32.] CHAPTER II. THE TISSUES. 159 



cap are successively split off and exfoliated by the pressure of the 

 internal growing tissues. 



Hairs (see p. 64), are frequently developed on the primary 

 tegumentary tissue, and are generally formed each as an outgrowth 

 of a single superficial cell (Fig. 1 22 ; see also Fig. 40, p. 65 ; and 

 Fig. 108, p. 143). 



The hairs of the subaerial parts of plants are, like the epider- 

 mal cells, cuticularised. In many cases the protoplasmic contents 

 disappear at an early stage (as in Cotton, the hairs on the outer 

 coat, or testa, of the seed of Gossypium) and are replaced by air. 

 Sometimes the cell- wall contains deposits of lime or of silica. The 

 hairs are frequently glandular (see p. 142). 



The root-hairs (Fig. 123 ; also see p. 65) are developed each 

 from a single cell of the epiblema or piliferous layer ; they are not 

 developed in the immediate neighbourhood of the growing-point 

 but at some little distance behind it. Moreover, as they grow 

 older, the root-hairs die off ; hence they are only to be found on a 

 very limited region of a primary or a secondary root. 



32. The Fundamental Tissue-system, or Ground-tissue, 

 is constituted by the tissue which belongs neither to the epidermis, 

 on the one hand, nor to the vascular tissue on the other. Two 

 regions of this tissue-system are distinguishable, according to their 

 relation to the stele, as extra-stelar and intra-stelar; the former 

 being developed from the periblem, the latter from the plerome, 

 of the growing-point. 



The Extra-stelar Fundamental Tissue. Morphology. The limits, 

 of this tissue vary with the structure of the part concerned. 

 When the part, whether it be stem, leaf, or root, has a true epi- 

 dermis (see p. 154), the external limit of the extra-stelar tissue 

 is the layer of cells lying immediately beneath the epidermis; 

 when, however, there is no true epidermis, the extra-stelar tissue 

 extends to the surface, and the superficial tegumentary layer is 

 merely its external layer. Again, when the member is mono- 

 stelic, the internal limit of the extra-stelar tissue is the layer 

 termed the endvdermis, which abuts upon the central stele ; in this 

 case the extra-stelar tissue consists of several layers of cells 

 bounded externally by the true epidermis (if present), or reaching 

 to the surface, and bounded internally by the stele, when it is 

 spoken of as the cortex of the member of which it forms part. In 

 a polystelic member, the internal limit of the extra-stelar tissue is 



