1. Warmup 11/2/15 Describe how a root grows Objective Tonight’s Homework To learn the structure and function of plant stems pp 314: 1, 2, 3, 4

2. Notes on Stems This week we look at stems. Stems can be extremely varied, but they come in two basic modes of support. - Erect These are like tree trunks or stems on flowers. They stand upright. - Climbing These are far more flexible and wrap around things for support or to spread.

3. Notes on Stems Additionally, we can also group stems by how hard they are. - Herbaceous These are green and flexible and usually go with plants that die in fall. - Woody As the name suggests, these stems are far harder and are what we normally consider to be “wood”.

4. Notes on Stems Branching Patterns In a lot of plants, as the stem grows, it can branch. Plants follow 3 main branching patterns: Excurrent: In these plants, the meristem at the top of the stem has dominance over the ones on the side. This means that plant has a dominant straight and tall structure. We see this mostly in things like pine trees.

5. Notes on Stems Deliquescent: In these plants, the meristem at the top is dominant only when the plant is young. This gives it a solid trunk at a young age. As the plant grows older, the lateral buds grow more dominant, causing the tree to spread. Perfect examples of this are things like oak trees.

6. Notes on Stems Columnar: The last category includes plants that have a crown of leaves only at the top. In these plants, the lateral buds have absolutely no growth until the very final stage. This gives us plants like the familiar palm tree.

7. Notes on Stems Dormant Woody Stems If you look at the tip of a woody stem, you will see the apical meristem. As we’ve discussed before, this is the part of the plant that can divide and become anything.

8. Notes on Stems Dormant Woody Stems If you look at the tip of a woody stem, you will see the apical meristem. As we’ve discussed before, this is the part of the plant that can divide and become anything. Meristems that survive the winter form dormant buds. We call the one at the tip an apical bud.

9. Notes on Stems The bud has a number of bud scales that form in the fall to protect it.

10. Notes on Stems The bud has a number of bud scales that form in the fall to protect it. When spring comes, the scales open and fall off, so the meristem can continue growth until the next fall. This leaves a scar around the twig called a “bud scale scar”. You can count the age of a twig by the number of scars.

11. Notes on Stems Structure and Growth of Woody Stems The structure of woody stems is quite similar to that of roots. As such, we won’t go over the structure. Growth is another matter. In the first year, there is little secondary growth on a woody stem. Remember, secondary growth is growth outward, like rings in a tree.

12. Notes on Stems In the spring and summer as the plant expands, it builds new xylem and phloem. As it keeps expanding, we have a new term for the interior xlyem. Wood!

13. Notes on Stems In the spring and summer as the plant expands, it builds new xylem and phloem. As it keeps expanding, we have a new term for the interior xlyem. Wood! Looking at the image to the right, you can see the growth of each year clearly. Each year, the plant adds new xylem and phloem, pushing outward.

14. Notes on Stems The Growth of Bark On the outer edge of the woody stem, the vascular cambrium produces phloem. Secondary phloem is arranged as a ring of pyramids outside this. In between these cells are sclerenchyma to strengthen the whole thing. All these tissues together form the inner bark. This is necessary to keep the plant alive. Without it, there would be no phloem.

15. Notes on Stems Outer bark – what we normally call bark – is made of dead cork cells filled with a fatty substance called suberin. The inner edge of the outer bark is constantly forming new outer bark. This pushes the outer bark farther out. As distance increases, it becomes unstable since the bark isn’t alive. This causes the bark to split and gives it rough texture.

16. Notes on Stems There’s just one problem here. Plants need oxygen and CO 2. As such, there are tiny openings in the bark called lenticels to allow the transport of gas.

17. Notes on Stems

18. Herbaceous Dicot Stems Lastly, we take a look at stems that aren’t woody, such as flowers. These stems do not produce cork and thus keep their outer epidermis. However, this also limits their secondary growth, so they rarely grow thicker. It also means these plants don’t have xylem and phloem in rings. Instead, they have them in clusters called fibrovascular bundles.

19. Notes on Stems

20. Exit Question Which of the following trees below is excurrent? a) A pine tree b) A potato c) A bush d) An oak tree e) A palm tree f) None of the above