Micrographs of the stem

Stems of kidney bean plants continue to stay green through maturity. However, they may lose the pith. The branch stems have a different shape (see Fig. 27).

A cross-section (transverse section) through the main stem of a bean plant showing secondary growth of a red kidney bean stem.  

Figure 22. A low magnification view of a cross-section of a bean plant stem.  A cross-section (transverse section) through the main stem of a bean plant showing secondary growth. The center tissue is the pith (white arrow). The next layer of dark green tissue is the initial vascular bundles (yellow arrow) and the pale green tissue is the recent maturing xylem (blue arrow) and the dark green band (red arrow) is the cortex. Taken Day 58. Magnified 33 times. Credit: M.E. Conley

Fig. 23. SEM of a bean plant stem.  In this scanning electron microscope view, the pith (yellow arrow) appears empty and secondary xylem (white arrow) is more easily recognizable. The large holes are vessels and the small holes are tracheids or tracheary elements.  Both are part of the xylem tissue. Note that the shape of the stem is circular. Credit: E.T. Paparozzi


Figure 24. A cross-section of a bean plant stem. In this cross-section, you can see the cortex (yellow arrow) which is made up of living cells which contain chloroplasts.  Chloroplasts give a plant its green color. The epidermis, which is often just a single cell layer, is also visible (blue arrow).  You can also see that the cells in the pith contain something (white arrow). A higher magnification will help identify these bodies. Slide prepared by the Nebraska Veterinary Diagnostic Center. Magnified 50 times. Credit: E.T. Paparozzi

Figure 25. SEM of the tissues of a bean plant stem.  This cross-section shows that the cortex (yellow arrow) which is made up of living cells is much smaller than the xylem. The epidermis is also visible in this 3-dimensional view (white arrow).  Credit: E.T. Paparozzi

Figure 26. SEM of the pith in a bean plant stem. These are the cells of the pith. Note that they are hexagons (6-sided). Thanks to this magnification, starch grains (arrow) are identifiable. To be sure these are starch grains, a stem can be stained with potassium iodide solution. If it turns blue/black, then there are starch grains present. Credit: E.T. Paparozzi

Day 48. The branch stems, which arise off the main stem of the red kidney bean plant are not cylindrical. They are almost hexagonal – just like the cells in the pith. The shape is due to the presence of fibers (sclerenchyma tissue) that give the stem strength and shape. 

Figure 27. The branch stem of a bean plant. Note the clear pith in the center and the hexagonal shape. Magnified 48 times. Credit: M.E. Conley

Figure 28. SEM of a branch stem of a bean plant.  Note the hexagonal shape of the branch stem and the presence of trichomes (arrow). Credit: E.T. Paparozzi

Figure 29. A close-up SEM of a branch stem. Note the pith (yellow arrow), xylem (blue arrow) and epidermis (red arrow) with trichomes. Credit: E.T. Paparozzi

Figure 30. A high magnification SEM of a branch stem. Note the pith (yellow arrow). The cells of the xylem (blue arrow) include vessels, tracheids and tracheary elements (dead, strong cells) as well as parenchyma cells which are the same live cells as in the pith. Credit: E.T. Paparozzi

Figure 31. Starch grains in the pith of a bean stem. Plants produce sugars via the process of photosynthesis.  Any sugars that are not immediately used by respiration may be stored in the cell as starch. Credit: E.T. Paparozzi

Figure 32. A close-up of starch grains in the pith of a bean stem. Staining a plant stem with iodine is a way to determine if starch is present. A drop of potassium iodide will turn dark blue/black when it encounters starch. Credit: E.T. Paparozzi