28] 



CHAPTER II. THE TISSUES. 



ill 



form of tissue is the main seat of the protoplasm in the plant, and 

 it is in the cells of this tissue that the chemical processes con- 

 nected with nutrition are more particularly carried on. It is 

 especially well-developed in fleshy and succulent parts (e.g. leaves, 

 fruits, tubers, tuberous roots, etc.). 



2. Thick-walled parenchymatous tissue. Of this there are two 

 principal forms: (1) that in which the cell-walls are lignified ; 

 (2) that in which they are not lignified but consist of cellulose. 

 The former occurs as wood-parcnchyma_in. the secondary woodj 

 and in the secondary medullary rays, of Dicotyledons. The latter 

 commonly occurs as collenchyma just below the surface of her- 

 baceous parts such as mid-ribs of leaves, petioles, young shoots, 

 etc., and serves to give them firmness (Fig. 69). Both forms of 

 this tissue retain their protoplasm 



for a long time after complete dif- 

 ferentiation. The middle lamella 

 (p. 88) of thick-walled paren- 

 chyma with cellulose walls, con- 

 sists of a peculiarly dense form of 

 cellulose. 



3. Cuticularised tissue consists 

 of cells of various form, generally 

 pareuchyinatous 1 the. walls of which 

 have undergone more or less com- 

 plete cuticularisation (see p. 76). 

 The most conspicuous examples of 

 this tissue are the .epidermis, and 

 the cork ^ in_the former, the cuti- 

 cularisation is confined almost ex- 

 clusively to the external wall of 

 the cell (Fig. 69 e), and the_cells re^ 

 tain their cytoplasm ; in the latter, 

 the, cuticularisation extends over 

 the whole cell-wall, and the cyto- 

 plasm is soon lost. In both cases 

 the cuticularisation is most marked 



in the external layers of the cell- wall. In cork-cells there is a 

 certain amount of lignification of the walls as well. The middle 

 lamella (p. 88) of cuticularised tissue consists entirely of cutin 

 (or suberin). Whilst cuticularisation generally occurs in the walls 

 oTTree cells (e.g. spores), or of the superficial cells (epidermis) 



FIG. 69. Transverso section nf part 

 of leaf-ttalk of a Begonia (x 650: after 

 Sachs), e epidermis, the cellsof which 

 have thickened and cuticularised ex- 

 ternal walls ; c cuticle. B Collenchy- 

 matous tissue, with walls thickened at 

 tbe angles v ; the walls of the epidermal 

 cells are similarly thickened where they 

 abut on the collenchyma ; cl individual 

 collenchymatous cells ; cM chloro- 

 plastids ; p large thin-walled parenchy- 

 matous cell. 



