48 BOTANY TART i 



or elongating the cells may behave independently (Fig. 174), so that 

 the ends of some which elongate greatly push past, or in between, other 

 cells (SLIDING GROWTH) ( 39 ). 



The permanent tissues may be classified in various ways. Thus 

 according to their origin primary and secondary permanent tissues 

 may be distinguished arising from the corresponding meristems. 



A morphologically useful division of the permanent tissues is 

 obtained when all the differences of the component cells are taken 

 into consideration together. 



It was formerly usual to take the dimensions of the cells into special considera- 

 tion, and on this ground PARENCHYMA and PROSENCHYMA were distinguished. By 

 parenchyma was understood a tissue the cells of which were isodiametric or, if 

 elongated in one direction, were separated by transverse walls. Prosenchyma was a 

 tissue the elongated cells of which were spindle-shaped and pointed at both ends, 

 which fitted between those of the associated cells, These two groups do not, however, 

 suffice to give a survey of the variety of kinds of tissues, and the underlying 

 conceptions are out of date, especially in the case of parenchyma. 



On examining the tissues of the higher plants comparatively there 

 is found in the first place a tissue which, like that composing the 

 lowest multicellular plants, consists of cells with living contents 

 and thin cellulose walls, and is capable of performing a diversity of 

 functions ; this will be termed PARENCHYMA. Other tissues may 

 be sharply distinguished from this parenchyma by peculiarities of 

 structure and special functions. The most striking tissues in the light 

 of their main functions are the BOUNDARY TISSUE, the MECHANICAL 

 TISSUE, and the CONDUCTING TISSUE. The PARENCHYMATOUS SYSTEM, 



the EPIDERMAL SYSTEM, the MECHANICAL SYSTEM, and the CONDUCTING 



SYSTEM correspond on the whole to these tissues. In addition the 

 SECRETORY TISSUE and GLANDULAR TISSUE may be recognised. 



The permanent tissues are frequently divided into epidermis, vascular bundles, 

 and ground tissue. 



1. Parenchyma. Parenehymatous System. The parenchyma 

 cell is characteristic of this type of tissue, the relative primitiveness of 

 which has been referred to above (cf. p. 45). It may be isodiametric 

 or elongated and of various shapes, and possesses the following further 

 characters (cf. Figs. 3 B, 9, 40, 41). The cell wall, which as a 

 rule consists of cellulose, is only moderately thickened and provided 

 with simple round or elliptical pits ; it thus facilitates the diffusion of 

 substances from cell to cell. Living protoplasm is usually present, 

 and the large vacuole may contain a considerable amount of nutritive 

 material. The chromatophores, which have the form of chloroplasts 

 or leucoplasts, often contain starch. Parenchyma is usually traversed 

 by a ventilating system of intercellular spaces. Parenchyma may 

 form part of other primary or secondary tissues and serves a 

 variety of functions. The most important vital processes of the 



