no BOTANY PAHT I 



Systems of Tissue 



A mass of tissue so united in the body of a plant as to form a 

 higher histological unit constitutes a tissue system. In the more 

 highly organised plants three such systems may be distinguished — 



the TEGUMENTARY SYSTEM, the VASCULAR BUNDLE SYSTEM, and the 

 FUNDAMENTAL TISSUE SYSTEM. 



The tissues which make up the different tissue systems are dis- 

 tinguished as PRLMARY and SECONDARY, according to their origin. 

 The primary are those which are derived from the promeristem and 

 primary meristems before growth in length has been completed ; the 

 secondary are those which owe their origin to the primary or 

 secondary meristems after growth in length is finished. 



The primary constituents of the tissue system will be considered 

 first. 



A. The Primary Tissues 



The Tegumentary System. — In the Pteridophytes and Phanero- 

 gams the plant body is covered by a distinct outer layer of cells or 

 EPIDERMIS ; this is wanting in plants of the lower groups. A 

 typically developed epidermis consists of one layer of cells (Fig. 81 e), 

 the outer walls of which on the free surfaces of the plant are more 

 strongly thickened. This is especially the case in all aerial parts of 

 plants adapted for a long life, but on the more perishable parts of a 

 plant, such as the floral leaves, or on subterranean parts, such as the 

 roots, the cells of the epidermal layer are generally thin-walled or 

 only slightly thickened. For such reasons the epidermis of the root 

 was formerly termed the epiblem. Even when the external walls 

 of the epidermal cells are considerably thickened, the side walls, at 

 least in part, remain unthickened. The external Avails are also more 

 or less cuticidarised, while their outermost layer, which is more 

 decidedly cuticulai'ised and capable of withstanding even the action 

 of concentrated sulphuric acid, extends as a cuticle continuously 

 over the surface of the epidermis. This cuticle appears to be always 

 wanting from the epidermis of roots. The cuticle has its origin in 

 the primary walls of the younger epidermal cells, which, during the 

 increase in size of the plant, grow in area, and at the same time are 

 strengthened by the deposition of cutin. The cuticle of leaves in the 

 tropics is often specially smooth and shining. It reflects a portion 

 of the sunlight, giving rise to the sparkling appearance characteristic 

 of tropical foliage, and serves to protect the plant from too strong 

 insolation. The cuticle frequently becomes folded, and so assumes a 

 striated appeai'ance (Fig. 124). 



Tlie remarkable blue iridescence of some leaves and Iruits depends, according 

 to G. Gkntner, on dilFuse reflection in the outer walls of the epidermal cells. It 



