30.] 



CHAPTER IT. THE TISSUES. 



145 



the glandular tissue-system is developed in relation with either 

 the vascular, the fundamental, or the tegumentary tissue-system. 



It is, however, impossible to arrive at the essentially morpholo- 

 gical relations of the tissue-systems without a study of their 

 development, a study which will now be entered upon with the 

 consideration of the apical growing-points of the stems and roots 

 of the sporophyte of the Vascular Plants. 



As already pointed out (p. 18), the growing-point consists, in 

 these plants, of embryonic tissue, the cells of which may be of 

 approximately uniform size, constituting a small-celled primary 

 meristem ; or there may be at the organic apex a cell conspicuously 

 Inrger than the rest, the apical cell ; or a group of several larger 

 initial cells. 



a. Grow ing -points consisting 

 of small-celled meristem are, 

 with rare exceptions, to be 

 found in the roots and stems 

 of Phanerogams, as also in 

 the root of Lycopodium and 

 Isoetes among the Pterido- 

 phyta. Although the cells 

 are all embryonic, they never- 

 theless present such a degree 

 of differentiation as to make 

 it possible to distinguish three 

 well-defined rudimentary tis- 

 sue-systems. 



In the stem, a growing- point 

 of this kind usually presents 

 the appearance shown in Fig. 

 110. It consists, in the first 

 place, of a well-defined super- 

 ficial layer which, on being traced backwards, is seen to be con- 

 tinuous with the primary tegumentary tissue (epidermis) of the 

 older parts ; this layer is, in fact, the embryonic epidermis or 

 dermatogen (tZ) ; it is quite distinct, morphologically, from the 

 subjacent cells, and is characterized by the fact tha-t its cells 

 only undergo division in planes perpendicular to the surface (arc/i- 

 clinal), and not in any plane parallel to the surface (periclinal}. 

 Consequently, whilst the dermatogen increases in area, so as to 

 keep pace with the growing tissues within, it does not become 

 many- layered, but remains a single layer of cells. 



v. s. B. L 



Fm. 109. The tissue-systems in a cross- 

 section of [the petiole of Helleborus ( x 20). 

 Epidermis ; g fundamental tissue ; / vascular 

 system ; x xylem ; c bast ; b sclerenchymatous 

 tissue. 



