PHOTO STUDY 10-1 Botrychium (grape fern): l.s. root tip and apical cell (median section)

Low Power photo: At a level in the section where structural zones are distinguishable, recognize the stellar region, cortical region, and root cap. Follow each of these, in turn, to its origin in the root tip, and see that all (including epidermis which isn’t shown in this view) have diverged out of just one large initiating cell. This is called the apical cell.

PHOTO STUDY 10-1 Botrychium (grape fern): l.s. root tip and apical cell (median section)

High Power photo: Note that the apical cell has the shape of a pyramid with a triangular base and three sloping sides. The base of the pyramid is directed toward the cap, the apex is directed upward. Cells newly cut off from the three sides are added to the stellar zone and the cortical zone. Outer cell layers in the latter zone then divide a number of times periclinally, giving rise to the epidermis and all of the root cap except its central core. These relationships are indicated by the regularity in cell rows that have developed on either side of the initial, and parallel with its cutting faces. Cells added successively to the central core of the root cap are those that were cut off from the base (front) of the initial cell.

PHOTO STUDY 10-1 Botrychium (grape fern): l.s. root tip and apical cell (median section)

Oil Immersion photo: Since there are no separate origins for the several structural zones of the root, there can be no recognizable boundaries between them at the growing point; and even above the growing point distinctions are not sharp, especially between cortex, epidermis, and root cap. It seems that the epidermis is merely what is left of the root cap after most of the latter has been worn away form the sides by soil abrasion. At the top of the cap the cells collapse. At a slightly higher level, even the epidermal cells are gone and the cortex is exposed. What serves the function of an epidermis is, from now on, the outer layer of the cortex.

PHOTO STUDY 10-2 Phoenix (palm): l.s. root tip

Identify the meristem tissue. This time it is a whole group of initiating cells. Note the forming stellar region, and the cortical region. Trace each of these, and the root cap as well, to the group of initiating cells. Although boundaries between root cap, epidermis, cortex, and stele are reasonable clear some distance away from the initiating cells, they become indistinguishable as you approach the group of intials. Note the directions in which initiating cells must divide in order to feed new cells into the several structural zones of the root. Note also that the root cap, when developed from this kind of growing point, is hardly a separate structure at all. Like that in Botrychium, it appears to be only an extremely thick epidermis that, at higher levels, is reduced in thickness by soil abrasion to a single layer of cells.

PHOTO STUDY 10-3 Canna (canna): l.s. root tip (median section)

Low Power Photo: Follow each structural zone of the root from a level where it can be recognized, down to the initiating cells of the meristem. This time, discover that the epidermis can be traced down to a single, definite layer of initiating cells, the dermatogen. Similarly, the whole cortex can be traced to a single cell layer that lies immediately above the dermatogen. This second layer is the periblem. Proceed upwards from the meristem this time, and see how the numerous cell rows fan out from the single layer of periblem. The stele can, in turn, be traced to a single layer of initials, the plerome, which lies above the periblem. Discover that cells of the root cap occur in regular rows that match, in position, the cells of the dermatogen. This indicates that the root cap has resulted from a proliferation of dermatogen cells.

PHOTO STUDY 10-3 Canna (canna): l.s. root tip (median section)

High Power photo: Recognize the 4 histogens more clearly in this higher power. In what direction (or directions) must cell divisions in the dermatogen occur, in order for that histogen to be maintained as single layer? Answer the same question for periblem and plerome. What do you conclude as to the directions of cell divisions derived from these histogens?

PHOTO STUDY 10-3 Canna (canna): l.s. root tip (median section)

Oil Immersion photo: Note the 4 histogens are much more clearly seen. Name them and their derivatives.

PHOTO STUDY 10-4 Canna (canna): t.s. Primary meristem region

Study the transverse section through a root tip at a level about even with the top of the root cap. Recognize protoderm, ground meristem and procambium areas: the first from dermatogen, the second from periblem, and the last from plerome. Why should cells of ground meristem be in such regular radial rows? Where are new cells being added to this tissue as it increases in thickness? Compare with what you saw in the longitudinal section. See how derivatives of the dividing ground meristem cells increase their tangential (horizontal) dimensions as they are pushed farther out from the stele, thus keeping pace with the increasing circumference of the root. Many of these cells finally divide anticlinally, thus increasing the number of cells in their respective, circumferential layers. In the stele, identify early stages in the differentiation of the first vessels of the metaxylem.

PHOTO STUDY 10-5 Allium (onion): l.s. Primary meristem region

Allium has a cluster of initials which, although are difficult to recognize the clarity of origin that Canna’s histogens show, the differentiation is obvious into root cap, protoderm, ground meristem, and procambium. Note that differentiation of a procambial strand that will become a vessel originates soon after divisions of the promeristem.

PHOTO STUDY 10-6 Zea (corn): l.s. root tip and primary meristem region.

See if you can interpret the root tip and its derivatives, indicating all observations you can make based on your studies of the root tip so far.

PHOTO STUDY 10-7 Species?: l.s. through differentiation zone of a root.

Note the various xylem elements in their procambial and early primary tissue stages. Identify the various wall thickenings in the xylem. Which side is the central parenchyma, and which side is the phloem? How can you tell?