THE ANGIOSPERMAE : STEMS 845 



of which onlv the lower pair are enclosed, while the top bud is left free to 

 grow into a temporary branch which is replaced next season by a permanent 

 branch from the upper of the two protected buds. 



That the axillary position gives, in all cases, valuable protection to the 

 bud is no doubt one reason for the universal adoption of this arrangement 

 in the Angiosperms. The relationship between bud and leaf is, however, 

 physiological as well as morphological, and as the nutrition of the bud must 

 depend largely upon the neighbouring leaf, this is readily understandable. 

 It is well known that the presence of the leaf has a repressive effect on the 

 growth of its axillary bud, at least in woody perennials, where the postpone- 

 ment of bud development may be biologically important. This fact comes 

 out in the behaviour of cuttings, in which the removal of a leaf may be the 

 condition necessary to ensure the growth of its axillary bud. 



The interesting experiments of Dostal with the Enchanter's Nightshade 

 {Circaea lutetiaua) show that the eifect of the leaf on the growth of the bud 

 mav be qualitative as well as quantitative. He took cuttings from various 

 heights on the stem. Each piece, including a node with a pair of opposite 

 leaves, was split lengthwise, and the leaf was removed from one half only. 

 From the pieces with leaves the buds developed as follows : from the lowest 

 nodes, as plagiotropic stolons ; from those higher up, as plagiotropic branches 

 which later became vertical ; and from the upper nodes as vertical flowering 

 shoots. From the leafless pieces, however, the buds grew into vertical leafy 

 shoots at all levels. 



Truly adventitious branches, that is to say, branches produced abnormally 

 without relation to axillary buds, are ver}' rare on angiospermic stems, though 

 they are not uncommon on leaves. The few cases recorded are from the 

 hypocotyls of seedlings. Thev may also develop from wound callus, as we 

 shall describe later in this chapter. Most of the apparently adventitious 

 branches arise from dormant buds buried in the bark, as we have already 

 described. These buds were originally exogenous so that the branches 

 formed from them are also truly exogenous. Endogenous branching of 

 Angiosperms is practically unknown. An exception must be made for the 

 flower shoots which grow from the main trunk, i.e., in the " cauliflorous " 

 trees of the Tropics. There is evidence that these may arise from the pericycle, 

 like the endogenous branches of roots. 



Before leaving the important subject of branching we should glance at 

 the special case of unbranched plants. They may be either primitively 

 unbranched as in the Palms, or have assumed this habit by reduction from 

 a branched type, as in the tiny uniaxial ?^Iousetail {Myosiirus). In practically 

 all cases the habit is associated with large leaves, large, at any rate, relative 

 to the size of the plant, so that a small number of them suffice for its nutrition 

 The habit is commonly associated with very large or massive plants. It has 

 been called " megaphytism," and is attributed by Cotton to the possession 

 of a giant apical meristem which exerts a highly repressive influence, 

 owing to the amount of auxin which it produces, on the growth of all 

 axillary buds. 



