76 MERLSTEMATIC SYSTEM 



tissues are correlated with their physiological functions. Meristematic 

 cells are invariably thin-walled: this property not only permits of any 

 degree and form of secondary growth in thickness, but also facilitates 

 that rapid influx of nutritive materials in the absence, of which a 

 meristematic tissue would soon be rendered inactive by exhaustion. 



Another distinguishing feature of meristematic cells is the massive 

 development of their protoplasts: usually the cell-cavities are completely 

 tilled with homogeneous protoplasm, vacuoles and sap-cavities of ordinary 

 dimensions as well as larger inclusions, such as coarse-grained starch 

 or drops of oil, being entirely absent. In other words, no storage of 

 plastic materials takes place in meristematic cells, on account of their 

 intense metabolic activity. The relatively large size of the nuclei in 

 meristematic cells has already been mentioned and explained in terms 

 of the probable functions of the nucleus (p. 27). Chromatophores, 

 when present, are generally represented by leucoplasts, less frequently 

 by small pale-green chloroplasts (Fig. 5 p., c). Finally, the small size 

 of the cells and their great capacity for division may be included in 

 this list of the general characteristics of meristematic tissues, although 

 as a matter of fact, both these features are tacitly comprehended in the 

 accepted definition of a meristematic tissue. 



When a meristem is converted into permanent tissue, the relative 

 positions of the constituent cells become altered in a variety of ways. 

 For one thing, the cells of adult tissues vary greatly in size and shape : 

 furthermore, the physiological requirements of the different cells and 

 cell-masses often necessitate more or less extensive modifications of the 

 spatial arrangement of the meristematic elements from which they are 

 derived. The developing cells thus often undergo a very considerable 

 amount of displacement owing to idiosyncrasies of growth : such indi- 

 vidual or acth r e displacements must not be confused with the passive 

 or mechanical displacements imposed upon the cells as a result of the 

 growth of the entire organ. Active displacement occurs, for exam] tie, 

 when the branches of non-articulated latex-tubes push their way into 

 adjacent tissues, when the pointed ends of bast-fibres grow past 

 one another, when the hypodermal crystal-cells in the leaves of 

 species of Citrus penetrate between the over-lying epidermal elements, 

 or, finally, when expanding tracheae or sieve-tubes force apart and 

 disarrange the cells which surround them. 



Krabbe was the first to demonstrate the widespread occurrence of 

 this phenomenon of displacement of cells, and to recognise its import- 

 ance as a factor influencing the internal structure of plants. Krabbe 

 refers the displacement in question to a process of sliding growth.' 1 ''' 

 He supposes that large areas of a cell-wall may simultaneously undergo 

 growth in surface; such extensive surface-growth, however, cannot 



