Botany - B.Ye. Yakubenko 2017

Part Three. Kingdom Plantae
Chapter IX. Division Angiosperms, or Flowering Plants (Angiospermae, Magnoliophyta)
Laboratory Session Topic 14.38. Types of Inflorescences

General Remarks. Only a few plants have solitary flowers (such as the poppy, magnolia, and tulip); more frequently, flowers are grouped on a common axis, forming an inflorescence.

An inflorescence is a group of flowers arranged in a definite pattern on a peduncle. Based on their branching patterns, inflorescences are divided into two major groups: indeterminate (or monopodial) and determinate (or sympodial).

Monopodial inflorescences are characterized by an indeterminate growth of the main axis. They can be simple, when flowers are borne singly on the axis in the axils of bracts or on pedicels, or compound, when the first-order lateral axes bear simple inflorescences. Types of simple indeterminate inflorescences (Fig. 102): a raceme is an inflorescence with flowers on pedicels of nearly equal length distributed along both sides of the axis (bird cherry, lily of the valley); a spike is an inflorescence with sessile flowers along the axis (plantain, sedge, birch); a spadix is an inflorescence with a thickened, fleshy axis bearing sessile flowers (female inflorescence of maize); a corymb is an inflorescence whose main axis bears flowers on pedicels of varying lengths, bringing them to the same level (pear, apple). An umbel has a greatly shortened axis, with flowers having nearly equal pedicels arising from a single point (primrose, onion). A HEAD (capitulum) is an inflorescence with a shortened and slightly expanded axis bearing flowers on short pedicels (beech, clover). A capitulum (or head) is an inflorescence with a disc-like shortened and expanded axis bearing sessile flowers surrounded by an involucre of one or more rows of bracts (dandelion, sunflower).

Types of compound indeterminate inflorescences (Fig. 103): a compound spike is an inflorescence where simple spikelets are borne on the main axis (rye, wheat); a panicle is an inflorescence where simple racemes or corymbs are arranged along the main axis (rowan, lilac, oat); a compound umbel has lateral axes that terminate in simple umbels (carrot, dill).

Types of determinate inflorescences: a dichasium is an inflorescence where the main axis terminates in a flower, and further growth proceeds via opposite lateral axes that also terminate in flowers, and so on (chickweed, carnation); a cincinnus (or scorpioid cyme) is an inflorescence where the main axis ends in a flower, and growth continues through lateral axes produced alternately on opposite sides, each ending in a flower (avens, gladiolus); a bostryx is structured similarly to a cincinnus, but all lateral axes are directed to one side (lungwort, potato); a pleiochasium (false umbel) structurally resembles a dichasium, but produces several lateral axes rather than two opposite ones (spurge) (Fig. 104).

Class="center">

Figure 102. Types of simple indeterminate inflorescences:

a — raceme; b — simple spike; c — spadix; d — simple umbel; e — head; f — capitulum (basket); g — corymb

Figure 103. Types of compound indeterminate inflorescences:

a — compound spike; b — panicle; c — compound umbel

Thanks to inflorescences, small flowers are easily noticed by pollinating insects, while in anemophilous plants, the capture of wind-borne pollen is enhanced.

Figure 104. Types of determinate inflorescences:

a — cincinnus; b — dichasium; c — bostryx; d — pleiochasium

The evolution of various types of inflorescences should be viewed as an adaptive mechanism for cross-pollination.

Objects: inflorescences of common plants (bird cherry, plantain, maize, clover, pear, cherry, sunflower, spiraea, rye, oat, carrot, chickweed, bugloss, spurge, etc.).

Tasks:

1. Examine and study the set of herbarium specimens of monopodial simple inflorescences.

2. Examine and study the set of herbarium monopodial compound inflorescences, determine the inflorescence type, and name it.

3. Study the set of herbarium specimens with determinate inflorescences.

4. Draw all types of simple indeterminate inflorescences (raceme, spike, spadix, umbel, head, capitulum, corymb); compound indeterminate inflorescences (compound spike, panicle, compound umbel), and determinate inflorescences (dichasium, cincinnus, bostryx).

Equipment and Materials: living plant specimens, herbarium material.

Macroscopic studies of various types of inflorescences. Examine the Structural Features of various inflorescence types using herbarium material or living plants. Begin with simple indeterminate inflorescences. Pay attention to the main axis of the inflorescence and the length of the pedicels: in a raceme, flowers on the axis have pedicels of nearly equal length; in a spike, the flowers are sessile; in a spadix, they are also sessile, and the axis is thickened; in a capitulum, the axis is disc-like; in a head, the axis is club-shaped and the pedicels are short; in a corymb, the pedicels are of varying lengths, placing the flowers at the same level; in an umbel, the axis is shortened, and the pedicels arise from a single point.

Next, examine compound indeterminate inflorescences: compound spike, panicle, and compound umbel. These are easily identified if The Structure of the simple spike, umbel, and raceme is well understood. Finally, examine determinate inflorescences. These feature a completely different branching pattern of lateral axes—sympodial: in a dichasium, the lateral axes are opposite; in a pleiochasium, they are whorled; in a cincinnus and bostryx, they are spiral. Sketch schematic diagrams of all the described inflorescence types in your lab notebook and provide Examples of plants that possess them.

Review Questions

1. What is the term for an inflorescence in which flowers with pedicels of equal length are arranged along the main axis?

2. What is the term for an inflorescence in which sessile flowers are arranged along an elongated main axis?

3. Name the types of determinate inflorescences.

4. To which type do inflorescences belong that have indeterminate growth of the main axis? 5. What is the term for an inflorescence with a disk-shaped expanded axis and sessile flowers?