The leader.--We have seen that there are certain definite arrangements of buds on the stem of a young tree, and that its upward growth is continued either (1) from the bud at its tip, or (2) from one of the side buds, or (3) from two side buds, or (4) from one or two
of the lower side buds.
(1) If the stem is continued each year from the tip of the previous year's formation, it may be likened to a telescope drawn out section by section (Fig. 144).
(2) If, however, the stem is continued from one of the side buds, the straightness Fig. 144 of its line is interrupted each season in one of the following ways. (a) If the shoot that is to pose as the true " leader " springs in successive seasons from the same side, a series of curves repeating one another breaks the continuity of the stem (Fig. 145). (b) If, however, the pseudo" leader " is formed year by year from right and left of the stem, a line of alternate curves or breaks is produced (Fig. 146). More often the stein is continued for a year or so from one side and then from another, giving us infinite variety in its curves.
(3) It is the habit of some species to continue the stem by shoots from the two side buds instead of from the terminal bud ; in other words, the stem forks and two main limbs take its place, and they in turn fork until they ramify into branchlets and twigs (Fig. 147). In more rare instances only those buds at the base of the shoot grow, and the whole of the shoot above them dies (Fig. 14F). The Guelder-Rose is the most noteworthy example of this habit. If this method is repeated each season, it is obvious that the dimension of a bush will hardly be extended ; but it is not to be understood that every shoot dies back to the base ; many of them are continued by the upper side buds (Illus. 85).
The stem--thus formed on one of the above plans--each season produces a set of side buds on the new portion. These buds become branches which grow in the following year, and repeat with their branches the system followed by the stem, which again, in turn, is repeated by the branchlets and the twigs they produce. The stem itself increases yearly in girth and in length between the points where branches are given out, and a similar addition in length and thickness occurs on the branches and newer formed growths (Fig. 149).
The annually formed junctions and curves in the stem just described become softened and less abrupt with each year's increase in girth ; and in those cases where it forks, one limb of the fork generally predominates so that its neighbour appears to be only a side bough (Fig. 150). Additional notes of difference between the species will be found in the various angles formed between bough and stem, and in the lines which the branches and twigs take in reaching the extent of their growth ; also in the form of the outline itself (Fig. 151).
Angles between stem and bough.--The angles formed between the larger and the lesser parts of the tree-structure vary from the right angle found in an Oak to the acute one of a Birch (Fig. 152). The predominant angle, as a rule, is more obvious in the space left between the twigs than in that between the trunk and boughs. In the latter case the sharpness of the angle becomes lost by a yearly addition to the girth of the limbs and trunk, and a sort of bracket between the two is formed (Fig. 153). In some species the smaller forms are joined to the larger by a gentle curve--Ex. : Holly (Fig. 153). In other species the junction is formed by nearly straight lines (Fig. 151).
ILLUS. 85. EXAMPLE OF GROWTH CONTINUED BY A FEW OF THE BUDS - GUELDER ROSE
ILLUS. 86. THE GREY POPLAR
ILLUS. 87 THE BOUGHS OF AN OLD SPANISH CHESTNUT
ILLUS. 88. BRANCH SYSTEM OF AN APPLE TREE
The proportion of boughs, branches, and twigs.--The gradation in girth from trunk to bough, bough to branch, and branch to twig is carried out by regular stages step by step (Fig. 155). The trunk (if one excepts the bole) is of equal thickness throughout its length until it gives forth a bough. Its next length (diminished in thickness) is again of even width up to the point where the next bough springs. It is noticeable that, if a drawing were made of the portion of the trunk that is above the bough, an increased thickness of the trunk below would have to be added on the side under the bough (Fig. 156). This even gradation from large to small forms gives delicate lines of great charm, and is especially noticeable in the case of the Goat Willow, the Birch, and the Ash, though the rule is found in all trees. The trunks of some conifers (especially those of the Spruce and Larch) appear at first sight not to observe the rule ; they seem to taper from the base upwards, instead of - being reduced in girth stage by stage, after the fashion of a telescope (Fig. 157). This tapering appearance is partly due to the lack of branches (that in other trees would accentuate the successive
reductions in the thickness (Fig. 158)), and partly to the shortness of the lengths of trunk between the scars that mark the position of former branches, so that the stages by which the trunk was diminished are hardly apparent (Fig. 159).
It may be stated on a rough calculation that, when a trunk divides, the limbs of the fork measure together rather more in diameter than the trunk below the division (Fig. 160). If the main limbs of a tree are conspicuous, and the boughs suddenly ramify into a great number of twigs, the whole effect will be vastly different from that of a tree where the twigs are fewer in number ; as in the latter case the succession from the large to the smallest part is is more gradual (Fig. 161). A comparison of the anatomy of an Elm and an Ash will show the importance of this observation.
The position taken by twigs.--The rigidity or flexibility of twigs accounts for some variations in the position of future boughs. One sees the supple shoot of a Scots Pine bent with the weight of the cones ; also the shoots of a Birch unable to bold themselves upright under their slight burden of catkin-fruited twigs (Illus. 90). From the upper side of such pendent branchlets new buds give life to sturdy twigs, which, in turn,
Fig. 162.--Hanging Bough of Pear
bend down and add new leverage to the parent branchlet. In this way a succession of curved steps, as it were, are formed, leading the branch outward curve by curve. This becomes a more noteworthy feature as time goes on ; for the few branchlets that were formed on the inner side of the pendent branch die away for want of light. On the other hand, the rigid shoots of an Oak or an Ash can hold themselves in a position
ILLUS. 89. EXAMPLE OF GRADATION FROM LARGE TO SMALL BRANCHES
ILLUS. 90. TWIGS OF BIRCH WEIGHED DOWN BY CATKIN'S
ILLUS. 91. PENDENT BRANCH OF APPLE TREE IN FLOWER
unattainable by the supple twigs of the former trees. It is not implied that pendent branches formed in the manner described are only to be found on trees with flexible twigs ; for the lower boughs of Pear trees are commonly constructed on this plan ; though here it may also be conjectured that the unusual weight of the fruitage may be in part responsible for the position.