CHAPTER XIII THE EFFECTS PRODUCED ON THE BRANCH ANATOMY OF A TREE BY (1) OPPOSITE BUDS--THE CONSTANT FAILURE OF BUDS ; (2) (a) BUDS ARRANGED SINGLY IN TWO ROWS ; (b) LV THREE OR MORE ROWS ; (c) BUDS CLUSTERED IN GROUPS--TWIGS ARRESTED IN GROWTH-- ADDITIONAL BUDS--DIFFERENT BRANCH SYSTEMS OF YOUNG AND OLD TREES
WE must next examine into the effects produced by these differing bud-arrangements upon the branch anatomy of the tree.
1. Opposite buds.--Where the buds are found in opposite pairs, we should have the formal arrangement shown on Fig. 128 if everyone produced a new shoot in the following season.
A. 1st and 3rd pair of side-shoots.
B 2nd and 4th pair of side-shoots.
Each of these new shoots in their turn would be furnished with buds, and in the year following would produce a similar set of shoots. Every bough would end in a central shoot set between two others like a trident (Fig. 129). This, with slight modifications, is actually what happens with some of the trees already mentioned, i.e. the Sycamore, Spindle-tree, and Privet. Anyone who works out an imaginary sketch of this arrangement to represent some few years of growth will find the branches becoming inconveniently crowded. This is the result in actual fact ; some of the shoots die for want of light and air, others take an unwonted direction in search of these necessities, others again produce fruit in place of twigs. Not withstanding these accidental modifications, the rule remains apparent.
Differentiation of a more regular kind begins where the terminal
* 1 The base of a twig is thicker than its tip, consequently the buds at the base project farther than those above them. This is shown in the following plans. In the former plans the twig was supposed to have an equal thickness throughout its length (to save confusion).
branch (the " leader " which prolongs the shoot) makes a more or less rapid growth than the side-branches. A marked deviation from the
The upper illustration shows the various ways in which buds fail. The lower illustration shows the effect in each case on the branch formation. The diagrams represent an Ash.
branch-type may be expected to follow where the buds in certain positions die away habitually year by year (Figs. 130, 131, and Illus. 74). Failure of buds.--If the bud at the tip of the shoot withers, the
ILLUS. 74 EXAMPLE OF GROWTH BEING ARRESTED AT THE POINT WHERE FLOWERS ARE FORMED - SHOOT OF WAYFAIRING TREE
ILLUS. 75. EXAMPLE OF IRREGULARITY IN THE BRANCH FORMATION OF AN ASH
The reason for it is given in Figs. 130, 131
growth may be continued by the upper pair of lateral buds ; and every year we find a succession of forks, in place of the tridents which should be looked for if all three buds came to maturity. If one of the side-buds perishes, as well as the terminal bud, an angle is the result. Both these forms occur on the Guelder-Rose (Illus. 85) and the Fig. 132 Fig. 133 Fig. 134 Elder. There the central bud produces a flower instead of a leaf-bud, and growth is continued by one or both
ILLUS. 76. SHOOT OF CORNEL SHOWING THE Two YOUNG SHOOTS, ONE ON EITHER SIDE OF THE TERMINAL BUD. AN EXAMPLE OF THE FORMATION OP A FORK
of the side-buds. In the Guelder-Rose it is not uncommon for a whole season's growth to die back, and the branches be renewed in the following season by a fresh set of buds--a habit which results in a bush-like tree with a frame full of abrupt angles. The Horse-Chestnut, when a young tree, closely follows the plan of the Sycamore ; but before long the terminal bud is often replaced by a pair of side-buds. In old specimens, the drooping habit of the lower boughs, which must still bear A upright new shoots, produces an irregularity unmistakable by its very marked deviation from the original type. The Ash, again, starts with the same equipment of buds as the Sycamore, but its individual habit of growth soon makes an appearance (Fig. 135).
One of the side-buds replaces the terminal bud, and forms a " leader," while its companion bud produces a comparatively insignificant twig. The main line of the bow is continued in this way by one of the gentle curves which are such an attractive feature of the tree.
2. (a) Buds arranged singly on alternate sides of the twig.--In the trees which fall into this class, it
should be noted that bud and shoot lie in one plane, which is not the case where the buds are arranged in opposite pairs. If every twig, developed from the buds, carried on this habit consistently, all the boughs would. be in flat layers--after the manner of a fern ; a cursory glance at the bough of a Lime or a Beech will show how nearly this comes to pass (Illus. 77). The zigzag line of the shoots is also a noteworthy feature. In some cases the tips recurve, either in the direction of the apex or towards the base of the spray. This is one cause of the variations found in these flat layers of twigs ; another is due to the fact that some, buds produce minute shoots only, while others are making long lengths of young wood. It is possible, however, for one of these arrested twigs (which season after season has hardly made any head way) suddenly to develop long shoots ; the closely packed rings at their base, which represent the remains of former leaves, alone serving as a reminder of less vigorous days (Illus. 79, 84).
ILLUS. 77. THE ARRANGEMENT OF THE BUDS ON ELM AND THE BRANCHES THEY GIVE RISE TO
ILLUS. 78. BRANCH OF ELM
ILLUS. 79. RINGS SHOWING ARRESTED GROWTH AT THE BASE OF AN APPLE TWIG
(b) Buds on three sides of the shoot.--The ramification that follows this plan does not show the simplicity of lines observable in No. 1 arrangement, where the boughs are formed in decussate pairs.
Nor are the flat layers of branches of No. 2 arrangement to be found. The boughs are more evenly distributed without much interlacing or confusion. Trees of this group rely more upon the difference of angle between bough and stem for their identification than do some other groups.
Buds on five or more sides of a shoot. -- The want of order, rather than its observance, becomes the feature
to look for here. The branch anatomy becomes too intricate for any close application of botanical facts, though often in young trees they are easily discernible (Illus. 80, 81).
ILLUS. 81. BLACK POPLAR - TO EXPLAIN FROM WHAT POINTS ON THE STEM THE BRANCHES SPRING
(See Buds, Chap. XII. Illus. 71)
Buds clustered in groups.--Where the buds are clustered in groups at certain points on the stem, with long intervals between, the branches to which they give rise appear to radiate from each of these points like the spokes of an umbrella from the shaft. On the subsequent branches this comparison does not hold good ; for to prevent crowding, and to obtain light and air, the twigs take a more or lass horizontal position. Therefore on a horizontal bough the twigs forming on the under side curve upwards, while those on the upper side grow outwards to avoid confusion with the boughs above.
Additional buds.--The aspect of certain trees is nearly
as much altered by the presence of additional buds as it may be by the habitual loss of others. The old wood of the Pines is not capable of bearing buds below those already formed,
ILLUS. 82. ADVENTITIOUS TWIGS ON THE BOLE OF AN ALDER
ILLUS. 83. EXAMPLE OF SPINES (ARRESTED TWIGS) FORMED INSTEAD OF TWIGS-BUCKTHORN
and the loss of a bough cannot be replaced by new branches, but this is rather the exception. Many trees when lopped, notably the Willows and the Elms, are capable of putting out quite a crop of new shoots ; and the suckers from the roots of the Poplars and the Elms, and the adventitious shoots on the Alder and the Holly, contribute largely to their characteristic appearance, since their upward direction comes in direct contrast to the older forms. A fallen Holly, where the roots have not been severed, often shows a row of new vertical shoots springing from the old trunk; and our pollard Willows, Copses, and Osier beds have a charm specially their own, and show how this power has been
ILLUS. 84. TWIGS OF SYCAMORE
Compare the two figures in this drawing and also Illus. 47
utilised in the case of the Beech, Chestnut, Alder, and some other trees.
Twigs arrested in growth.--A somewhat similar effect is produced upon trees belonging by their normal bud-arrangement to other classes, and is shown in the spurs and short stumpy shoots common on the Hawthorn, Sloe, Crab-Apple, Buckthorn. On these trees are found the two extremes, very long shoots bearing many scattered buds, and short dwarf shoots reduced to sharp spurs bearing tufts of buds. This is also to be seen in the case of the Pines and the Larch.
Differences in the branch-system of old and young trees.--The habit of branching shown by a young tree becomes considerably altered in middle life or old age. This is partly explained by the fact that some species attain a fair height before bearing fruit, their energies being entirely expended on making long shoots. In greater age the rule is reversed ; short, much-branched twigs, arrested at each point where fruitage occurs, take the place of the long shoots and subsequent branches. An example will be found in the number of short, forked twigs which terminate the boughs of an old Sycamore (Illus. 84). Trees on attaining their full height continue to expand laterally, instead of in an upward direction, while their lower boughs decay. We have already given a comparison of a young Scots Pine with an old tree where a tall spire has been replaced by a flat mop-like head.