May 31, 1888] 



NA TURE 



109 



from the compressing influence of the cortex and bark, 

 and owing to this release of pressure it begins to grow out 

 at the edges into a cushion or "callus," as shown in Figs. 

 23*and 24. A very similar " callus " is formed in the 

 operation of multiplying plants by " cuttings," so well 



18 G 7- 73 



Fig. 22.— The same in longitudinal section. /', the pith of stem and branch ; 

 on either side of this are the twelve annual zones of wood produced 

 during the years 1867-78, as marked. The cambium, C, separates these 

 from the cortex, B. 



known to all : the cambium at the cut surface of the 

 " slip" or " cutting," is released from the pressure of the 

 cortex, and begins to grow out more rapidly in the direc- 

 tions of less pressure, and forms the callus. 

 Now this callus (Fig. 23, Cal)\s\r\ all cases somethingmore 



Fig. 23. — The same piece of stem four years later. The cushion-like deve- 

 lopment, Cal, resulting from the overgrowth of the cambium and cortical 

 tissues of the cut branch, has extended some distance from the edges, 

 and is covering in the exposed wood. B is the dead outer corky tissue, 

 incapable of growth, and partially cracked under the pressures exerted by 

 the thickening of the stem. The latter is somewhat swollen trai s- 

 versely. owing to the release of pressure in this region enabling the 

 cambium to develop a little more actively here ; the quicker growth of the 

 occluding cushion in the horizontal direction is due to the same cause. 



than mere cambium — or rather, as the cambium extends 

 by cell-divisions from the cut edge of the wound, its outer 

 parts develop into cortex, and its inner parts into wood, 

 as in the normal case. The consequence is that we have 

 in the callus, slowly creeping out from the margins of the 



wound, new layers of wood and cortex with cambium 

 between them (Fig. 24) ; and it will be noticed that each 

 year the layer of wood extends a little further over the 

 surface of the wound, and towards the centre of the cut 

 branch ; and in course of time, provided the wound is 

 not too large, and the tree is full of vigour, the margins of 

 the callus will meet near the middle, and what was the 

 exposed cut surface of the branch will be buried beneath 

 layers of wood and cortex, between which lies the cam- 

 bium, now once more continuous over the whole trunk of 

 the tree (Figs. 25 and 26). 



It is not here to the purpose to enter into the very in- 

 teresting histological questions connected with this callus- 

 formation, or with the mechanical relations of the various 

 parts one to another. It is sufficient for our present 

 object to point out that this process of covering up, or 

 occlusion, as I propose to term it, requires some time for 

 its completion. For the sake of illustration, I have num- 

 bered the various phases in the diagram, with the years 

 during which the annual rings have been formed ; and it 



Fig. 24. — The same in longitudinal section : P, B, and C as before. The four 

 new layers of wood formed during 1879-82 are artificially separated 

 from the preceding by a stronger line. On the left side of the figure it 

 will be noticed that the cambium (and therefore the wood developed from 

 it) projected a little further over the cut end of the branch each year, 

 carrying the cortical layers (Cor) with it. At X , in both figures, there is 

 necessarily a depression in which rain-water, &c-, is apt to lodge, and 

 this is a particularly dangerous place, since fungus-spores may here settle 

 and develop. 



will be seen at a glance that, in the case selected, itre- 

 quired seven years to cover up the surface of the cut 

 branch (cf. Figs. 21-26). During these seven years 

 more or less of the cut surface was exposed (Fig. 24) to 

 all the exigencies of the forest, and it will easily be under- 

 stood that abundant opportunities were thus afforded for 

 the spores of fungi to fall on the naked wood, and for 

 moisture to condense and penetrate into the interior ; more- 

 over, in the ledge formed at X in Figs. 23 and 24, by the 

 lower part of the callus, as it slowly creeps up, there will 

 always be water in wet weather ; and a sodden condition 

 of the wood at this part is insured. All this is, of course, 

 peculiarly adapted for the germination of spores ; and, 

 since the water will soak out nutritive materials, nothing 

 could be more favourable for the growth and development 

 of the mycelium of a fungus. These circumstances, 

 favourable as they are for the fungi, are usually rendered 

 even more so in practice, because the sawyers often allow 

 such a branch to fall, and tear and crush the cambium 

 and cortex at the lower edge of the wound. These and 



