84 



COMPENDIUM OF GENERAL BOTANY. 



plane ; those bundles placed in a somewhat more lateral position 

 penetrate less deeply and describe a smaller curve ; those still more 

 lateral enter least deeply and soon extend vertically downward, 

 In monocotyledons the course of the vascular bundles in the tan- 

 gential plane may be described as follows : Each radially curved 

 bundle inclines in a tangential direction. Among dicotyledons a 

 tangentially inclined course of the vascular bundles is the rule ; 

 they unite laterally after they have taken an isolated course down- 

 ward for some distance. Fig. 48 represents the tangentially in- 

 clining course of the leaf-bundles of Sedum reflexum (dicotyledon) 

 upon the surface of a cylinder. 



There is a leaf at every node and every leaf contains vascular 

 bundles. The radially diagonal course 

 of the vascular bundles in monocotyle- 

 dons is shown in Fig. 49, which is a dia- 

 gramatic median longitudinal section. 

 Only a few of the leaf-bundles are rep- 

 resented in order to illustrate the rela- 

 tions explained above. The dotted lines 

 are* intended to show the tangentially 

 inclined course of the radially curved 

 bundles. The arrangement and distri- 

 bution of vascular bundles, according to 

 the requirements of nutrition, that is, 

 for the uniform distribution of water 

 and of food-substances, have received 

 special attention from HABERLANDT. 1 

 The numerous anastomoses of vascular 

 bundles in the leaves are of great im- 

 portance in cases of local injury, in that 

 the neighboring bundles are thereby en- 

 FlG - 49 - abled to take up the work of those de- 



stroyed. (The circulatory system in man is similarly arranged.) 



The following is of importance in regard to the arrangement 

 and structure of the vascular bundles in the leaves. Green leaves: 

 are for the purpose of assimilation. Therefore the mechanical 

 elements must maintain these flattened organs in a suitable position 

 with regard to the light (see pp. 67 and 68 in regard to the 

 1 Phys. Pflanzen-Anatomie. 



