Lecture 22

Flower – Reproductive Cycle

I. Introduction

A. Angiosperms have a diplontic (or diplohaplontic) life cycle where the sporophytic generation is dominant (image). The gametophytic part of the generation has few cells and exists for a shorter period of time. Review: gymnosperms also have a diplontic life cycle. Bryophytes, lycophytes and ferns have alternation of generations type of life cycle.

B. All seed plants (angiosperms and gymnosperms) are heterosporous, meaning their spores are differentiated into microspores (male) and megaspores (female). The male and female gametophyes are small (pollen is only 2 cells - image of Lilium) and are dependent upon the sporophyte for nutrition and support.

II. Microsporogenesis

A. Definition: the production of haploid microspores (male spores – NOT gametes!) via meiosis.

B. Terminology (be sure to know the hierarchical relationships of the structures referred to by these terms)

1. Microsporophyll = stamen (2N)
2. Microsporangium = pollen sacs (thecae)
3. Microsporocyte = microspore mother cell (2N)
4. Microspore = the uninucleate stage of pollen (1N)

C. Development of the microsporangium (Figure 21.1 of Gossypium)

1. Just below the protoderm of a young anther is a layer composed of the archesporial cells. These divide periclinally and produce the primary parietal layer and the primary sporogenous cells.
2. Those two layers themselves divide periclinally. The primary parietal layer makes two secondary parietal layers, and the outer layer then divides and produces three parietal layers. The outermost is the future endothecium, a middle layer, and the innermost tapetum.
3. The primary sporogenous cells either divide or function directly as microsporocytes. Image of Lilium microsporocytes.
4. Microsporocytes (= microspore mother cells, diploid) undergo two meiotic divisions, Meiosis I and Meiosis II, to produce haploid microspores.
5. Microsporocytes during meiosis prophase I lose intercellular connections, cells walls, gain callose-type polymer, become round in shape. Connected now via cytoplasmic bridges through the callose.
6. The above process converts the mass of microsporocytes into a coenocyte – a group of interconnected cells (like some algae!). This image possibly shows the coenocytic association. This facilitates movement of nutrients and growth substances and allows coordination of Meiosis I cell divisions. These connections are lost in Meiosis II.
7. Meiosis can lead to two different arrangements of microspores (Figure 21.2)

a. tetrads tetragonal (image of Lilium)
b. tetrads tetrahedral (image Nelumbo)

D. Wall layers of microsporangium

1. Tapetum. The external tapetum is derived from the primary parietal layer. Internal tapetum from metamorphosed cells of the connective tissue. Anther cross section from Lilium showing external tapetum.
2. Tapetal cells may be polyploid or multinucleate.
3. Tapetum functions in nourishing the differentiating pollen grains.
4. Two types of tapetum

a. Secretory (glandular). The most common type. During meiosis, an increase in dictyosome vessicles occurs as well as ER. After meiosis, tapetum cells break down, including the protoplast. The cellular remains are called tryphine which is deposited on pollen as a coating.
b. Amoeboid. Here the tapetum cells break down but the protoplast remains intact and may fuse with others forming a periplasmodium (Figure 21.3E).

5. Middle layer – usually one cell layer – gets crushed and degenerates between the endothecium and tapetum.
6. Endothecium. Common in anthers with dehiscence along slits, not pores. Secondary walls have thickenings (Figure 20.2). Involved in anther dehiscence. The endothecium matures late in anther development.

III. Pollen

A. Structural terminology (exine, intine, etc.) Figure 21.4. There is a great web site called the Glossary of of Pollen and Spore Terminology HERE.

B. Pollen has pores called apertures (Figure 21.6, Figure 21.7) through which the pollen tube grows upon germination. Pore number varies, but two common types are:

1. one pore = monocolpate, common in magnoliids and monocotes
2. three pores = tricolpate, common in eudicots. Example: Elaeagnus.

C. Pollen wall composed of sporopollenin – a polymer of carotenoids that is highly resistant to decay. This is why the fossil record contains so much well-preserved pollen.

D. Development. Figure 21.5. Begins when the tetrads are still in callose (Figure 21.3B). ER is flattened against a portion of the wall that marks where the aperture will be. Primexine is deposited elsewhere, replaced by probaculum which becomes the exine by the deposition (first) of protosporopollenin and later sporopollenin. The last stage is the formation of exine and intine.

IV. Male Gametophyte (Microgametophyte)

A. Gametogenesis. Definition: development of the gametophyte – so here – microgametogenesis. For the female, megagametogenesis (later).

B. Before pollen is dehisced, one mitotic division gives rise to a vegetative nucleus and a generative nucleus. Image of Lilium pollen.

C. A cell division gives rise to the generative nucleus (Figure 21.6) which has its own membrane, attached to the pollen wall (intine). But it then buds off, rounds up, and is surrounded by the vegetative cell. So, this produces a cell within a cell. In some plants the generative wall disappears, in others it stays, e.g. Monotropa (Figure 21.7).

D. Generative cell undergoes mitosis producing two sperm nuclei. In Beta (beet, Figure 21.8) and Hordeum (barley) the sperm are cells enclosed in two unit membranes (one their own, one from the vegetative cell). They may contain plastids. Note: plastids are usually thought to be inherited maternally, i.e. via the egg cell. Plastids carried on the sperm would result in paternal or biparental inheritance.

E. Pollen tube. Normally pollen germinates on the stigma, but will also germinate in culture (Figure 21.9 of Scilla). It growns rapidly (2 mm/hr. in Gossypium). Its wall is composed of cellulose or a β (1,3)-polyglucan in Linum. Cytoplasm accumulates at the tip as do the vegetative and sperm nuclei (or generative nucleus).

V. Megasporogenesis

A. Ovule. Basic organization Figure 21.10. Composed of:

1. nucellus – vegetative cells making up the main body of the ovule, enclosing the sporogenous cells
2. integuments – Encloses the ovule, may be one (unitegmic) or two (bitegmic) [or none, as in some parasitic plants]
3. funiculus – the stalk that connects ovule to placenta
4. chalaza – a region where the above three tissues meet (adj. form, chalazal, e.g. chalazal end of ovule as opposed to the micropylar end).

B. Archesporial cell differentiates very early in the young ovule (image) into the megasporocyte (image). Integuments grow, envelop the nucellus, leaving an opening, the micropyle.

C. Ovules come in various shapes, and there are names for these (of course!). See Floral Morphology2 page.

D. Megaspores are produced from the megasporocyte via meiosis. Various types, we will look at the Polygonum type in Figure 21.11 for Solanum.

E. Meiotic divisions give four haploid megaspores, three degenerate, one (chalazal end) enlarges. Callose is deposited on this cell. It is this one cell that then continues on to megagametogenesis (diagram).

VI. Megagametogenesis (Producing the Female Gametophyte)

A. Mitotic divisions gives 2, 4 (image1, image2) and then 8 nuclei (Figure 21.11 E-H) in 7 cells. This is called the embryo sac or the megagametophyte Diagram. It is composed of:

1. egg (1) – the female gamete
2. synergids (2) – involved in attracting the pollen tube to the embryo sac (the “bait” !!)
3. antipodals (3) – chalazal end of embryo sac – don’t do much
4. polar nuclei (2) – haploid (1N) cells that may fuse and form a diploid (2N) secondary endosperm nucleus

B. Variations in embryo sac formation. Many types, but two main categories: monosporic and tetrasporic (Figure 21.13). Note that in the Polygonum type, a shown in the diagram in V. E., is the monosporic type. Lilium, which was also used to illustrate the angiosperm life cycle, is actually the Fritillaria type, which is tetrasporic.

C. Synergid complexities – Figure 21.12B. Produce a filiform apparatus (Figure 21.15 A,D).

D. Egg cell. Highly vacuolated (Figure 21.12 B, C). May have a wall or a partial wall.

VI. Fertilization

A. Pollen tubes grow through the transmission tissue of the style (image1, image2, through the placenta, eventually reaching the micropyle and entering the ovule. Figure 21.14.

B. Pollen tube enters a synergid (destroying it) through the filiform apparatus, discharging the sperm. Found its way here by a chemotropic attraction.

C. Sperm leaves the synergid in a coiled state (Figure 21.15).

D. One sperm fuses with the egg, the other with the secondary endosperm nucleus. These two fertilizations (double fertilization - image) produce a zygote and a primary endosperm nucleus, respectively.

E. The zygote undergoes changes. Its wall is completed, stores starch, increases the number of ribosomes, vacuole decreases in volume. The whole zygote cell may increase or decrease in size depending upon the species.

Last updated: 14-Oct-22 / dln