INTRODUCTION TO PLANT HISTOLOGY 453 



Elementary Tissues : Parenchymatous Types 



Cells of the permanent tissues are loosely classified as either parenchyma, 

 which are more or less isodiametric, or prosenchyma, which are elongated 

 cells, usually of some specialized type ; they make up the conducting or 

 vascular tissues and fibres. 



The parenchyma cells are the least differentiated and collectively form 

 the ground tissue, which is usually divided into the medulla, or pith, 

 inside the vascular ring, and the cortex, outside it. The simple parenchyma 

 cell (Fig. 441) is, in fact, the typical mature cell, as we have described it in 

 Chapter IL The principal features of its differentiation from a meristem 

 cell are : — 



(a) Considerable increase in size, which is due to ; 



(b) The multiplication and expansion of the vacuoles, which finally unite 



and occupy all the central part of the cell, thus limiting the proto- 

 plasm to a thin layer, or utricle, and pushing the nucleus to one 

 side. 



(c) The stiffening of the cell wall by the growth of a secondary cellulose 



layer inside the original wall. 



(d) Rounding and contraction at the edges and corners of the cell, so 



that a system of narrow intercellular spaces appears, which 

 contain air and play a part in the supply of oxygen to the tissues. 



(e) In many cases the development of plastids, either leucoplasts in the 



deeper-seated cells or chloroplasts in those exposed to light. 



As the parenchyma cell is a living cell, with a considerable internal hydro- 

 static pressure, its form is fundamentally spherical, that is to say, the figure 

 of minimal area, and it is the physical necessity for assuming this figure, 

 under the influence of internal pressure, which no doubt accounts for the 

 separation of the cells at their edges as they become vacuolated. The sides 

 of the cells are somewhat flattened by mutual pressure, giving the isolated 

 cell the twelve-sided figure of a rhombic dodecahedron. This is the 

 geometrical figure, with all its sides plane and similar, which has the least 

 surface for its volume, and it is therefore the simplest form into which a given 

 space can be completely partitioned. In section this figure is hexagonal, 

 and therefore the cells of a perfectly uniform ground tissue will always appear 

 approximately hexagonal in sections of plants organs. This outline is subject 

 to deformation by the internal strains due to differing growth rates, but it 

 can be very generally observed and should be kept in mind when making 

 drawings of cellular structure. 



The fascinating study of the physical principles of cell form and growth 

 cannot be further pursued here, but will be found fully treated in " Growth 

 and Form," by D'Arcy Thompson (Second Edition, 1942). 



Parenchyma cells of irregular form are found occasionally, principally 

 in the spongy tissue of the leaf, where neighbouring cells touch only at a 

 few points, and there are large intercellular spaces. In some Monocotyledons 



