March 29, 1888] 



NATURE 



515 



of certain layers of cells along a plane parallel to the 

 surface of the stem or branch. These cells {C.Ca) behave 

 very much like the proper cambium, only the cells divided 

 off from them do not undergo the profound changes suffered 

 by those which are to become elements of the wood and 

 inner cortex. The cells formed on the outside of the line 

 C.Ca in fact simply become cork-cells ; while those 

 formed on the inside of the line C:Ca become living cells 

 \CI) very like those I am now going to describe. 



(2) Inside this cork-forming layer is a mass of soft, 

 thin-walled, " juicy" cells, /tz, which are all living, and most 

 of which contain granules of chlorophyll, and thus give 

 the green cobur to the young cortex — a colour which 

 becomes toned down to various shades of olive, gray, 

 brown, &c., as the layers of cork increase with the age of 

 the part. It is because the corky layers are becoming 



thicker that the twig passes from green to gray or brown 

 as it grows older. Now these green living cells of the 

 cortex are very important for our purpose, because, since 

 they contain much food-material and soft juicy contents 

 of just the kind to nourish a parasitic fungus, we shall 

 find that, whenever they are exposed by injury, &c., they 

 constitute an important place of weakness — nay, more, 

 various fungi are adapted in most peculiar ways to get at 

 them. Since these cells are for the most part living, and 

 capable of dividing, also, we have to consider the part 

 they play in increasing the extent of the cortex. 



(3) The third of the partly natural, partly arbitrary 

 portions into which we are dividing the cortical jacket 

 is found between the green, succulent cells \pa) of the 

 cortex proper (which we have just been considering), and 

 the proper cambium, Ca^ and it may be regarded as 



Fig. 20. —A piece of the cambium and cortical jacket of a young oak, at the end of the first year. It may be regarded as consisting of three parts, in addition 

 to the cambium (Co). Beginning from the outside, we have : (i) cork-cells (X), formed from the cork-cambium (C.Cd) : the cells developed on the inside 

 of the latter (C/) are termed coUenchyma, and go to add to the cortex. (2) The cortex proper, consisting of parenchyma<ells (J>a), some of which 

 contain crystals. (3) The inner or secondary cortex (termed phloem or bast), developed chiefly by the activity of the cambium (Ca) : this phloem 

 consists of hard bast fibres (Jib), sieve-tubes (5), and cells (c), and is added to internally by the cambium (Co) each year. It is also traversed by 

 medullary-rays (Mr), which are continuations of those in the wood. The dotted line (0) in the cortical parenchyma indicates where the new cork- 

 cambium will be developed : when this is formed, all the tissues (e.g. pa, CI), lying on the outside of the new cork will die, and constitute (together 

 with the cork) the true bark. 



entirely formed directly from the cambium-cells. These 

 latter, developed in smaller numbers on the outside, 

 towards the cortex, than on the inside, towards the wood, 

 undergo somewhat similar changes in shape to those 

 which go to add to the wood, but they show the important 

 differences that their walls remain unlignified, and for the 

 most part very thin and yielding, and retain their living 

 contents. For the rest, we may neglect details and refer 

 to the illustration for further particulars. The tissue in 

 question is marked by S, c, lib in the figure, and is called 

 phloem or bast. 



A word or two as to the functions of the cortex, though 

 the subject properly demands much longer discussion. 

 It may be looked upon as especially the part through 

 which the valuable substances formed in the leaves are 

 passing in various directions to be used where they are 



wanted. When we reflect that these substances are the 

 foods from which everything in the tree — new cambium, 

 new roots, buds, flowers, and fruit, &c. — are to be 

 constructed, it becomes clear that if any enemy settles in 

 the cortex and robs it of these substances, it reduces not 

 only the general powers of the tree, but also— and this is 

 the point which especially interests us now — its timber- 

 producing capacity. In the same way, anything which 

 cuts or injures the continuity of the cortical layers results 

 in diverting the nutritive substances into other channels. 

 A very large class of phenomena can be explained if 

 these points are understood, which would be mysterious, 

 or at least obscure, otherwise. 



Having now sketched the condition of this cortical 

 jacket when the branch or stem is still young, it will be 

 easy to see broadly what occurs as it thickens with age. 



