THE LIKENESS BETWEEN TWIG AND BOUGH-THE DIFFERENCE BETWEEN SHOOT AND TWIG-THE ARRANGEMENT OF BUDS ON THE SHOOT: (1) OPPOSITE BUDS; (2) BUDS ARRANGED SINGLY; (3) BUDS CROWDED IN GROUPS
IT may seem to you, whose purpose it is to become a painter of fine landscape, of trivial importance whether the buds you see on a twig are arranged in systematic way or not. All you wish to know is how the large boughs ramify, and in what manner one species of tree is different from another. It is exactly this that you will find out from an examination of buds and by following carefully the life history for a few years of those buds and the growth they give rise to. To understand this, you must know that a bud is nothing else than a young shoot with its leaves tightly compressed into a compact space ; when the shoot and leaves have broken their confinement and become fully grown, they will form the annual addition at this point to the growth of the tree. We illustrate here (Illus. 64 and 65) two shoots of different trees (Sycamore and Sallow) : you notice that the arrangement of the buds on the former and latter are quite unlike ; but if you collected a number of shoots, you would find that the buds on all the Sycamore shoots were placed in the same way as on this Sycamore shoot, and also that all the shoots on the Sallow would have their buds placed alike. Each species of tree has its buds arranged on the twig in a definite manner. There are about forty distinct species of trees, but there are not forty different methods of bud-arrangement. There are about six or seven. Each species conforms to one or another of these plans, while every individual tree has its buds set on the plan observed by its species. Some of the arrangements merely differ in matters of detail, and the species they embrace can be grouped together, except when an exact representation is required of the smaller parts ; then a little observation will make clear the points of deviation, or they can be ascertained by reference to a botanical treatise.
There is a rather interesting cause of differentiation in the branch anatomy of some species of trees that have a precisely similar arrangement of buds. On one species, certain buds consistently fail to produce
ILLUS. 64. CATKIN BUDS OF SALLOW ILLUS. Go. SYCAMORE BUDS
Ex. : DISSIMILAR BUD ARRANGEMENTS SEEN TOGETHER
a shoot, while on another species all the buds produce their shoots with an almost unfailing regularity. In this way two species, equally equipped for the formation of a similar branch system, are productive of systems that have little in common, except the young shoots that are formed each season. Nature seems unsatisfied until she has exploited every possible variety ; and other species add to the variation just mentioned yet another by giving out extra buds in addition to those that regulate its anatomy. It would not be surprising if individual trees should " sport " and not strictly conform to the habits of their species ; but it is curious that all the individuals forming a species should consistently be wayward in following out the plan arranged for them. Since a new shoot with its buds will, in the following season, have become part of a twig (carrying in its turn new shoots), and the twig will in a few years have developed into a branch (bearing in its turn twigs and shoots), and the branch in time becomes the bough supporting the branch, twig and shoot, we should expect to find a considerable likeness between their old and young formations. Generally speaking, the twig does bear a very remarkable likeness to the larger parts ; in many cases it is indeed a replica in miniature of the bough ; the predominant lines of the bough are repeated, with slight though endless variations, throughout the structure of the tree.
If, on the other hand, we examine a shoot of one season's growth, and assume that each of the buds it bears will in time produce a new shoot ; and if we then compare it with a twig which represents the growth of several seasons, the result is not in all cases quite what we had taught ourselves to expect. The general resemblance between the twig and the bough of which we have spoken is modified in various accidental ways--by the loss of certain parts of the bough through overcrowding, by injury, by a change in the direction of growth, or by the position of some one part (Illus. 66). Owing to these accidents, the true character lines are often more noticeable in the twig than in the bough, a twig of two or three years' growth giving the best material for determining what are the main points of identification of its species. A study of the development of the twig from the shoot is necessary, if we are to understand the origin of those differentiating features. The first step in that study must be a classification of the various ways in which the buds are arranged on the shoot.
(1) Opposite buds.--By an arrangement very commonly found, the shoot ends in a bud bearing beside it other buds,
one on either side of the shoot and opposite one another. Other buds are arranged at intervals along the shoot, every bud being paired with its fellow on the opposite side of the shoot, and each pair
ILLUS. 66. EXAMPLE OF CHANGE IN DIRECTION-HORNBEAM
being set at right angles to the pair above and below it. A plan of this arrangement (as seen from the tip of the shoot) would be as follows :
A. The 1st and 3rd pair of side buds.
B. The 2nd and 4th pair of side buds.
It is the arrangement found on a shoot of the Ash, Horse Fig. 118 Chestnut, Sycamore, Maple, Elder, Guelder-Rose, Spindle-tree, Dogwood, Wayfaring-tree, and Privet (Illus. 67) (Fig. 117).
(2) Buds arranged singly.--By another ordinary method the buds, instead of being in pairs, are arranged singly, though still on a definite plan. If we follow up the buds in order from base to tip of the vertical shoot, we find that each is placed a little higher up than the last ; but the points from which they spring are not in a straight
line one above the other.
(a) In cases where they spring alternately from the near and far side of the shoot, two straight lines drawn from the apex down the whole length of the shoot would cut all the buds ; a plan of this arrangement would be as follows : Buds 1, 3, 5 on one side and buds 2, 4, 6 on the other (Fig. 119).
Such an arrangement is the normal one in the case of the Beech, Elm, Hazel, Lime, and Hornbeam, and occurs, though less consistently, on the Birch and on upright shoots of the Sweet Chestnut (Illus. 70 and Figs. 119, 120, 121).
(b) In some species, however, the buds spring from three sides of the shoot, and three lines would have to be drawn
from apex to base to cut through them all.
Buds 1, 4 ; Buds 2, 5 ; Buds 3, 6 (Fig. 122).
This can be seen on a shoot of Alder, but it occurs on some other trees also.
(c) In other cases five or even eight lines would have to be drawn down the shoot in order to cut all the buds (Illus. 71 and Figs. 124, 125).
The buds of the following species are arranged singly on one of the plans thus described (under the headings of (b) and (c) : the Bird Cherry, Oak, Willows and Poplars (Illus. 71), Walnut, Apple, Plum, Pear, Cherry, Hawthorn (Illus. 70), Sloe and Alder Buckthorn. The
ILLUS. 67. OPPOSITE BUDS
ILLUS. 68. BUDS ARRANGED IN TWO ROWS
ILLUS. 69. BUDS IN THREE OR MORE ROWS
ILLUS. 70. ARRANGEMENT OF BUDS ON THORN TREE
ILLUS. 71. BLACK POPLAR
ILLUS. 72. SPANISH CHESTNUT
ARRANGEMENT OF SINGLE BUDS
ILLUS. 73. EXAMPLE OF CLUSTERED BUDS--SPRUCE FIR
(See Branches developed on this plan, Fig. 143)
Oak (Illus. 69) and Apple and Cherry included in these are examples of buds forming five rows.
(3) Buds crowded in groups.--The two arrangements described above (1 and 2), with their modifications, occur in about thirty-five species of trees ; but in some other cases the buds cluster round the shoot at certain points, and long empty spaces are left between them.
Fig. 126 - Crowded buds of spruce.
This is most noticeable in the Spruce (Illus. 73), (especially in the young trees grown in a nursery garden for " Christmas trees "), the Scots Pine, Silver Fir, and Holly. The Oak has groups of buds clustered on the tip of the shoot. A more accurate description of these " clustered " buds would be " buds arranged singly on a spiral, so short as to make them appear to radiate from points at the same level on the shoot."