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A TEXTBOOK OF THEORETICAL BOTANY 



is not consistent with fact. Frequently, indeed, the three zones cannot be 

 distinguished, especially at the apex of the stem. 



Modern studies have interpreted the meristem of stems as composed of 

 two zones only (Fig. 440). These are the tunica, which consists of one or 







\* 



^^f 



d/ 





Corpus 



K 





Fig. ^^o.^Solamim crispum. Longitudinal section 

 of stem apex showing tunica and corpus. 



several peripheral layers of cells, enclosing the corpus or central core of 

 tissue. The distinction between them reflects two difi^erent modes of growth. 

 In the tunica, surface growth predominates and cell divisions are uniformly 

 anticlinal, that is, perpendicular to the surface, with the result that the cells 

 are regularly oblong in longitudinal section and each tunica layer remains 

 distinct.* In the corpus, by contrast, volume growth is predominant and the 

 planes of cell division and cell arrangement tend to be highly irregular. The 

 maintenance of balance between surface and volume growth leads to constant 

 adjustments which find their expression in the rhythmical alternation of 

 minimal and maximal areas of the growing point, the latter being associated 

 with the formation of external folds or leaf primordia (see also Chapter XXI). 

 These zones are not " histogens " in Hanstein's sense but represent the distri- 

 bution of growth patterns, which may vary considerably in different species. 

 Immediately below the corpus may often be seen a tissue which is called 

 the rib meristem, distinguished by short, vertical rows of cells in process 

 of vacuolization. These are formed from cells of the corpus which have 

 repeatedly divided horizontally. As each row is produced from one original 

 cell it has a common wall surrounding it. This tissue continues downwards 

 into the pith. 



* As the tunica cells have no vacuoles their mutual pressure is only the lateral pressure 

 due to growth. They therefore take the form of hexagonal prisms with their long axes 

 anticlinal. 



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