126 



BOTANY 



PART I 



The Terminations of the Vascular Bundles. — In lecaves, particu- 

 larly in the foliage leaves of Angiosperms, the vascular bundles 

 become much branched until finally they are reduced to extremely 

 fine strands. In the leaves of Gymnosperms this branching of the 

 bundles does not usually take place, but instead, a single vascular 

 bundle frequently runs throughout the whole length of the leaf. 

 The vascular bundles of the reticulately- veined leaves of Dicotyledons 

 illustrate the most extreme form of branching. 



The minute distribution of the bundles in the leaf- lamina facilitates the 

 regular conduction of water to all parts of the leaf-tissue, and at the same time 

 renders easier the removal of the assimilated products. As the ramifica- 

 tions of the vascular bundles are continiied, 

 the bundles themselves become attenuated 

 and simpler in structure (Fig. 136). The 

 vessels first disappear, and only spirally and 

 reticulately thickened tracheides remain to 

 provide for the water conduction. The phloem 

 elements undergo a similar reduction. In 

 Angiosperms, in wliich the sieve-tubes are ac- 

 companied by companion cells, the sieve-tubes 

 become narrower, whilst the companion cells 

 retain their original dimensions. Finally, in 

 the cells forming the continuation of the sieve- 

 tubes, the longitudinal division into sieve-tubes 

 and companion cells does not take place, and 

 TRANSITION CELLS are formed (^'■^). "With these 

 the phloem terminates, although the vascular 

 portion of the bundles still continues to be 

 represented by short spiral tracheides. The 

 ultimate branches of the bundles either termin- 

 ate blindly or anastomose with other vascular 

 bundles. 



Fio. 136. - Termination of a vascular The Fundamental TissuB System 



bundle in a leaf oT Impatiens parvi- ij r .i ■ in rj.u 



flora (x240) usually fomis the main bulk of the 



primary tissues of the body of a plant. 

 The whole tissue of the lower plants, as it shows no internal differentia- 

 tion, may, in a certain sense, be considered fundamental tissue. The 

 other tissues have gradually arisen from the fundamental tissue in the 

 course of phylogenetic development. The fundamental tissue in the 

 higher plants is enclosed by tegumentary tissue, and traversed by 

 the vascular bundle system. While the tegumentary tissue protects 

 the plant externall}', and the vascular ])undle system perfoi'ms the 

 office of conduction, and also of mechanically strengthening the 

 plant, the duty of providing for the nutrition of the plant and of 

 storing reserve food material falls chiefly to the fundamental tissue. 

 The fundamental tissue consist.s, therefore, for the most part of 

 parenchymatous cells containing chlorophyll, at least to such depth as 



