 

 rings will be crowded with starch, every cell of the 

 wood-pareuchyma and medullary rays being full. 



It is, of course, impossible to go into the details of 

 Hartig's experiments and measurements, but he found 

 that under ordinary circumstances the main mass of 

 stored-up starch does not move at all : contrary to the 

 received opinion that the starch is all, or nearly all, dis- 

 solved in early summer, and stored up again in autumn, 

 the astonishing fact comes out that during the develop- 

 ment of the current year's annual ring, the cambium only 

 takes starch from the next inner ring (and sometimes the 

 next but one) in June and July, and that before the middle 

 of September it is all restored. 



In other words, only the two preceding annual rings 

 yield starch to start the cambium : the completion of 

 the new ring, its stores of starch, and the restoration of 

 the borrowed starch, are at the expense of the work of the 

 leaves of the current year. 



Light is thrown on the subject by the following experi- 

 ment—an admirable instance of the progress which is 

 being made in the study of the physiology of plants. Two 

 trees were completely deprived of branches and leaves^ 

 and then allowed to stand otherwise untouched : one was 

 felled at the end of twelve months, the other at the end of 

 two years. In both cases it was found that during the 

 first year after the mutilation a new ring was formed by 

 the cambium, but the mass of wood in this was only about 

 5 per cent, of the normal increment which would have 

 occurred if the tree had remained intact : no trace of 

 further increment was observable in tree No. 2 during the 

 second year. , „ , 



This 5 per cent, increment was at the expense of all the 

 starch stored in the medullary rays and wood-parenchyma 

 of the stem ; in other words, the quantity of starch held 

 stored in each of these trees was equivalent to the quantity 

 of woody substance in a ring containing 5 per cent, of 

 the normal annual amount : in other experiments the 

 amount rose to 1 5 per cent, or more, but never approached 

 that of a complete normal ring. It is noteworthy that 

 the cainbium only acquires the power to attract the whole 

 of this stored starch under such special conditions of 

 hunger as are induced by stopping its supplies from the 

 leaves. 



Some similar experiments, with modifications m the 

 special cases, led to the result that the starch which 

 comes down from the leaves— even when only sufficient 

 to partly fill one layer of wood— is rapidly distributed over 

 the whole sheet of wood, both above and below. 



The question. What are the stores of starch for, if not 

 to feed the cambium? is answered by the following. 

 Weber's analyses show that the nitrogenous substances 

 decrease from without inwards in the wood, just as does 

 the starch in a normal tree ; but the total proteid sub- 

 stances remain practically unaltered (at least they suffer 

 no diminution) because they are not used up in building 

 the cell-walls. Any drain on the proteids by the cambium 

 seems to be paid back in due course by the travelling of 

 the proteids from cell to cell. 



Now it is a well-known fact that the beech, like other 

 similar forest trees, only yields seed after attaining an 

 age of 50 to 60 years, and that what are termed good seed- 

 years are separated by considerable pauses. It is also 



