THE STEM 219 
bundle will be seen an area of soft, small-celled sieve tubes, 
companion cells and phloem parenchyma. The entire bundle is 
enclosed by a several layered ring of sclerenchyma fibers. 
HistoLocy or Stems oF Grasses (MoNOcOTYLEDONOUS).— 
The stems of grasses are solid in their terminal and nodal regions; 
6a 
Sau: 
tec 
Pee, . 
36 
~ 
Fic. 149.—Structure of a Grass Stem. ‘Transverse section of the underground 
stem (rhizome) of the couch grass, Agropyron repens, X 35. ep, epidermis; h, hypo- 
dermis; co, cortex; 6, concentric bundle; en, endodermis; scl, sclerenchymatous ring 
of several rows of sclerenchyma fibers in which are imbedded an irregular, inter- 
rupted circle of undeveloped vascular bundles; ér, trachea and ph, a phloem strand 
of a closed collateral bundle; cb, closed collateral bundle; m, pith. Note the hollow 
space in the center due to the disintegration of the pith in centrifugal fashion. 
but as we pass backward from the terminal internode into the 
older internodes the central pith is seen to disintegrate in cen- 
trifugal fashion, leaving a hollow cavity in the central region 
bordered by parenchyma cells. A distinctly differentiated 
endodermis may or may not be present. The vascular bundles 
may be entirely closed collateral, as in corn, but are frequently of 
two kinds, namely, small, phlocentric bundles in the cortex and 
