132 



PART IT. THE INTIMATE STRUCTURE OF PLANTS. 



[29. 



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-parenchyma in the secondary wood, 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. 91). Both forms of this 

 tissue retain their cytoplasm for a long time after complete differen- 

 tiation. The middle lamella (p. 

 128) of thick-walled parenchyma 

 with cellulose walls, consists of a 

 peculiarly dense form of cellulose. 

 3. Cuticularised tissue consists of 

 cells of various form, generally 

 parenchymatous, the walls of 

 which have undergone more or less 

 complete cuticularisation (see p. 

 106). The most conspicuous ex- 

 amples of this tissue are the epi- 

 dermis, and the periderm ; in the 

 former, the cuticularisation is con- 

 fined almost exclusively to the ex- 

 ternal wall of the cell (Fig. 91), 

 and the cells retain their cyto- 

 plasm ; in the latter, the cuticu- 

 larisation extends over the whole 

 cell- wall, and the cytoplasm is soon 

 lost. In both cases the cuticulari- 

 sation is most marked in the ex- 

 ternal layers of the cell- wall. In 

 cork-colls there is a certain amount of lignification of the walls as 

 well. The middle lamella (p. 128) of cuticularised tissue consists 

 entirely of cutin (or suberin). Whilst cuticularisation generally 

 occurs in the walls of free cells (e g. spores), or of the superficial 

 cells (epidermis, periderm) of a multicellular body, it occurs 

 sometimes in internal tissues (e.g. endodermis). 



4. Sclerenchymatous tissue, or sclerenchyma, consists typically of 

 proscnchymatous cells which lose their cytoplasm relatively early, 

 and then contain only water or air, and are distinguished as fibres-, 

 but in some cases they retain their cytoplasm, and are then dis- 



Fio. 91. Transverse section of part 

 of leaf-stalk of a Begonia (x 550: after 

 Sachs). E Epidermis, the cellsof which 

 have thickened and cuticularised ex- 

 ternal walls; c cuticle. B Collenchy- 

 matous tissue, with walls thickened at 

 the angles v ; the walls of the epidermal 

 cells are similarly thickened where they 

 abut on the collenchyma ; cl individual 

 collenchymatous cells ; tW chloro- 

 plastids; p large thin-walled parenchy- 

 matous cell. 



