MORPHOLOGY 



107 



vascular bundles of the reticulate- veined leaves of Dicotyledons illus- 

 trate the most extreme form of branching. 



The fine 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. An extended distribution of 

 the bundles in the leaves is thus evidently of advantage to a plant. In the same 

 degree as the ramifications of the vascular bundles are continued, the bundles 

 themselves become attenuated and simpler in structure (Fig. 122). The vessels first 

 disappear, and only spirally and reticulately thickened tracheids remain to provide 

 for the water-conduction. The phloem elements undergo a similar reduction. In 

 Angiosperms, in which the sieve-tubes are accompanied 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 discontinues, and transition cells are 

 formed. With these the sieve-tubes terminate, although the vascular portion of 

 the bundles still continues to be represented by short spiral tracheids, until finally 

 they too disappear, either terminating blindly or anastomosing with other vascular 

 bundles. 



The Fundamental Tissue System usually forms the principal part 

 of the primary tissues of the body of a 

 plant. The whole tissue of the lower 

 plants, as it shows no internal differ- 

 entiation, may, in a certain sense, be 

 considered fundamental tissue. The 

 other tissues have gradually been de- 

 veloped 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 externally, and the 

 vascular bundle system performs the 

 office of conduction, and also of me- 

 chanically strengthening the plant, the 

 duty of providing for the nutrition of 

 the plant and of storing reserve food 

 material falls chiefly to the funda- 

 mental tissue. The fundamental tissue Fie. 

 consists, therefore, for the most pact of 

 parenchymatous cells containing chloro- 

 phyll, at least to such depth as the light penetrates ; but internally, 

 and wherever the tissues are so situated as to be unaffected by the 

 influence of the light, a colourless parenchyma is found. The funda- 

 mental tissue system also takes part in providing for the mechanical 

 rigidity of plants, and in connection with this function it possesses 



122. — Termination of a vascular 

 bundle in a leaf of Impatient* pawl- 

 flora. (x240.) 



