Botany - B.E. Yakubenko 2017

Part Three. The Plant Kingdom (Planthae)
Chapter VIII. Division Gymnosperms, Pine family — Pinophyta
Laboratory Class Topic 13.36. Class Conifers, or Pinopsida

Conifers belong to seed plants and represent the most evolutionarily advanced group among gymnosperms. Their defining feature is a high degree of xeromorphlessness: the wood consists exclusively of tracheids, and the needle-like leaves feature Stomata sunken within hypodermal pits, among other adaptations. When studying them and examining their developmental cycle, It is important to note that microsporophylls are aggregated into male cones, while megasporophylls form female cones or strobili. An exception is the yew family (Taxaceae), which do not form true cones. In pine families, pollination occurs without the involvement of a liquid-droplet medium. The male gametophyte germinates to form a pollen tube, through which sperm Cells are delivered to the archegonium. As a result of egg Cell Fertilization, a diploid embryo develops from the zygote. The primary endosperm forms prior to fertilization. Another characteristic feature is that both male and female gametophytes develop directly on the sporophyte rather than existing as free-living independent structures, unlike horsetails or ferns.

Objects of Study. Order — Pinales. Representative species — Scots pine (Pinus sylvestris L.)

Tasks:

1. Examine and draw The Structure of a three-year-old pine branch bearing male and female cones.

2. Examine and draw the structure of a male cone.

3. Independently prepare a pollen slide, examine, and draw the structure of

a pollen grain.

4. Examine and draw the structure of a female cone with ovules.

5. Examine and draw the structure of an ovule.

6. Examine and draw the structure of a seed.

Equipment and Materials: Microscope, magnifying Glass, dissecting needles, forceps, scalpels, herbarium specimens, fixed female and male cones, seeds.

Macroscopic studies of a three-year-old pine branch. A casual inspection of a three-year-old pine branch reveals the presence of Two Types of shoots — long and short (dwarf) shoots. Long shoots correspond to the current year's spring growth; they are succulent, with a well-developed cortex and covered in scales. In the axils of these scales, short shoots develop, which have limited growth, live for three years, and bear two needle-like leaves each.

At the Base of the previous year's shoots, clusters of yellowish male cones (microstrobili) are visible (Fig. 94). At the apex of the young SHOOT, reddish solitary first-year cones (macrostrobili) can be seen. On certain specimens, find larger (up to 20 mm) greenish female cones of the second year and large brown (up to 70 mm) cones of the third year. Draw the three-year-old branch and label all the aforementioned components.

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Figure 94. Three-year-old pine branch:

1 — first-year female cone; 2 — needle leaf; 3 — third-year female cone; 4 — short shoot; 5 — cluster of male cones;

6 — second-year female cone

1

Microscopic Study of the female cone. Examine the structure of a female cone using fixed or fresh material. For this purpose, a second-year cone is best suited. Using a dissecting needle and forceps, carefully separate individual seed scales on a glass slide and examine them under a magnifying glass. It will be observed that the STRUCTURE OF THE female cone differs from that of the male cone because it possesses two types of scales: bract scales and the much larger seed scales, each bearing two ovules (future seeds). The female cone can be studied in greater detail using a permanent slide of its longitudinal section. Placing the slide under low magnification reveals that the female cone consists of an axis and two types of scales: a bract scale, and in its axil, a seed scale bearing ovules. Switching the microscope to high magnification allows for a closer examination of the arrangement of the ovules. Draw a seed scale and clearly indicate the ovules (Fig. 95).

Microscopic studies of the male cone. Prepared permanent slides of longitudinal sections of a male cone clearly demonstrate its microscopic structure. Under low magnification, the cone is seen to consist of an axis to which microsporophylls bearing sporangia are attached. Yellowish microspores are clearly visible on the sporangia (Fig. 96). Draw the Cytology/practical/54.html">Longitudinal section of the male cone and use arrows to indicate the cone axis, microsporophylls, and microsporangia containing microspores.

Figure 95

Using fixed material, gently use forceps to detach a single cone from a cluster of male cones. Place it on a glass slide and examine it under a magnifying glass. Extract pollen using a needle, place it on a glass slide, cover with a coverslip, and examine first under low and then under high magnification of the microscope.

Figure 96

Within the field of view, it is evident that each pollen grain possesses two walls: an outer exine and an inner intine (Fig. 97). Two air sacs are visible on the sides of the pollen grain, which aid in wind dispersal. The Nucleus inside the pollen grain is quite distinct. Furthermore, by searching for pollen grains that are close to germination, one can distinguish two nuclei — a vegetative nucleus and a generative nucleus, with the latter located close to The Cell wall. The vegetative nucleus is involved in The formation of the pollen tube, while the generative nucleus participates in the formation of male Gametes, known as sperm cells.

Study of ovule structure. The detailed structure of the ovule can be examined using figures and tables. The pine ovule consists of an integument and a nucellus (Fig. 98). The nucellus is a homologue of the megasporangium; it has an egg-like shape and is partially fused with the integument. Only the apex facing the cone axis remains free, where the integument forms the micropyle. The young nucellus consists of diploid cells, one of which eventually transforms into a megasporocyte capable of meiotic division. It gives rise to four linear megaspores, three of which degenerate, while one develops into a multicellular female gametophyte—the primary endosperm with two archegonia at the apex. Draw a diagram of the ovule structure and indicate with arrows: the integument, micropyle, nucellus, female gametophyte, and archegonia.

Thus, having studied the structure of the sporogenous Organs of pine, we can conclude that the pine pollen grain is a highly reduced, underdeveloped male gametophyte developing inside the microspore, whereas the female gametophyte is the primary endosperm developing within the nucellus.

The pollen grain is carried by the wind to the ovule, where the final Development of the male gametophyte takes place. The following spring, the pollen grain that has landed on the ovule germinates. Its vegetative cell, together with the intine, forms a pollen tube; the Nucleus of the antheridial cell penetrates the tube and divides mitotically into a generative cell and a stalk cell. Two sperms are formed from the generative cell just prior to fertilization. The pollen tube grows through the micropyle into the nucellus and reaches the female gametophyte, where one of the sperms fertilizes the egg cell. An embryo subsequently develops from the zygote, and the ovule transforms into a seed. The mature seed combines a dual nature—that of the gametophyte (haploid primary endosperm) and the sporophyte (diploid embryo).

Fig. 98

Study of the pine seed. To prepare a slide of a pine seed, carefully make a longitudinal section of the seed using a sharp scalpel or razor blade. Place one of the halves or a thin section onto a glass slide and examine it under a magnifying glass or at low microscope magnification. On the section, you will see a thin brownish seed coat externally, surrounding the whitish, oily seed content (Fig. 99). In the middle of it lies the embryo with several cotyledons covering the plumule. With a careful examination of the seed, you will also observe the suspensor, which anchors the embryo.

Fig. 99

Draw the structure of the pine seed and label the seed coat, primary endosperm, and embryo.

Conclusion. Gymnosperms are the most ancient seed plants. They evolved along two evolutionary branches: microphyllous (conifers) and macrophyllous (seed ferns, cycads). Ovules are located on the seed scales of female cones. Microspores and pollen are formed in male cones. The ovule contains a single megasporangium (nucellus) surrounded by an integument. The Development of the female gametophyte, fertilization, and the Early stages of sporophyte development occur inside the ovule and seed, which represents a major evolutionary advance compared to external fertilization in Pteridophytes.

Self-Control Questions

1. What sporogenous organs does Scots pine possess, and how do they develop?

2. What are the constituent parts of a spore, and what develops from them?

3. Where does the female gametophyte of pine develop, and what is its pathway of formation?

4. What types of shoots have you identified in Scots pine?

5. Which gymnosperm species grows in the deserts of Africa?

6. Into which classes is the division Gymnospermae divided?

7. Which fossil gymnosperms possessed a bisexual cone?

8. Name the Representatives of the division that lack archegonia.

9. What are the constituent PARTS OF THE pine seed and embryo?

10. Why are gymnosperms classified as archegoniate plants?