838 A TEXTBOOK OF THEORETICAL BOTANY 



enables the leaves to be disposed to the greatest advantage. The amount of 

 branching is inversely related to the size of the leaves. Unbranched plants 

 have either large leaves, e.g., Palms, or no leaves at all, e.g., Cacti. 



Theoretically the simplest way of branching is by the division of the 

 apical growing point into two. This method, known as Dichotomy, is not 

 uncommon among lower groups, and occurs also in some Angiospermic 

 roots, but is very rare in stems, the only definite examples being in two 

 Palms, Hyphaene thebaica and Chamaedorea martiana. The former is common 

 in North Africa and shows very marked dichotomy, with an angle-leaf at the 

 stem junctions, as in SelagineUa. 



The normal method of branching is axillary, that is, from buds which 

 arise in the leaf axil, i.e., the angle between leaf and stem. Axillary branches 

 are said to be subtended by the related leaf. Every axillary bud resembles 

 the apical bud in structure and has a similar power of growth by the elonga- 

 tion of its internodes. Axillary buds seldom commence developing 

 immediately they are formed. There is commonly a resting period before 

 unfolding begins, which may, however, be only a few days in an annual. 

 In woody perennials the buds formed in one season do not generally grow 

 until the following year, and they may remain dormant indefinitely. This, 

 as we shall see in Volume IV., is conditioned by the physiological dominance 

 of the apical bud. In all cases where this bud retains permanently its 

 capacity for active growth, the branches which develop from axillary buds 

 remain lateral and subordinate to the main axis, and this is called mono- 

 podial branching (Fig. 832), e.g., Acer platanoides. 



Neither the main axis nor the branches have, however, always the power 

 of unlimited growth. Frequently the apical bud is transformed into a flower 

 or flowers, which, being themselves of limited growth, close its career. Again, 

 in some of our common trees, notably the Elm and the Lime, the tip of each 

 young branch dies back to a node during the summer and drops ofi^, leaving 

 a small round scar to mark the place. Whatever may be the fate of the 

 terminal bud, the effect of its disappearance is to leave subsequent growth 

 to the axillary shoots. This is called sympodial branching (Fig. 833). 



In trees or shrubs with opposite leaves the uppermost pair of axillary 

 buds may develop next season with equal vigour and hence arises a semblance 

 of dichotomy to which we have referred above. Where the leaves are set 

 singly, however, as in the Elm and the Lime, there is only one bud at the 

 node to which the shoot dies back, and its subsequent growth carries on 

 the axis in the original line, so that an appearance of monopodial growth 

 is preserved. The same appearance of monopodial growth in a sympodial 

 system may also be seen where a terminal flower is pushed to one side by 

 the growth of the nearest axillar\' bud, which continues the line of growth 

 of the main axis. When the shoots of a tree end in flowers, as in the Horse 

 Chestnut, the branching may be monopodial for some years, until the 

 flowering stage is reached, and then becomes sympodial. 



The sympodial method, by which the responsibility for active growth is 

 regularly passed on from branch to branch, permits a degree of specialization 



