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Figure 91.
Stomata (plural: stomata) are openings in plants that allow them to absorb carbon dioxide while 
also releasing oxygen and water vapor. A dicot's closed stoma is depicted in (a) colorized scanning-electron 
micrograph. Each stoma has two guard cells that control its (b) opening and closing. The (c) guard cells are 
located within the epidermal cell layer. (credit a: modification of work by Louisa Howard, Rippel Electron 
Microscope Facility, Dartmouth College; credit b: modification of work by June Kwak, University of 
Maryland; scale-bar data from Matt Russell) 
Vascular Tissue 
The xylem and phloem that comprise the stem's vascular tissue are organized into distinct 
strands known as vascular bundles that run up and down the length of the stem. The vascular bundles 
of dicot stems are arranged in a ring when viewed in cross section. Individual bundles grow together 
and produce the characteristic growth rings in plants with stems that live for more than a year. The 
vascular bundles in monocot stems are randomly distributed throughout the ground tissue bundle, and 
the phloem is located on the outside. The vascular bundles are capped by sclerenchyma fibers. 
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(credit: modification of work by "(bio photos)"/Flickr; scale-bar data from Matt Russell)

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Vascular bundles of xylem and phloem tissues are scattered throughout the ground tissue of (b) 
monocot stems. 
There are three types of cells in xylem tissue: xylem parenchyma, tracheids, and vessel 
elements. The latter two types conduct water and die when they reach maturity. 
Tracheids 
are xylem 
cells with lignified secondary cell walls. Water flows from one tracheid to the next via pits on the 
side walls, where secondary walls do not exist. 
Vessel elements
are xylem cells with thinner walls; 
they are shorter than tracheids. Each vessel element is linked to the next by a perforation plate located 
on the element's end walls. Water flows up the plant through the perforation plates.
Sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers make up phloem 
tissue. A series of