PHOTO STUDY 13-1 Equisetum (horsetail): stem tip (apex) l.s. LP

In a median section, you will see that the tip of the stem is occupied by a single initiating cell: it is called the apical cell. In this section view it appears to be triangular, with the apex of the triangle directed downward, and the base upward. Actually, the cell is pyramid-shaped; the base, directed forward, is actually triangular, so the pyramid has three lateral sides that slope to the backward-directed apex. See that the apical cell possesses a very large nucleus. Note also that cells recently derived from the apical cell have been cut off from its two sloping sides (actually three). See that the walls between derived daughter cells are slanted parallel with the sides of the apical cell.

PHOTO STUDY 13-2 Equisetum (horsetail): stem tip (apex) l.s. HP

The first division to occur in a new segment is anticlinal, dividing the segment into an upper and a lower daughter. The former will contribute to a node of the stem, while the lower will contribute to an internode. The next division in the two-celled segment is periclinal, dividing the two cells into outer and inner daughters. The inner daughters contribute to the pith. While the outer ones contribute to everything outside the pith, including the stele.

PHOTO STUDY 13-3 Equisetum (horsetail): stem tip (apex) l.s.

Back from the apical cell, recognize older, and older leaves, and also the growing points of lateral buds.

PHOTO STUDY 13-4 Pinus nigra (pine): t.s. superficial initials stem apex

At the apex is a whole group of initiating cells, rather than a single apical cell. A short layer of initials occurs at the surface of the apex. Anticlinal divisions in this layer produce new cells on the flanks of the group, and these are contributed to the protoderm that will eventually mature into the epidermis.

PHOTO STUDY 13-5 Pinus nigra (pine): t.s. mother cell initials, stem apex

Periclinal divisions also occur in this layer, adding new cells toward the back. These new cells enlarge somewhat, but remain meristematic. They divide in all directions, contributing new cells to all of the stem except the epidermis. Cells of this meristematic group are called mother cells, and in a sense they are initiating cells too. Apical initials, plus their relatively unmodified derivatives, constitute the promeristem.

PHOTO STUDY 13-6 Pinus nigra (pine): l.s. shoot apex with leaf primordia

Identify a leaf primordium: it appears as dome-shaped swelling on the side of the apex. Down from the apex, recognize older leaves. The primary meristem, protoderm, is easy to identify, since it is the uniseriate, superficial layer of cells of both stem and leaves. The ground meristem at the center is also easy to identify, for it is the large-celled tissue that will become pith. Many of its cells are resinous and they have taken a deep characteristic stain.

PHOTO STUDY 13-7 Pinus nigra (pine) Procambial strands t.s. age 1 LP

PHOTO STUDY 13-8 Pinus nigra (pine): t.s. apex at the level of procambium LP

At approximately the 4th leaf below the apex, the third primary meristem, procambium, can also be recognized. Several procambial bundles are shown in this photo. Since they are to become vascular tissue, they occupy the position of stele and leaf traces. Between procambium and protoderm is tissue that will become ground meristem and eventually the cortex, but at present it is promeristem which is simply late in differentiation. Note the resemblance of cells here to those at the stem apex.

PHOTO STUDY 13-9: Pinus nigra (pine) Procambial strands t.s. age 2 LP

PHOTO STUDY 13-10: Pinus nigra (pine) Procambial strands t.s. age 2 MP

PHOTO STUDY 13-11: Pinus nigra (pine) Procambial strands t.s. age 3 HP

PHOTO STUDY 13-12: Pinus nigra (pine) Procambial strands l.s. age 1 HP

PHOTO STUDY 13-13: Pinus nigra (pine) Procambial strands l.s. age 2 HP

PHOTO STUDY 13-14:Pinus nigra (pine) Vascular bundle in l.s. age 3 HP

For this study, all of the procambium has matured except a few cells at the center, between xylem and phloem. Recognize the spiral elements of protoxylem and, external to them, numerous tracheids with bordered pits. This later-formed xylem matured after stem elongation had stopped; it is called metaxylem. See that metaphloem has been added to the original protophloem too, though distinction between the two kinds is less obvious than that between the two kinds of xylem. Procambium cells between xylem and phloem will give rise to the vascular cambium. Xylem is said to be endarch when all metaxylem is centrifugal to the protoxylem.

PHOTO STUDY 13-15: Coleus (coleus): l.s. stem tip (apex)

PHOTO STUDY 13-16: Coleus (coleus): l.s. stem tip series 1

PHOTO STUDY 13-17 Coleus (coleus): l.s. stem tip (apex)

At the apex of the stem, identify four obvious layers of meristem cells. In order for these cells to be consistently retained in layers, what would have to be the direction of cell divisions in them? These four layers of initials constitute what is called the tunica. Proceed back from the tunica. See that its surface layer contributes cells to the protoderm only, while the three deeper layers contribute to the part of the ground meristem that will mature into the outer portion of the cortex. Since divisions in these cells of the tunica are anticlinal, they produce a tissue that primarily, has a covering effect: it expands in length and width, but not in thickness, so it can cover an increasing area. At the apex, but immediately beneath the innermost layer of the tunica, is another group of initials that is less obviously layered. This group is called the corpus.

PHOTO STUDY 13-18: Coleus (coleus): l.s. stem tip bud age 1

PHOTO STUDY 13-19: Coleus (coleus): l.s. stem tip bud age 2