5i6 



NATURE 



IMarck 29, 1888 



In the first place, it is clear that the continuous sheet 

 of cork {Co) must first be extended, and finally ruptured, 

 by the pressure exerted from within : it is true, this layer 

 is very elastic and extensible, and impervious to water or 

 nearly so — in fact it is a thin layer or skin, with properties 

 like those of a bottle cork — but even it must give way as 

 the cylinder goes on expanding, and it cracks and peels 

 off. This would expose the delicate tissues below, if it were 

 not for the fact that another layer of cork has by this time 

 begun to form below the one which is ruptured : a cork- 

 forming layer arises along the Hne ^, and busily produces 

 another sheet of this protective tissue in a plane more or 

 less exactly parallel with the one which is becoming 

 cracked. This new cork-forming tissue behaves as 

 before : the outer cells become cork, the inner ones add 

 to the green succulent parenchyma-cells {pa). As years go 

 on, and this layer in its turn splits and peels, others are 

 formed further inwards, and if it is remembered that a 

 layer of cork is particularly impervious to water and air, 

 it is easy to understand that each successive sheet of cork 

 cuts off all the tissues on its exterior from participation in 

 the life processes of the plant : consequently we have a 

 gradually increasing (^ar/& proper, formed of the accumulated 

 cork-layers and other dead tissues. 



A great number of interesting points, important in their 

 proper connections, must be passed over here. Some of 

 these refer to the anatomy of the various " barks " — the 

 word " bark " being commonly used ih commerce to mean 

 the whole of the cortical jacket— the places of origin of 

 the cork-layer, and the way in which the true bark peels 

 off: those further interested here may compare the plane, 

 the birch, the Scotch pine, and the elm, for instance, with 

 the oak. Other facts have reference to the chemical and 

 other substances found in the cells of the cortex, and 

 which make " barks " of value commercially. I need 

 only quote the alkaloids in Cinchona, the fibres in the 

 Malvaceae, the tannin in the oaks, the colouring-matter 

 in Garcinia (gamboge), the ^tta-percha from Isonandra, 

 the ethereal oil of cinnamon, as a few examples in this 

 connection, since our immediate subject does not admit 

 of a detailed treatment of these extremely interesting 

 matters. 



The above brief account may suffice to give a general 

 idea of what the cortical jacket covering our timber is, 

 and how it comes about that in the normal case the 

 thickening of the cylinder is rendered possible without 

 exposing the cambium and other delicate tissues : it may 

 also serve to show why bark is so various in composition 

 and other characters. But it is also clear that this jacket 

 of coherent bark, bound together by the elastic sheets of 

 cork, must exert considerable pressure as it reacts on the 

 softer, living, succulent parts of the cortex, trapped as 

 they are between the rigid wood cylinder and the bark ; 

 and it is easy to convince ourselves that such is the case. 

 By simply cutting a longitudinal slit through the cortex, 

 down to near the cambium, but taking care not to injure 

 the latter, the following results may be obtained. First, 

 the bark gapes, the raw edges of the wound separating 

 and exposing the tissues below ; next, in course of time 

 the raw edges are seen to be healed over with cork— pro- 

 duced by the conversion of the outer cells into cork-cells. 

 As time passes, provided no external interference occurs, 

 the now rounded and somewhat swollen cork-covered 

 edges of the wound will be found closing up again ; and 

 sooner or later, depending chiefly on the extent of the 

 wound and the vigour of the tree, the growing lips of the 

 wound will come together and unite completely. 



But examination will show that although such a slit- 

 wound is so easily healed over, it has had an effect on the 

 wood. Supposing it has required three years to heal over, 

 it will be found that the new annual rings of wood are a 

 little thicker just belov/the slit ; this is simply because the 

 slit had released the pressure on the cambium. The con- 

 verse has also been proved to be true— /.^. by increasing 



the pressure on the cambium by means of iron bands, the 

 annual rings below the bands are thinner and denser than 

 elsewhere. 



But we have also seen that the cambium is not the only 

 living tissue below the bark : the cortical parenchyma 

 {pa), and the cells {c) of the inner cortex (technically the 

 phloem) are all living and capable of growth and division, 

 as was described above. The release from pressure affects 

 them also ; in fact, the " callus," or cushion of tissue 

 which starts from the lips of the wound and closes it 

 over, simply consists of the rapidly growing and dividing 

 cells of this cortex, i.e. the release from pressure enables 

 them to more than catch up the enlarging layer of cortex 

 around the wound. 



An elegant and simple instance of this accelerated 

 growth of the cortex and cambium when released from 

 the pressure of other tissues is exhibited in the healing 

 over of the cut ends of a branch, a subject to be dealt 

 with later on ; and the whole practice of propagation by 

 slips or cuttings, the renewal of the " bark " of Cinchonas, 

 and other economic processes, depend on these matters. 



In anticipation of some points to be explained only if 

 these phenomena are understood, I may simply remark 

 here that, obviously, if some parasite attacks the growing 

 lips of the " callus" as it is trying to cover up the wound, 

 or if the cambium is injured below, the pathological dis- 

 turbances thus introduced will modify the result : the 

 importance of this will appear when we come to examine 

 certain disturbances which depend upon the attacks of 

 Fungi which settle on these wounds before they are 

 properly healed over. In concluding this brief sketch of 

 a large subject, it may be noted that, generally speaking, 

 what has been stated of branches, &c., is also true of 

 roots ; and it is easy to see how the nibbling or gnawing 

 of small animals, the pecking of birds, abrasions, and 

 numerous other things, are so many causes of such 

 wounds in the forest. H. Marshall Ward, 



{To be continued.) 



NOTES. 



On Friday, the 23rd inst., Sir Henry Roscoe drew attention 

 in the House of Commons to the Woolwich regulations, the 

 mischievous nature of which we have repeatedly exposed. Sir 

 Henry Roscoe was cordially supported by Sir Lyon Playfair ; 

 and Mr. Stanhope, we are glad to say, dealt with the subject in 

 a fair and conciliatory spirit. He promised to discuss the matter 

 with men of science, and the result, we may hope, will be that 

 new regulations will soon be drawn up, securing that scientific 

 candidates shall not be placed at a disadvantage as compared 

 with students of language and literature. 



In replying to Sir Henry Roscoe, Mr. Stanhope made a 

 statement to the effect that the regulations now in force for 

 Sandhurst, which were issued in 1884, were recommended by 

 upwards of fifty head masters. We have before us the Report 

 of the Head Masters' Conference for 1883, in which the recom- 

 mendations of their Committee are printed, and these recommend- 

 ations differ in certain important respects from the regulations 

 actually adopted by the War Office. The head masters appear, 

 from their suggestions, to have desired to retain a more import- 

 ant place for Latin and Greek than those subjects occupy in the 

 War Office regulations ; and they also attached a higher value 

 to higher mathematics. On the other hand, they placed modern 

 languages and experimental science on a lower but relatively 

 less unequal footing than the War Office has done. Although, 

 therefore, in consequence of the position of classics, the actual value 

 of science was, on the whole, perhaps not better under the head 

 masters' suggestions than under the regulations issued by the War 

 Office, it is hardly correct to speak of the head masters as having 

 recommended a scheme of which the predominating fault is, as 



