Flower Morphology of Plants

You are already familiar with a number of aspects of the vegetative structure of angiosperms from your previous laboratory work. Now you will concentrate on flower structure, since these are the parts of the plant that are most closely associated with sexual reproduction, and are widely used in the classification of the angiosperms. Over 250,000 species of angiosperms are recognized, and they include the principal plants covering the land surface and provide the great majority of the plants used for a variety of economic purposes.

I. General Flower Structure

Although in gross appearance, the flowers of the angiosperms show a tremendous diversity, the parts which make up the flower are basically the same throughout. The most widely accepted interpretation of the nature of the flower is that it is a specialized branch; a stem with leaves. The following parts are recognized:

The RECEPTACLE is the stem portion. It is found at the base in the center of the flower. Its internodes are short and the number of leaves is usually small, so the receptacle is not usually a large or obvious part of the flower.

SEPALS make up the lower (or outermost) whorl of floral leaves. They are frequently, but not always, green and rather leaf-like in appearance, although they usually are smaller than the foliage leaves of the plant. Collectively, all sepals of a flower constitute the CALYX. The major function appears to be protection of the flower parts during early development.

PETALS are attached above (inside) the sepals on the receptacle. They are usually larger than the sepals, brightly colored, and rather leaf-like in that they are broad and rather thin organs. Collectively they constitute the COROLLA. They serve as additional protection and may also attract insects and/or facilitate the pollination of the flower when the insect reaches it. The two leaflike appendages, the sepals and petals (calyx and corolla), together make up the PERIANTH of the flower. They may be similar in appearance to one another, or one or the other or both may be missing from the flower.

STAMENS, which are found above (inside) the corolla, are rarely leaflike in appearance. Each usually is composed of two distinct parts: an elongated narrow FILAMENT rising from the receptacle and a terminal enlarged ANTHER which contains many POLLEN GRAINS. All stamens of the flower, taken together, make up the ANDROECIUM.

CARPELS, one or more, are located in the center (top) of the flower. Collectively they constitute the GYNOECIUM. Each carpel has three parts. Starting at the base these are: the OVARY, an enlarged hollow region bearing one or more OVULES; a narrower, short to long STYLE; and a terminal STIGMA of a variety of sizes and shapes. If more than one carpel occurs in a flower, all may be separate from one another, but more frequently they are joined to one another to form a compound structure. The term PISTIL is often used in conjunction with the gynoecium of a flower; this may or may not be synonymous with carpel. If carpels are independent (unattached to one another) in a flower, either a single carpel or several independent ones, each carpel can be called a pistil and the flower has one or more SIMPLE pistils. If, however, more than one carpel occurs in the flower and these are all joined to one another, the flower has a single COMPOUND pistil. The cavity in the ovary portion of carpel or pistil is called the LOCULE. There is one locule in a simple pistil and one or more in a compound pistil. The portion(s) of the ovary to which the ovule(s) is (are) attached is (are) the PLACENTA(E). The number of ovules varies from one to several hundred per locule, depending upon the species.

Carpels are not the only flower parts that may be joined to one another. Members of each whorl in the flower may be fused to other members of the same whorl. There may also be fusion between whorls; for example, the stamens of many flowers appear to diverge from the surface of the corolla. In an extreme case, the perianth and stamens are fused to the ovary wall and become free from it only at the top of the ovary. In flowers where the floral whorls diverge below the gynoecium, the ovary is said to be SUPERIOR and the flower HYPOGYNOUS. In contrast, when the free floral whorls arise from the top of the ovary, the ovary is called INFERIOR and the flower EPIGYNOUS. An intermediate condition can be found in which the several floral whorls are free from the ovary, diverging below it, so that the ovary is basically superior. All the external whorls (sepals, petals, and stamens) are fused to one another, however, forming a FLORAL CUP which surrounds the gynoecium. The organs separate at the top of the cup. Such a flower is called PERIGYNOUS.

Flowers are PERFECT, if both stamens and carpels are present, IMPERFECT if one or the other is lacking. In the latter case, both staminate and carpellate flowers may occur on the same plant (MONOECIOUS) or may be on separate plants (DIOECIOUS). Flowers may be regular, exhibiting RADIAL symmetry, or be irregular and show BILATERAL symmetry. Note, that in a longitudinal section such as the one below, it is not possible to be sure of the symmetry.

Use as many of the terms in bold type in the description above as possible to label the longitudinal section below (there should be at least 24 labels!):

Dissecting Alstroemeria or Gladiolus flowers

Examine one of the flowers provided; draw and label the flower below to show all parts and their relation to one another. Then answer the following questions about your flower:
What is the symmetry of your flower?  radial       bilateral  
Are both calyx and corolla present?  yes       no  
The sepals are:  free       fused  
The petals are:  free       fused  
How many sepals are there? _____ Petals? _____ Stamens? _____ Pistils?_____
Where are the stamens in relation to the sepals and petals? 
If one, the pistil is  simple       compound  
If compound, of how many carpels is it composed? 
How did you determine this? 
The ovary is:  superior       inferior  
The flower is:  hypogynous     perigynous     epigynous  
How many ovules are there?  one     less than 20     many  
Where is/are the placenta(e) to which they are attached?  axile     radial     parietal
Give the complete flower formula for your flower (8 pts): 

Diagram and label your flower in the space provided below. Use as many structure name labels as possible based upon the description in Part I above (16 labels required).

III. Sex and Development in the Flower

A. MICROSPOROGENESIS AND POLLEN GRAINS. Look carefully at the stamen of the flower.
How many anther sacs do you find? 
In most species there are four, one in each corner of the anther, but at maturity the two in each half fuse. Cells produced in the anther sacs divide by meiosis to produce four microspores each. These separate from one another and each divides once by mitosis to produce a POLLEN GRAIN composed of a TUBE OR VEGETATIVE CELL and a smaller GENERATIVE CELL. It is at this stage that the anther sac opens, most often longitudinally, and the pollen is shed.

Observe the demonstration slide showing a cross section of a Lilium anther containing mature pollen grains.

How can you distinguish the tube nucleus from the generative nucleus?
The tube nucleus is 
The generative nucleus is 

Examine fresh pollen from one of the flowers in the lab.

In the remaining space draw a diagram of the pollen grain (either fresh or from a prepared slide) and label completely.

B. MEGASPOROGENESIS AND THE EMBRYO SAC. The carpel bears OVULES which consist of a NUCELLUS surrounded by one or two protective INTEGUMENTS which meet at the tip of the nucellus leaving a narrow opening or MICROPYLE. One cell within the tip of the nucellus divides by meiosis to form a linear row of four megaspores. The three megaspores closest to the micropyle disintegrate and the fourth microspore undergoes a series of three mitotic divisions to form the eight nuclei of the EMBRYO SAC. These organize into seven cells: three cells at the micropylar end (two SYNERGIDS and one EGG), three antipodal cells at the opposite end, and the one CENTRAL CELL containing two POLAR NUCLEI (that fuse in some species to form a single diploid SECONDARY NUCLEUS).

Observe the demonstration slide(s) of the embryo sac in Lilium.

In the remaining space, draw a diagram of the embryo sac and label completely.

C. POLLINATION AND SPERM PRODUCTION. When the pollen grain is shed from the anther, it is carried (usually by insect, bird, or wind) to the stigma of a carpel. The pollen grain germinates there, sending out a tube which grows through the tissues of the stigma and style to reach the ovary.

Keeping in mind the very small size of the pollen grain, and its non-green color, what is the source of nutrition of the POLLEN TUBE?

The nuclei remain near the tip of the pollen tube as it passes through the style, and the generative nucleus undergoes mitosis to form two sperm nuclei. On reaching the ovary, the pollen tube enters an ovule via the micropyle, penetrating a synergid and there bursting at the tip. The two sperm nuclei are discharged.

Observe the demonstration slides showing pollen tubes.

D. FERTILIZATION. One of the two sperm nuclei moves to the egg cell and fuses with its nucleus to form the ZYGOTE. The second sperm nucleus fuses with the two polar nuclei (or the secondary nucleus) to form a PRIMARY ENDOSPERM NUCLEUS. This DOUBLE SYNGAMY is characteristic of all angiosperms.

If each nucleus in the fertilization has one set of chromosomes, how many sets would be found in the zygote? 
How many sets of chromosomes would be found in the primary endosperm nucleus? 

Observe the demonstration slide showing double fertilization in Lilium.

The primary endosperm cell, the zygote, and the rest of the ovule form the SEED, a subject for future study.

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