Exercise VI 

 PARENCHYMA CELLS 



I. Introduction. — The term "parenchyma" is used in a rather 

 abstract or loose sense to designate a wide variety of living 

 cells -which occur in many dilt'erent regions of the plant body. 

 Parenchyma cells may appear in groups scattered among highly 

 specialized conducting elements, as for example the cells of vas- 

 cular rays and the vertical files of phloem and xylem parenchyma 

 cells. Often, however, parenchyma cells form homogeneous or 

 "simple" tissues which may constitute a large part of the softer 

 regions of leaves, stems, roots and fruits. From these illustra- 

 tions it should be clearly evident that under the concept of 

 "parenchyma" are included cells which differ markedly in their 

 position, origin and functions. For this reason, "parenchyma" 

 is merely a convenient and long-established anatomical category 

 within which are included cell types which are not necessarily 

 either homologous or analogous. 



In an effort to characterize parenchyma tissue more precisely, 

 it is commonly described as being composed of cells essentially 

 isodiametric in form, separated by more or less conspicuous inter- 

 cellular air spaces, and with thin walls and active protoplasts. 

 To appreciate the merits as well as the limitations of this set of 

 characteristics, the following critique will be found useful. 



1. Form of cell. The recent work of Marvin (1939) and of 

 Marvin and Matzke (1939) has shown that the form of paren- 

 chyma cells in some regions (e.g., the pith of Enpatorium) is 

 approximately isodiametric and that with respect to the number 

 of faces of contact made with neighboring cells, "the cells show 

 an economy of surface to volume approaching, and in some cases 

 equaling, that of an orthic tetrakaidecahedron or a rhombic 

 dodecahedron of equal volume." This conclusion is particularly 

 interesting in view of Tupper-Carey and Priestley's (1924) state- 

 ment that the cells of the apical meristem approach the form 



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