VI.

If we turn our attention for a moment to the illustrations in the first

article, it will be remembered that our typical log of timber was clothed

in a sort of jacket termed the cortex, the outer parts of which constitute

what is generally known as the bark. This cortical covering is separated

from the wood proper by the cambium, and I pointed out that the cells

produced by divisions on the outside of the cambium cylinder are employed

to add to the cortex.

[2] Continued from SUPPLEMENT, No. 644, page 10281.

Now this cortical jacket is a very complicated structure, since it not

only consists of numerous elements, differing in different trees, but it

also undergoes some very curious changes as the plant grows up into a

tree. It is beyond the purpose of these articles to enter in detail into

these anatomical matters, however; and I must refer the reader to special

text books for them, simply contenting myself here with general truths

which will serve to render clearer certain statements which are to follow.

[Illustration: 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, C_a_. Beginning from the

outside, we have: 1. Cork cells, X, formed from the cork cambium, C.C_a_:

the cells developed on the inside of the latter, C_l_, are termed

collenchyma, and go to add to the cortex. 2. The cortex proper, consisting

of parenchyma cells, _pa_, some of which contain crystals. 3. The inner or

secondary cortex (termed _phloem_ or bast), developed chiefly by the

activity of the cambium, C_a_: this phloem consists of hard bast fibers,

_hb_, sieve tubes, S, and cells, _c_, and is added to internally by the

cambium, C_a_, each year. It is also traversed by medullary rays, M_r_,

which are continuations of those in the wood. The dotted line, [psi], in

the cortical parenchyma indicates where the new cork cambium will be

developed: when this is formed, all the tissues (e.g. _pa_, C_l_) lying on

the outside of the new cork will die, and constitute (together with the

cork) the true _bark_.]

It is possible to make two generalizations, which apply not only to the

illustration (Fig. 20) here selected, but also to most of our timber

trees. In the first place, the cortical jacket, taken as a whole, consists

not of rigid lignified elements, such as the tracheids and fibers of the

wood, but of thin-walled, soft, elastic elements of various kinds, which

are easily compressed or displaced, and for the most part easily killed or

injured--I say for the most part easily injured, because, as we shall see

immediately, a reservation must be made in favor of the outermost tissue,

or cork and bark proper, which is by no means so easily destroyed, and

acts as a protection to the rest.

The second generalization is, that since the cambium adds new elements to

the cortex on the inside of the latter, and since the cambium cylinder as

a whole is traveling radially outward--_i.e._, further from the pith--each

year, as follows from its mode of adding the new annual rings of wood on

to the exterior of the older ones, it is clear that the cortical jacket as

a whole must suffer distention from within, and tend to become too small

for the enlarging cylinder of rigid wood and growing cambium combined.

Indeed, it is not difficult to see that unless certain provisions are made

for keeping up the continuity of the cortical tissues, they must give way

under the pressure from within. As we shall see, such a catastrophe is in

part prevented by a very peculiar and efficient process.

Before we can understand this, however, we must take a glance at the

structural characters of the whole of this jacket (Fig. 20). While the

branch or stem is still young, it may be conveniently considered as

consisting of three chief parts.

(1) On the outside is a thin layer of flat, tabular cork cells (Fig. 20,

C_o_), which increase in number by the activity of certain layers of cells

along a plane parallel to the surface of the stem or branch. These cells

(C.C_a_) 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.C_a_ in fact simply become cork cells; while

those formed on the inside of the line C.C_a_ become living cells (C_l_)

very like those I am now going to describe.

(2) Inside this cork-forming layer is a mass of soft, thin-walled "juicy"

cells, _pa_, which are all living, and most of which contain granules of

chlorophyl, and thus give the green color to the young cortex--a color

which becomes toned down to various shades of olive, gray, brown, etc., 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, etc.,

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, C_a_, and it may be regarded as entirely formed

directly from the cambium cells. These latter, developed in smaller

numbers on the outside, toward the cortex, than on the inside, toward 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_, _hb_ 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, etc.--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.

In the first place, it is clear that the continuous sheet of cork (C_o_)

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 line [phi] 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

inward, 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 _bark_ 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 in 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 fibers in the malveceae, the tannin in the oaks, the coloring

matter in _Garcinia_ (gamboge), the gutta 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--produced 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 vigor 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 below the slit; this is simply because the

slit had released the pressure on the cambium. The converse has also been

proved to be true--_i.e._, 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 disturbances

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, etc., 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.

(_To be continued._)