TISSUE SYSTEMS 835 



enlarged at its base, b, by an irritating i5uid /, /, which it con- 

 tains, and tapering upwards to near its apex, wliere it again 

 expands into a rounded or pointed liead, s. The enlarged base 

 is closely invested by a dense layer of epidermal cells, w e, 

 which forms a kind of case to it. In touching a Nettle lightly 

 the knob-like head, .s, is broken off, and the sharp point of 

 the sting then left enters the skin, while the irritating fluid is 

 pushed up at the same time into the wound by the pressure 

 occasioned by the elastic force of the surrounding epidermal 

 cells, iu e. 



The Ground or Fundamental Tissue System. 



Underneath the epidermis in the gametophyte of all cellular 

 plants which show any differentiation of tissues, we find a mass 

 of cells which constitutes the ground or fundamental tissue. 

 In the sporopliyte of the vascular plants, which is in their 

 case the prominent form, we can distinguish in addition a 

 central stele or steles, largely composed of vascular tissue, but 

 containing some fundamental or conjunctive tissue interspersed 

 among the latter [figs. 716 and 717). The fundamental tissue 

 in these forms may be spoken of as exira-stelar and intra-stelar 

 respectively, the former derived from the periblera, the latter 

 from the plerome. If there is a single stele the extra-stelar 

 fundamental tissue constitutes what is known as the cortex, and 

 has the form of a hollow cylinder. It is the region which is 

 continuous with the periblem of the growing point (Jig. 701). 



Extra-stelar Orouncl Tissue. 



Generally ground tissue is composed of parenchyma, the cells 

 being polygonal in outline and thin-walled, with intercellular 

 spaces between the cells. Sometimes the walls are slightly 

 thickened, and then show pits upon their surfaces. The tissue 

 may contain, in addition, fibres or masses of solerenchyma or 

 collenchyma, especially when the vascular bundles of the stele 

 are not well developed. 



These masses are arranged in different ways, sometimes as 

 a sheath underlying the epidermis, sometimes as isolated 

 strands in the substance of the cortex, or the perioycle or external 

 layer of the stele, sometimes as sheaths or masses, in contact 

 with the vascular bundles. They may be connected together in 

 various ways, and thus form a supporting tissue supplementing 

 the vascular bundles. These sclerenchymatous and collenehy- 

 matous masses together with the hardened elements of the 



