PHOTO STUDY 14-1 Gleichenia (gleichenia): t.s. rhizome (Haplostele) LP

Exhibited here is a haplostele, which is a subtype of protostele. Identify the epidermis, a single layer of cells at the outside. Next inside the epidermis is a thick cortex in which you should distinguish three tissue regions: a thin outer zone of parenchyma tissue, then a thicker middle zone of sclerenchyma, and finally another zone of parenchyma, this one the thickest of the three.

PHOTO STUDY 14-2 Gleichenia (gleichenia): t.s. rhizome (Haplostele) LP

The inner limit of the cortex is marked by the endodermis, a cylindrical sheath one cell thick, whose radial and end walls are thickened by Casparian strips. Inside the endodermis layer is the pericycle, four or five cells in thickness. Cells of this layer are relatively small and thin-walled, and many are filled with deposits of waste products.

PHOTO STUDY 14-3 Gleichenia (gleichenia): t.s. rhizome (Haplostele) HP

Recognize the kinds of xylem cells: tracheids and parenchyma cells. The latter are abundant, and they intervene between groups of tracheids. Identify the several protoxylem areas. Is xylem exarch, mesarch, or endarch? The radial stele, characteristic of roots, is fundamentally a protostele. The procambium that matures into the stele is a solid cylinder with no ground meristem at its center.

PHOTO STUDY 14-4 Spiraea (spiraea): t.s. stem

Identify epidermis and cortex, and note the conspicuous endodermis that limits the cortex at the inner boundary. See how thin the cortex is, in contrast with the very thick cortex in stems that develop protosteles. See also the great diameter of the stele here, and the massive pith at its center. Recognize the phloem tissue, wholly external to the xylem, and enclosed by a pericycle six to eight cells in depth. Phloem is a complete cylinder, as you found it to be also in Adiantum. Within the cylinder of phloem is the cambium. The latter is not conspicuous to this level in the stem, though it has already produced some secondary tissues. Note the thinness of the xylem cylinder, in its radial dimension. Is the xylem exarch, mesarch, or endarch? Here also is a solenostele, but since phloem is restricted to a zone wholly external to the xylem, the solenostele is said to be ectophloic.

PHOT STUDY 14-5 Adiantum (maiden hair fern): t.s. rhizome LP

Recognize the xylem tissue and then, five or six cells outward from the xylem identify the endodermis. Next inside the endodermis is the pericycle. Between the latter and the xylem is the phloem, consisting of about two layers of large sieve tubes and smaller parenchyma cells. Xylem consists of both tracheids and parenchyma. Is the xylem exarch, mesarch, or endarch? Discover, within the xylem cylinder, a layer of internal phloem, then an internal pericycle and finally, against the pith, an internal endodermis. Study a section that was cut at the level of a leaf gap, and see if internal phloem, pericycle, and endodermis are continuous with corresponding external tissues.

A tubular stele, such as this, with a core of pith at the center, is a siphonostele. And a siphonostele whose xylem and phloem cylinders are continuous (without the interruptions of wide pith rays) is called a solenostele. Here, then, is a solenostele, and since the ph

PHOTO STUDY 14-6 Adiantum (maidenhair fern): ts rhizome leaf trace LP

Recognize the solenostele also becomes a leaf trace, and as the trace departs, it leaves a gap of thick walled parenchyma.

PHOTO STUDY 14-7 Adiantum (maidenhair fern): ts rhizome leaf trace HP

Recognize the xylem tissue and then, five or six cells outward from the xylem identify the endodermis. Next inside the endodermis is the pericycle. Between the latter and the xylem is the phloem, consisting of about two layers of large sieve tubes and smaller parenchyma cells. Xylem consists of both tracheids and parenchyma. Is the xylem exarch, mesarch, or endarch? Discover, within the xylem cylinder, a layer of internal phloem, then an internal pericycle and finally, against the pith, an internal endodermis. Study a section that was cut at the level of a leaf gap, and see if internal phloem, pericycle, and endodermis are continuous with corresponding external tissues.

PHOTO STUDY 14-8 Helianthus (sunflower): t.s. young stem

Here is an example of a dissected siphonostele: it is called an ectophloic eustele. Note the wide pith, thin cortex, and separate vascular bundles, the latter separated from one another by medullary rays. Identify the endodermis, which again is the innermost layer of the cortex. The thick “cap” of fibers at the outer limit of each bundle is primary phloem; not pericycle as one might expect. Here, then, pericycle is missing; vascular bundles border directly on the cortex (endodermis). In aerial stems of most of the higher seed plants, both endodermis and pericycle fail to differentiate. What is the position of protoxylem here?

PHOTO STUDY 14-9 Acorus : (sweetflag), t.s. stem

Acorus is a monocotyledon, and its stem develops the type of stele common to most plants of that group. Here, as also in Helianthus, the stele is “broken up” into discrete vascular bundles, separated from one another by parenchymatous ground tissue. But discover that, unlike Helianthus, the bundles are irregularly scattered: They are not distributed in a single circular series around a central pith. In fact, pith, pericycle, and cortex are not even distinguishable. Despite their irregular distribution vascular bundles are uniform in the orientation of their constituent tissues, ie, the xylem tissue in all of them is directed toward the stem center, while phloem is toward the outside. A stele of this sort has been called a meristele, though it is actually a modified eustele.