MODIFICATIONS OF FORM 



173 



Chestnut, which retain the radial structure of their leading shoot. 

 Their lateral branches are slightly dorsiventral, 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 lead, in many trees 

 may itself become dorsiventral as growth proceeds, while the lateral 

 branches 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. 129 bis.) 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 disti- 

 chous leaves (in place of the 

 decussate arrangement) appears 

 in the second year of the seed- 

 ling, and continues from then 

 onwards. But though the whole 

 tree is thus built up of dorsi- 

 ventral shoots, these may form 

 collectively a radial crown, as in 

 the Elm. Such examples of the 

 transition from radial to dorsi- 

 ventral symmetry are carried out 

 in each individual plant. They 

 indicate that the radial is the 

 more primitive state, and the 

 dorsiventral derivative. 



In many herbaceous plants the 

 dorsiventral habit is more pro- 

 nounced than in trees. The 

 whole shoot may take a creeping horizontal position, or a climbing 

 habit by lateral attachment, as in the Ivy. The lopsidedness then 

 becomes marked, and different degrees of it may be noted. Thus 

 in the creeping rhizome of Carex, while the leaves show the i-3rd 

 divergence, the axillary bud in the axil of each fourth downward- 

 directed leaf alone is developed. The apex of the main shoot then 

 turns upwards, while the axillary bud grows on, continuing the 

 false-axis (sympodium). Thus the individual shoot is radially con 

 structed, but the sympodial system is dorsiventral. (Fig. 130.) In 

 other cases the mature shoot may be itself dorsiventral, though that 



FIG. 129 bis. 



Strongly dorsiventral branch of Lime, seen from 

 above. Its leaves form a compact leaf-mosaic. 

 (Reduced to i.) 



