R2101 2.4 Structure of the Stem

Question 1

What’s a bud?

Answer 1

• A small lateral or terminal protuberance on the stem of a plant that may develop into a flower, leaf, or shoot.
• When the growing tip withers away or drops off, one side bud will develop into the pseudoterminal bud
• In most woody plants, the buds are usually protected by bud scales called scaly buds or winter buds. In deciduous trees and shrubs they help to protect the growing points during the winter. Protects from desiccation by reducing transpiration. Scales fall off when the buds start to grow.
• Often covered in fine hairs

Question 2

What are scars?

Answer 2

• Bud-scale scar – marks on the stem when an apical bud opens and the scales fall off in the spring.
• Leaf scar – the mark left by a leaf after it falls off, where the petiole attached to the stem. Typically found below a branch as branches come from axillary buds located above leaf scars.
• Bundle scar – a small mark on a leaf scar indicating a point where vascular bundle once connected with the stem.
• Number of groups of bud-scale scars on a twig can tell one how old the twig is.
• Scars and buds aid identification.

Question 3

What are lenticels?

Answer 3

• A small elliptical pore containing cells with large intercellular spaces in the periderm in bark and woody stems of dicots
• The means of gaseous exchange between the internal tissues and atmosphere through the bark
• The periderm replaces the epidermis as the outer cellular layer of stems and roots in plants exhibiting secondary growth
• Lenticels are analogous to the stomata of primary tissues and vary in size

Question 4

Give examples of stem adaptating to other functions.

Answer 4

Protection – epidermal outgrowths; Stem spines – Crataegus; prickles – Rosa

Climbing – e.g. Wisteria passiflora has thin, twining stems that reach upwards using other plants for support

Storage/perennation – e.g. corms (Crocus), stem tubers (Begonia)

Natural vegetative reproduction – e.g. stolons/runners (Fragraria); rhizomes (Festuca rubra), stem tubers (Solanum tuberosum)

Question 5

What are stem tubers?

Answer 5

• A swollen underground stem bearing minute scale leaves
• It allows a plant to survive from one season to the next and provides a means of propagation.
• Eyes which are the axillary buds
• Generally form near the surface of the soil
• Produce chlorophyll when exposed to sunlight

e.g. Solanum tuberosum, Begonia × tuberhybrida and associated cultivars

Question 6

What are corms?

Answer 6

• Short, swollen underground food-storing stem base.
• The swollen stem is surrounded by protective scale leaves.
• Grows vertically and the adventitious roots arise from the base.
• One or more buds in the axils of the scale leaves produce the new foliage leaves/flowers in the new season, using up the food stored in the stem.

• Different from a bulb as when you cut through it it is solid.

e.g. Crocus vernus

Question 7

What are rhizomes?

Answer 7

• Modified horizontal underground stems
• Grows horizontally just below soil surface, e.g. yellow loosestrife, Lysimachia punctata
• Adventitious roots arise from nodes. The buds are found in the axils of the scale leaves at the nodes and these send up shoots.
• Adapted for both food storage and vegetative propagation e.g. Iris germanica
• Can be thin and wiry as in couch grass (Elymus repens) or Festuca rubra (red fescue), or fleshy and swollen as in irises.

Question 8

What are stolens/runners?

Answer 8

• A slender stem that grows horizontally along the ground
• Where this touches the ground, it will develop adventitious roots and axillary buds at the nodes, and form a new plant
• e.g. Fragaria x ananassa; and Rubus fruticosus