Lecture Notes for Friday, October 16; BTNY 1210, Fall 1998

No new handout today

Outline

PRIMARY PLANT GROWTH (continued)

Specialized plant organs (continued)

SECONDRY PLANT GROWTH

PRIMARY PLANT GROWTH (continued)

Specialized plant organs (continued). A number of plants produce horizontal stems that are used for asexual reproduction (to produce another genetically identical individual some distance from the parent plant. Stolons (often called runners) develop above ground; examples include strawberries (p. 180) and the spider plant, a popular house plant. Rhizomes are underground stems (see Fig. 9-12 on page 179). Notice the node-internode structure of rhizomes and stolons. I showed a bamboo rhizome with nodes containing adventitious roots and axillary buds. A white potato is a tuber (Fig. 26-44), a swollen rhizome (the "eyes" on the potato are nodes with axillary buds). Sweet potatoes and yams are modified root tissue. Bulbs such as onions and tulips contain mostly leaf tissue modified to store food to support the production of a new shoot the following year. Gladiolus "bulbs" are not bulbs, but corms, short upright, underground stems modified to store food. Some leaves produce leaves that catch insects - Venus fly trap (Fig. 29-26), pitcher plant and sundew. These plants live in nitrogen-poor soils and produce these modified leaves to obtain nitrogen from the disintegrating insect bodies.

SECONDARY GROWTH
Secondary growth is a product of two different lateral meristems, the vascular cambium and cork cambium. Tissues produced by these meristems are secondary tissues, but they contain the same types of cells as found in primary phloem and primary xylem. Secondary growth increases the diameter of a plant part (both stems and roots - leaves do not have secondary growth). Secondary growth only occurs in woody plants. Except for a few rare cases, secondary growth does not occur in monocots. Palm trees may be quite large, but they are the result of a very broad apical meristem, not the result of secondary growth. Recall that a palm tree stem is the same diameter all the way up, whereas a dicot tree stem is narrower at the top than at the bottom. Compare and contrast the above characteristics of secondary growth to those of primary growth.

The vascular cambium arises between the primary xylem and the primary phloem and produces the secondary xylem (toward the inside of the stem) and the secondary phloem (toward the outside of the stem). Examine Fig. 26-10 again and this time notice the layer of dividing cells between the primary xylem and primary phloem - this is a very early stage in vascular cambium formation and activity. I showed a diagram from another book illustrating what your book shows in Fig. 27-5 - as the vascular cambium produces xylem cells toward the inside of the stem, the vascular cambium is displaced toward the outside of the stem. Carefully examine and understand Fig. 27-6b-e which illustrates the same thing. Find the xylem and phloem in the vascular bundle and notice how they become separated by the production of secondary xylem and secondary phloem by the vascular cambium. Also notice that as the stem expands, the epidermis ruptures and is replaced by cork tissue (called periderm in the figure). Examine Fig. 27-10 which shows cork cambium (the other lateral meristem) which forms from ground tissue cells which start dividing again to produce cork tissue, a secondary tissue. Cork contains dead cells which thick, lignin-containing cell walls which form a waterproof protective layer on the outside of the stem. Notice the lenticel in Fig. 27-10d. Lenticels can be seen as small dots on stems; they are spongy areas in the cork the allow for gas exchange through the cork layer.

Secondary xylem (wood) functions in water transport and support. Secondary phloem functions in sugar transport. Cork functions in protection (replacing the epidermis which dies and is sloughed off the plant).

Wood and Bark Examine Fig. 26-12 closely. Notice that everything inside the vascular cambium is secondary xylem, otherwise known as wood. The dark wood in the center of the stem is called heartwood; those cells have been filled in with rot-resistant chemicals and no longer function in water transport. The lighter colored wood (sapwood) continues to function in water transport. All the tissue to the outside of the vascular cambium is the bark. What botanists call wood and bark are the same things normal people call wood and bark. Bark consists of secondary phloem, cork cambium and cork. Examine Fig. 27-13 carefully. The outer bark has a layered appearance. From common experience we know that bark often flakes off in pieces. The reason is that new cork cambium form deeper within the bark over time as the outer layers of the bark are split by the pressure of the ever expanding stem. Flaking occurs because of weak zones represented by the old cork cambia. The inner bark contains the most recently form secondary phloem, the active secondary phloem. Older secondary phloem has been pushed out and is part of the outer bark. Bark does not become very thick because it flakes and wears away at its outer surface. Annual rings in wood (see Fig. 27-21) occur because of seasonal activity of the vascular cambium. One annual ring starts with early (spring) wood which is softer, lighter in color and contains bigger cells compared to late (summer) wood which is harder, darker in color and contains smaller cells. The transition from early to late wood is somewhat gradual, but the transition from late wood of one year to early wood of the next year is very distinct.

In lecture I diagrammed a stem as shown in November 1998 with two layers of secondary phloem to the outside of the vascular cambium and two layers of secondary xylem to the inside. In both cases the most recently formed tissues (in 1999) are found closest to the tissue that produced them - i.e. the vascular cambium. The tissue inside the vascular cambium is wood (secondary xylem). The tissue to the outside of the vascular cambium is the bark.

READ and STUDY: External features of twigs (page 657), Commercial cork (pages 658-9), Climate recorded in width of annual rings (pages 664-5), Knots in wood (page 668).