100 BOTANY PART i 



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 

 distinguished as PRIMARY and SECONDARY, according to their 

 origin. The primary are those which are derived from the pro- 

 meristem 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 systems 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. 74 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 walls are also more 

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

 decidedly cuticularised 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 ( 101 ). 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 appearance (Fig. 114). By the thickening and the 

 cutinisation of their outer walls the mechanical rigidity of the epi- 

 dermal cells is increased, and the loss of water by transpiration is 

 lessened. Plants in dry climates, or so situated that, for any reason, 

 transpiration from their outer surfaces must be diminished, are char- 

 acterised by the greatly thickened and cuticularised walls of their epi- 

 dermal cells. In some of the Gramineae, Equisetaceae, and many other 



