29] CHAPTER II. THE TISSUES. 101 



The secretum (which, may consist of mucilage, or gum-resin, or 

 ethereal oil, balsam, etc.) is accumulated either in the cavity of the 

 secreting cells (e.g, mucilaginous hairs at the 

 growing-point of Liverwort gametophytes and of 

 Fern-sporophytes), or between the external cuticle 

 and the deeper layers of the cuticularised cell- 

 wall (e.g. mucilaginous hairs [colleters] on the 

 buds of many Phanerogams, resinous hairs gener- 

 ally ; Fig. 82). 



29. General Morphology of the Tissue- 

 Systems. When a form of tissue constitutes a 

 complex which extends continuously throughout FIG. 82. Gian- 

 the body of a plant, or over considerable areas, dular hair from the 



J baseofaramentum 



it is spoken of as a tissue-system : for instance, O f A*pidium mix 

 a laticiferous system; a resin-duct system; a mas ( X2 <x>):, "* 



, . , ,. secretum, lying be- 



sieve-tube system; a mechanical tissue-system tween the outer and 

 (stereom) including collenchyma and scleren- inner layers of the 

 chyma ; a glandular tissue-system ; a tegumen- 

 tary tissue-system : or a more elaborate system may be produced 

 by the combination of two or more systems: for instance, the 

 sieve-tube system and the tracheal system taken together con- 

 stitute the vascular tissue-system. 



These tissue-systems are, however, characterised rather by their 

 function, that is physiologically, than by their development, that 

 is morphologically. From the latter point of view the following 

 primary tissue-systems are distinguished: (1) The Tegumentary 

 Tissue; (2) The Ground-Tissue; (3) The Stele. The study of 

 these tissues will be begun by the consideration of the structure 

 of the growing-point in stems and roots. 



As already pointed out (p. 8), the growing-point consists, in 

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

 of approximately uniform size, constituting a small-celled primary 

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

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

 initial cells. 



a. Grouping-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 Pteridophyta. Although the cells are all embryonic, 

 they nevertheless present such a degree of differentiation as to 

 make it possible to distinguish the three primary tissue-systems. 



