PHOTO STUDY 12-1 Ranunculus (buttercup): root stele: t.s. secondary root

Transverse section of an older root (slides labeled “old, 5-arch”). Sections were cut at a level where cambium was making its appearance.

PHOTO STUDY 12-1 B Ranunculus (buttercup): root stele: t.s. secondary root

Against the inner surface of a phloem group, see that a few parenchyma cells have divided in a tangential plane. These divisions have resulted in a short, curved layer of cells that, in this view, appear as narrow rectangles. These narrow cells remain meristematic and constitute the cambium. How thick is it in its radial dimension? How extensive is it around the stele? What intervenes between cambium and metaxylem? Discover that a few cells, which were formerly parenchyma lying against the metaxylem, have now matured into xylem. They are, of course, primary xylem, but have been late in maturing.

PHOTO STUDY 12-1 Ranunculus (buttercup): root stele: t.s. secondary root

Transverse section of an older root (slides labeled “old, 5-arch”). Sections were cut at a level where cambium was making its appearance. Against the inner surface of a phloem group, see that a few parenchyma cells have divided in a tangential plane. These divisions have resulted in a short, curved layer of cells that, in this view, appear as narrow rectangles. These narrow cells remain meristematic and constitute the cambium. How thick is it in its radial dimension? How extensive is it around the stele? What intervenes between cambium and metaxylem? Note the one large sieve tube in each phloem group, together with a few other smaller ones. Discover that a few cells, which were formerly parenchyma lying against the metaxylem, have now matured into xylem. They are, of course, primary xylem, but have been late in maturing. What changes do you see in the endodermis, as compared with that of a younger root? Recognize the pass

PHOTO STUDY 12-2 Ranunculus (buttercup): root stele: t.s. older secondary root

Transverse section of a root slightly older than Photo study 12-1. Here see that each cambium strip is flanked on one side by phloem, and on the other by xylem, with no intervening parenchyma at all. What happened to the parenchyma that once intervened between phloem and xylem?

PHOTO STUDY 12-2 B Ranunculus (buttercup): root stele: t.s. older secondary root

From an oil immersion magnification, one can see more clearly that xylem cells most recently added lie against the cambium, and moreover that they lie in direct line with cambium cells. Are these new cells primary, or secondary? They are so young that they may not yet have lignified their walls. How extensive is a cambium strip now? Does it reach the pericycle?

PHOTO STUDY 12-3 Actaea (baneberry): root stele: t.s. young root LP

Observe an Actaea young root so you can follow cambial development. Identify epidermis, hypodermis, cortex, endodermis, pericycle, xylem, and phloem. No secondary growth has begun.

PHOTO STUDY 12-4 Actaea (baneberry): root stele: t.s. young root MP

Transverse section of an young root. First, identify endodermis and pericycle; then, within the pericycle cylinder, recognize the three vascular bundles. See that each bundle consists of phloem, cambium, and xylem in collateral arrangement. Most, and perhaps all, of the xylem in a bundle is secondary, having been added by the cambium. This is indicated by the radial alignment of the xylem cells, and by their agreement in position with cells of the cambium. Identify the protoxylem-metaxylem groups between the four bundles. How do they happen to be in such locations? Is the xylem still exarch?

PHOTO STUDY 12-5 Actaea (baneberry): root stele: t.s. intermediate root

In a root with only primary tissues, the protoxylem “points”, as well as the phloem groups, are out directly against the pericycle. But in this root, after secondary tissues have been added, the pericycle is far out from the three protoxylem “points”. How do you account for this difference? The parenchyma tissue that intervenes between the neighboring bundles, and therefore lies opposite a primary xylem group, is called a ray. What regularity do you see in the arrangement of its cells? Note the regular alignment of phloem cells in a bundle, and also their agreement in position with cells of the cambium. Such regularity indicates that the phloem is cambium-derived, and therefore secondary. See that the phloem that lies directly against the pericycle in any bundle, show unmistakable signs of breakdown. The crushed, and partly resorbed, cells here are all that remain of the primary phloem. Identify sieve tubes and companion cel

PHOTO STUDY 12-6 Actaea (baneberry): root stele: t.s. old root MP

Note the three collateral bundles (two obvious in photo), the primary xylem groups which alternate with them, and the central pith.

PHOTO STUDY 12-7 Actaea (baneberry): root stele: t.s. old root pericycle ray

Opposite the primary xylem groups, broad rays of parenchyma tissue extend out to the pericycle. Does vascular cambium extend across these rays? How do rays increase their radial dimension to keep pace with the expanding radium of the stele? A ray is considerably broader, out near the pericycle, than it is near the center of the stele. How did this increase in breadth occur?

PHOTO STUDY 12-8: Raphanus (radish): t.s. root, hypocotyls cortex crushed

PHOTO STUDY 12-9: Raphanus (radish): t.s. hypocotyls, cortex split

PHOTO STUDY 12-10: Raphanus (radish): c.s. root (hypocotyl), approaching maturity