THE STEM. 



107 



h likewise varies greatly in 



to different conditions of 



found enormously developed 



as in some tubers, such as 

 In other plants, particularly 



132. (c) Pith.— The 1 



different plants according 

 growth. It is frequently found 

 in those parts of the stem which are used by the plant 

 for storing its reserve food, 

 the white potato and the yam. 

 those growing in water, it 

 suffers extreme reduction or is 

 often completely wanting, in 

 which case the bundles of the 

 stele are in close contact, and 

 the cortex usually shows a cor- 

 responding increase. In other 

 plants the cells constituting 

 the pith are greatly thickened, 

 so as to form a mechanical 

 tissue. The thickened areas 

 are usually either opposite the 

 bundles, forming a strand 

 closely adherent to their inner 

 faces, or they may extend to 

 the flanks of the bundles, thus 

 forming an arc embracing 

 each. Sometimes the thickened region becomes extended 

 between the bundles and joins the corresponding mechanical 

 tissues in the pericycle, or even those of the cortex, so as to 

 enclose completely the individual bundles (fig. 123). In 

 other plants the pith dies early and shrivels up. Very large 

 canals may thus be formed through it, or it may even dis- 

 appear entirely (fig. 122). Such early disappearance of the 

 pith produces the hollow stem characteristic of the grasses, 

 the sedges, and the various members of the sunflower family. 



133. Secondary structure. — Some stems retain throughout 

 their entire existence the primary structure which has just 



Fig. 123. — Transverse section Ol .1 bundle- 

 pair of Indian corn. r>, phloem bundle; 

 1. %\g, s, r, xylem bundle: p. pith; /, 

 an intercellular space formed by the 

 tearing of some of the .xylem tissues. 

 The bundle pair is surrounded by a 

 sheath of thick-walled mechanical 

 tissues. Magnified 235 diam. — After 

 Sachs. 



