Lecture 18

Lecture 19

Stem - Secondary Growth and Structural Types

I. Secondary Growth

A. We have already covered a number of topics that are in the beginning parts of this chapter: Chapter 8 xylem, 9 wood, 10 vascular cambium, and 12 periderm.

B. Review of the fascicular cambium (one associated with the vascular bundle), interfascicular cambium (between the vascular bundles) as shown in Lotus (Figure 17.1A) and Medicago (Figure 17.1B).

C. Excission experiment in Ricinus (Figure 17.2). Do existing vascular bundles influence the interfascicular cambium? An interfascicular segment was inverted and grafted into a 7 day old plant with no secondary growth. 10 days later, secondary growth occurred and cambial continuity was established between the segments. But, the xylem and phloem developed the normal way, not inverted. So the answer is that no, the existing vascular bundles did not influence the directionality of xylem and phloem development.

D. Review of transition from primary to secondary growth in Tilia stems (Figure 17.3). The primary and secondary tissues are connected throughout the whole plant (continuity).

E. Interpolation of secondary tissue between the primary xylem and phloem (vascular cambium) induces many changes in the plant.

Protoxylem is crushed and becomes nonfunctional (Figure 17.5B, Figure 17.6).
Parenchyma may remain intact and expand (dilates).
Primary phloem is pushed outward, becomes nonfunctional.
Protophloem fibers, metaphloem, secondary phloem (including fibers) shown in Figure 17.5B.
Dilated rays in phloem, including fibers (Figure 17.3, 17.4B, 17.7).
Cells can divide within the ray.
Axial system banded with secondary phloem and tangential bands of fibers.
Periderm / rhytidome formation may cut off layers of secondary phloem.

F. Effects on leaf gaps / traces. The process is illustrated in Figure 17.8. May take several years for secondary growth to incrementally fill in the gap. If the leaf trace is angular, it gets separated from the portion that connects to the stem xylem leaving a stub on the leaf side that is carried outward by secondary xylem growth. If the trace was horizontally oriented, it would become embedded in the secondary xylem.

G. Wound healing and grafing

1. Vegetative parts that are wounded heal by forming a cicatrice (dead cells that seal off the wound) and a closing layer – review from Chapter 12. Periderm then forms under the cicatrice. If the wounded tissue is secondary, callus forms first (parenchyma).
2. Grafts are similar to wound healing (Figure 17.9). Scion = donor tissue, stock = host tissue. Contact layer has necrotic (dead) cells along the cut surfaces. Callus fills the space between scion and stock. Works best if the two cambia are placed in close proximity. New cambial tissue forms in the callus, eventually connecting the scion to the stock. Phloem rays and xyem rays form the callus.

II. Types of Stems

A. Conifers. Pine stem in primary state has discrete vascular bundles. Leaf gaps are indicated by points of parenchyma projecting into the early secondary xylem in an older stem. Secondary phloem is harder to recognize. It contains no fibers.

B. Woody dicots. Review Figures 17.3, 17.4B, 17.7.

C. Most of these herbaceous dicots we have seen before and they exhibit fairly typical secondary growth

Hibiscus cannabinus (Figure 17.5).
Pelargonium (Figure 17.10A)
Helianthus (Figure 17.10B)
Medicago (Figure 17.1B)
Coleus (Figure 17.11)
Solanaceae and Cucurbita (Figure 17.12B) with bicolateral vascular bundles

D. Dicot vines. Often have wide rays, making secondary tissue in XS appear dissected.

1. Example of Vitis (Vitaceae) described (but not illustrated). Secondary phloem forms discrete masses that later collapse and form a protective layer around the stem (images).
2. Aristolochia (Aristolochiaceae, Figure 17.13). Photos of A. gigantea.

vascular tissue occurs in wedgelike blocks embedded in ground tissue, all surrounded by perivascular fibers (image).
The phloem has no fibers and in young stems, has a starch sheath (innermost layer of cortex, next to perivascular fibers).
Rays and perivascular fiber ring is ruptured by further secondary growth (at rays). Parenchyma invades this area by intrusive growth (image1, image2)
Periderm later forms in the collenchyma below the epidermis (image). Periderm layed down in vertical strips, later connected up, becomes thick, lots of phellem produced. Image3, image4 of the anomalous growth just beginning to form from the interfascicular cambium.

E. Dicots – anomalous secondary growth. Anomalous means it deviates from the “normal” condition, e.g. in the distribution of xylem and phloem.

1. Leptadenia (Apocynaceae) Figure 17.14A. Secondary phloem is scattered, intermixed with the secondary xylem.

2. Beta vulgaris (Chenopodiaceae). Multiple series of concentric cambia (supernumerary) – see Lecture 16 Roots – Secondary Growth.

3. Bougainvillea (Nyctaginaceae). Does not have a normal cambium but forms multiple ones (like Beta). The primary vascular strands remain discrete. Cylinders of meristematic cells arise outside the bundles (Figure 17.15). Forms conjunctive tissue, i.e. xylem and phloem in parenchymatous areas outside the vascular strands. Forms bands of tissue in growth increments, with xylem forming from inner derivatives, phloem from outer ones. Image of a young stem hand XS. Boerhaavia (Nyctaginaceae) shown in Figure 17.14B has a similar anatomy.

4. Bignoniaceae anomalous secondary growth (Figure 17.16 diagram). In addition to a normal cambium that is bidirectional (xylem inside, phloem outside), another cambium forms that gives off derivatives only on the phloem side. The anomalous seconary phloem forms panels embedded with the secondary xylem. Panels increase in width with further growth. The xylem is repeatedly fissured by the intrusion of the new phloem. Image1 and image2 of Bignonia capreolata.

5. Gossypium (Malvaceae). Dilatation tissue. Primary phloem cells that are pushed out by secondary growth divide and enlarge, forming proliferative tissue (image).

6. Monocots.

a. Grasses - Poaceae (Figure 17.17). The vascular bundles occur in two rings: outer and inner. This is true for Avena, Hordeum, Secale, and Triticum). Vascular bundles are scattered in Bambusa, Saccharum, Sorghum, and Zea). For the former, outer bundles embedded in a mass of sclerenchyma (fibers) and collenchyma. For latter, the sclerenchyma may be at deeper layers.

b. Potamogeton (Potamogetonaceae) (Figure 17.18). A water plant that shows aerenchyma in the stem and a compact vascular cylinder with an endodermis (image). The leaves are distichous (alternate, but in one plane). Vascular cylinder shows eight vascular bundles, the largest bundles are each composites of three leaf traces that are free higher up on the stem (image). Xylem lacunae are formed when the tracheids are destroyed by internode expansion.

c. Large monocots with secondary growth. Not from a vascular cambium, but in some large monocots such as Agave, Cordyline, Dracaena, Sansevieria, and Yucca, diffuse secondary growth occurs. A cambium, continuous with the primary thickening meristem, functions after stem elongation is complete. Arises in parenchyma cells outside the primary vascular bundles. Produces secondary vascular bundles and parenchyma to the inside. For Cordyline Figure 17.19, image1, image2.

Last updated: 14-Oct-22 / dln