188 



STEMS 



as to form distinct regions, such as pith, vascular cylinder, and 

 cortex, which are more or less distinct regions in Dicotyledons. 



When cross sections of the Corn stem are studied with the 

 microscope, such anatomical features as shown in Figure 166 

 may be seen. The cells of the rind are rectangular in shape, con- 

 sist of a number of rows, and their walls are thickened and made 

 woody for strength. The woody feature of the rind is character- 

 istic of Grasses and Sedges, being much less prominent in other 

 monocotyledonous stems, as, for example, in Lilies and Aspara- 

 gus. The outer row of cells of 

 the rind constitutes the epider- 

 mis, although in the Grasses the 

 epidermal cells differ very little 

 from other rind cells, except that 

 they have silica and cutin de- 

 posited in their outer walls. 

 The vascular bundles, contain- 

 ing numerous cells, show three 

 or four large openings which are 

 the large vessels of the xylem. 

 Besides the large size of the pith 

 cells as shown in the drawing, 

 other features not shown, such 

 as their storage function and 

 their being so loosely joined as 

 to form a spongy filling for the 

 stem, should be mentioned. 

 To study the complex structure of a vascular bundle, we must 

 turn to a more highly magnified cross section of the bundle as 

 shown in Figure 167. The vascular bundle consists of strength- 

 ening and conductive tissues, the latter of which are composed 

 of the xylem and phloem, the chief structures of all vascular 

 bundles. In respect to the character of the vessels composing 

 them, xylem and phloem show much uniformity throughout Flow- 

 ering Plants. 



In the xylem the conductive tissues consist mainly of large ves- 

 sels, known as spiral, annular, or pitted vessels according to the 

 character of the thickenings in their walls, as partly shown in Fig- 

 ure 168 and more fully shown in Figure 169. The woody thicken- 

 ings, which strengthen the cellulose walls of the vessels so that 



FIG. 166. A portion of a cross 

 section of a Corn stem much en- 

 larged, a, epidermis; 6, the band of 

 strengthening cells under the epider- 

 mis and often called cortex; v, vascu- 

 lar bundles; e, pith. After Stevens. 



