Stems~ Section 13.5 ~

Stem Function

1. To support the plant

2. To provide a transport link between the roots and the leaves

3. To store carbohydrates and water

Types of Stems1. Herbaceous Stem:

Thick, soft, green, short lived, containing little or not wood

Carries out some photosynthesis thus produces some carbohydrates

usually less than 1 metre tall

2. Woody Stem:

Perennial plants

Increases in diameter each year more and more vascular tissue is made

Xylem tissue becomes wood after it has died in the first year of growth

1. Monocot: vascular bundles are distributed randomly throughout the stem

2. Dicots: vascular bundles are in the stem are arranged in a ring, usually around the outside.

Patterns of Vascular Tissue in Stem

Plants: Form and Function 519

13.5

(a) monocotVascular bundlesare distributed randomlythroughout the stem.

(b) dicotVascular bundles inthe stem are arrangedin a ring.

(i) (ii) (iii)

Figure 1The very young stem structure of (a) a monocot and (b) a dicot(i) Drawings to show the distribution of vascular bundles in longitudinal stem sections(ii) Photomicrographs of young stem cross sections(iii) Highly magnified photomicrographs of vascular bundles as seen in cross sections

vascular bundles: collections of xylemand phloem tissue, separate from other col-lections, running longitudinally through stems

epidermissclerenchyma

epidermiscollenchymaparenchyma

ground parenchyma

phloem

phloem

xylem

pith

stele(b)

(a)

cortex

pith ray

cambium

xylemvascularbundle

vascularbundle

Figure 2Diagrams of a small portion of the crosssections of very young stems of (a) Zea,a monocot, and (b) Ranunculus, a dicot

Plants: Form and Function 519

13.5

(a) monocotVascular bundlesare distributed randomlythroughout the stem.

(b) dicotVascular bundles inthe stem are arrangedin a ring.

(i) (ii) (iii)

Figure 1The very young stem structure of (a) a monocot and (b) a dicot(i) Drawings to show the distribution of vascular bundles in longitudinal stem sections(ii) Photomicrographs of young stem cross sections(iii) Highly magnified photomicrographs of vascular bundles as seen in cross sections

vascular bundles: collections of xylemand phloem tissue, separate from other col-lections, running longitudinally through stems

epidermissclerenchyma

epidermiscollenchymaparenchyma

ground parenchyma

phloem

phloem

xylem

pith

stele(b)

(a)

cortex

pith ray

cambium

xylemvascularbundle

vascularbundle

Figure 2Diagrams of a small portion of the crosssections of very young stems of (a) Zea,a monocot, and (b) Ranunculus, a dicot

The First Year of Growth

In the first year of grow woody and herbaceous stems are similar. Growth occurs in the apical meristem.

Many dicots (mostly trees) grow year after year. Thus this secondary growth occurs in the vascular and cork cambia.

Cells in the vascular cambium divide by mitosis to produce secondary phloem to the outside and xylem to the inside

Secondary phloem tends to crush the previous years phloem, while secondary xylem is not due to its thick cells walls.

Producing Annual Rings

Secondary xylem thickens and forms wood tissue. The oldest xylem is in the centre while the youngest is found on the outside.

Each year secondary xylem acts to thicken the stem, add hardness and increase stem length.

Spring Xylem Cells: are usually larger and fast growing due to added available moisture

Summer-Fall Xylem: grows slower. An annual ring is the increase in the amount of secondary xylem from the first spring to the end of the summer-fall xylem.

The number of annual rings determines the age of the woody plant.

1. Cork: protects the stem, produced by the cork cambium, forms dead outside layers

2. Cork Cambium: lateral meristem, formed by the pericycle in dicots over 2 years old.

3. Cortex: parenchyma cells that store water and carbohydrates, have thick cells walls

Cross Section of a Woody Stem

4. Pericycle: lateral meristem surrounding the vascular cells.

5. Phloem: transports sugars in dissolved water.

6. Vascular Cambium: produces secondary xylem (to the inside) and secondary phloem (to the outside)

7. Xylem: Conducts water and minerals

8. Pith: parenchyma tissue in the centre used for storage.