PHOTO STUDY 11-1 Ranunculus (buttercup): t.s. young root with triarch stele.

The outermost cell layer is, of course, the epidermis. Beneath the epidermis is the very thick cortex, composed mostly of storage parenchyma. The outermost cortical cells (those next to the epidermis) are different from the rest; they are arranged in a pretty definite layer, the hypodermis. It continues the protective function of the epidermis when the latter is destroyed. Recognize the endodermis by the regular arrangement of its cells, by the slight thickening of its cell walls. This layer is commonly considered to be the innermost layer of the cortex. Next to it, on the inner side, is the single layer of thin-walled pericycle, and next inside that are the xylem and phloem tissues. Distinguish the four xylem "wings", and the four phloem groups which alternate with them. Between xylem and phloem are parenchyma cells that have not yet completely matured. Is there a central pith?

PHOTO STUDY 11-2 Ranunculus (buttercup): t.s. root stele

Concentrate your study on the stele and the enveloping endodermis. Identify Casparian strips in the radial walls of endodermis cells. Because xylem has differentiated on four radii (as has also the phloem), the stele is said to be tetrarch. In each xylem "wing" see that those cells that border on the pericycle are of smallest diameter, while those toward the center of the stele are the largest. Understand what this means in terms of development. One to perhaps three of the outer most and smallest xylem cells in each "wing" are protoxylem; the rest are metaxylem. All, of course, are primary, and probably all are vessels. Since the first xylem to mature is that at the periphery of the stele, all later-maturing xylem being centripetal to that, the xylem is said to be exarch. Unlignified central vessels, such as you saw developing in the root tip of Canna, are not found in dicotyledons, though they are common in monocotyledons.

PHOTO STUDY 11-3 Ranunculus (buttercup): t.s. triarch root stele

Note in this photo there are only 3 "archs"; said to be triarch. Find the endodermis cell at the tip of the right xylem wing. This cell shows the Casparian strip as a ribbon across the cell, rather than a thickening on the radial walls. Interpret. Phloem cells make up the compact strands that alternate with the xylem "wings". Phloem does not, however, fill completely the interval between any two "wings": parenchyma tissue fills in between a phloem strand and xylem. In a phloem group, recognize sieve tube elements and companion cells, the former having so little protoplasm that they appear to be empty. Some show end walls with sieve plates (red).

PHOTO STUDY 11-4 Actaea (baneberry): t.s. young root with triarch stele.

Recognize the epidermis, hypodermis, cortex (containing amyloplasts), and the triarch stele. Why is this called triarch?

PHOTO STUDY 11-5 Actaea (baneberry: t.s. young root with triarch stele.

Compare stellar structure with that in Ranunculus. Identify the endodermis. Are its cells thick-walled? Recognize Casparian strips. Identify pericycle, xylem, phloem, and unspecialized parenchyma of the stele. Because xylem and phloem are each differentiated on two radii, what would you call this stelar type? Is xylem here exarch? How can you tell? Study the three xylem “wings” in detail, then arrive at some interpretation of the pith-like parenchyma tissue at the center of the stele. Since the procambium cylinder of roots is solid (eg., without any enclosed ground meristem), it would seem that there should be no central pith. (This parenchyma has been interpreted differently by different anatomists.)

PHOTO STUDY 11-6 Actaea (baneberry) Young root diarch stele (HP)

Recognize that this photo has a diarch root. How does the central parenchyma of a triarch root compare with the center of a diarch root. Can you explain the difference?

PHOTO STUDY 11-7 Smilax (carrion flower): t.s. root cortex

Distinguish between epidermis and hypodermis of the root. Note the great quantity of food material stored in the cortex.

PHOTO STUDY 11-8 Smilax (carrion flower): t.s. stele

Identify the endodermis, the thick pericycle, the groups of xylem and phloem and, at the center, a core of parenchyma tissue. Such a stele is said to be polyarch.

PHOTO STUDY 11-9 Smilax (carrion flower): t.s. stele

Is this a younger or older section than the one you just observed? How can you tell?

PHOTO STUDY 11-10 Smilax (carrion flower): t.s. stele dark field

This unique photo is shown in dark field illumination. Several things can be gleaned from such a photo. For example, some structures such as secondary wall and amyloplasts are anisotropic, ie., they depolarize plane polarized light. If a polascreen is placed on either side of the image on a microscope and rotated, when light passes through, (not polarized), the section appears normal. When the polascreen in the eyepiece is rotated, so that the light is polarized, only optically active structures depolarize the light and light passes through these structures (secondary walls and amyloplasts). Optically inactive structures (primary walls) remain dark in polarized light.

PHOTO STUDY 11-11 Smilax (carrion flower): root cortex and stele HP

What walls of the endodermis are thickened? Note the laminated structure of these walls. Casparian strips are present, but they are obscured. Are pericycle cells thick-walled, or thin-walled? Can you see canal-like pits in pericycle cell walls? Where is the protoxylem? The innermost xylem differentiates earlier than protoxylem so has a chance to grow larger. Since these vessels were the last to mature, they are true metaxylem. In a phloem group, identify sieve tube elements, companion cells, and phloem parenchyma. The center of the root consists, again, of parenchyma tissue. Although these cells are thick-walled. they still possess living protoplasts. This central tissue is commonly called pith, though it should be understood that it is of procambial origin.