go OUTLINES OF PLANT LIFE. 



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 corresponding 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 vascular strands, 

 forming a strand closely adherent to their inner faces, or they may extend 



to their flanks, thus forming an arc 

 embracing each. Sometimes the thick- 

 ened region becomes extended between 

 the vascular strands and joins other 

 mechanical tissues of the stele, or even 

 those of the cortex, so as to enclose 

 completely the individual strands (fig. 

 80). 



In other plants the pith dies 

 early and shrivels up: Very large 

 canals may thus be formed 

 through it, or it may even disap- 

 pear entirely (fig. 79). Such 



Fig. 80.— Transverse section of a bundle i j • r , i • , i 



pair of Indian corn. 7., bast bun- early disappearance of the pith 



die ; X, g, g, s, r, wood bundle j /, , ^.i i n . ^ 



pith; /, an intercellular space formed produCeS the holloW Stem cha- 



by the tearing of some of the wood , - , ■ r ,i i 



tissues. The bundle pair is surrounded raCteriStlC 01 the graSSCS, the 



by a sheath of thick-walled mechanical , , . , . 



tissues. Magnified 235 diam.-After sedges, and various members of 



the sunflower family. 

 111. Secondary structure. — Some stems retain through- 

 out their entire existence the primary structure which has 

 just been described, undergoing only slight changes which 

 do not materially alter the structure. This permanence of 

 primary structure is frequent in the stems of monocotyledon- 

 ous plants. But the stems of the great majority of dicoty- 

 ledonous plants, as well as the conifers, quickly lose their 

 primary structure, adding tissues of considerable amount, so 

 as to bring about a more or less striking rearrangement of 

 the first formed tissues (fig. 8i). This is due chiefly to the 



