326 MANUAL OF BOTANY 



CHAPTER III. 



TISSUE SYSTEMS. 



The combination of cells into tissues which we have now 

 examined is an indication that in a cell mass such as con- 

 stitutes the body of most plants there exists a division of 

 labour ; that instead of each cell practically living for itself 

 alone, as is the case in the simplest forms, each takes a 

 definite share in the common life work of the whole organism. 

 This division of labour is the explanation of the differentiation 

 of the structure which we have seen, the forms and arrangement 

 of the cells being such as enable them to discharge their special 

 functions most advantageously. Generally in the sporophyte of 

 the Plianerogams and Vascular Cryptogams three main systems 

 of tissue can be distingiiished, each of which, though possessing 

 broadly characteristic features of its own, may include more 

 than one form of the tissues that have been described. These 

 are the epidermal or tegumentary, chiefly protective in character ; 

 the fibro-vascular, which is mainly concerned in the conduction 

 of fluids through the plant body; and the ground or fundamental, 

 which is limited externally by the epidermal, and through which 

 the fibro-vascular bundles pass. It is concerned principally 

 with the metabolic work of the plant, and to a less degree with 

 its mechanical support. None of these is exclusively the seat 

 of the function assigned to it as its chief work. Thus in most 

 of the higher plants the epidermis is comparatively short-lived 

 in many parts, aud the work of protection is then taken up by 

 particular tissues belonging to the fundamental tissue ; the 

 latter also often includes certain forms of conducting tissue. In 

 the gametophytes of these and of the lower plants the differen- 

 tiation into these three systems is not nearly so complete. In 

 most only the epidermal and ground tissues are represented ; 

 in some only the latter. In the Mosses there is a solid central 

 portion which though not truly vascular tissue is an indication 

 of it. 



