BOTANY OF THE LIVING PLANT 



While the main axis of the "Christmas Tree" is upright and 

 ,,1 the lat.-r,! branches appear flattened. This is due partly 



to the fact that the lateral 

 branches of higher order have 

 been developed from buds 

 seated right and left on the 

 main branches ; partly to the 

 curvature of the leaves upwards. 

 Nevertheless the construction 

 of these dorsiventral branches 

 is essentially radial. If the 

 terminal bud of the main stem 

 be removed, one or more of 

 the lateral buds, which would 

 normally grow into flattened 

 lateral branches, will grow up- 

 wards and take its place. With 

 the vertica-1 position, it regains 

 its radial symmetry. Very 

 much the same may be seen 

 in the Sycamore, or the Horse 

 Chestnut, both of which retain 

 the radial structure of the 

 leading shoot. Their lateral 

 branches are slightly dorsiven- 



Fig. 138. 

 Strongly dorsiventral branch of Lime, seen from 

 above. Its leaves form a compact leaf-mosaic. 

 (Reduced to J.) 



tral, but they do not modify the symmetry of the lateral branches 

 so much as the Spruce. 



On the other hand, the upright radial shoot, or leader, in many trees 

 may itself become dorsiventral as growth proceeds, while the lateral 

 \ "ranches are conspicuously so. Consequently the whole tree when 

 full grown will be made up of flattened shoots, the lateral branches 

 spreading out like fans. (Fig. 138.) In the Elm, Beech, and Lime 

 this change of the leading shoot comes early. Their seedlings are 

 radially constructed. In the Elm and Beech the dorsiventral structure, 

 with alternate distichous leaves (in place of the decussate arrangement) 

 app n the second year of the seedling, and continues from then 



onwards. Bui though the whole tree is thus built up of dorsiventral 

 shoots, these may form collectively a radial crown, as in the Elm. 

 Such examples of the transition from radial to dorsiventral symmetry 

 are carried out in each individual plant. They indicate that the 

 radial is the more primitive state, and the dorsiventral derivative. 



