192 



STEMS 



Closed vascular bundles and their scattered arrangement are 

 the chief distinguishing features of the anatomy of monocotyle- 

 donous stems. 



Structure of Herbaceous Dicotyledonous Stems 



Herbaceous Dicotyledons constitute an important group, for 

 they include many forage plants, notably the Clovers and Alfalfa, 

 some important fiber plants as Flax and Hemp, most vegetables, 

 and many greenhouse plants. In the tropical countries there 

 are a few Gymnosperms that are herbaceous, but in general 



features their anatomy is 

 quite similar to that of 

 herbaceous Dicotyledons. 



All stems of the herba- 

 ceous dicotyledonous type, 

 whether they are stems 

 strictly herbaceous through- 

 out or only the young 

 branches of woody stems, 

 have pith, vascular cylinder, 

 and cortex which occupy well 

 separated regions when well 

 FIG. 171. - Diagram of a cross section developed. Cross sections 

 of a well developed herbaceous stem, show- appear to the naked eye 

 ing the epidermis (a); band of tissue (6) about as shown in Figure 

 composed of cortex and phloem; xylem 171 The epidermis, cortex, 

 cylinder (c); and pith (d). and phloem form the goft 



outer zone, while the xylem forms the woody cylinder just within 

 the soft zone, and encloses the pith, which occupies the center of 

 the stem. In order to trace the development and study the 

 anatomy of the different tissues, we must turn to highly mag- 

 nified sections as shown in Figure 172. 



The Cortex, which is the larger part of the outer zone of tissues, 

 is covered by the epidermis, and includes the starch sheath as its 

 innermost layer. Just under the epidermis some of the cells of 

 the cortex are transformed into collenchyma cells, which are par- 

 ticularly abundant in the angles of the stem shown in the Figure 

 but more generally distributed around the stem in many other 

 plants. The collenchyma cells, often noticeable in sections on 

 account of their whitish glistening appearance, have much thick- 



