In this article we will discuss about the anatomy of selaginella.
Cut thin transverse sections of stem, root, rhizophore and root by inserting the material in pith, stain them separately in safranin-fast green combination, mount in glycerine and observe under microscope. Also compare your preparations from the prepared permanent slides of these parts.
T. S. Stem:
1. Anatomically, it is differentiated into an outer layer of epidermis, middle layers of cortex and centrally located stele (Fig.213B).
2. Outermost one-celled thick epidermis consists of cutinized cells with no stomata.
3. In most of the species, the cortex is differentiated into a few outer layers, of thick-walled sclerenchymatous hypodermis and many inner layers of thin-walled parenchymatous cells, while in very delicate species it is fully composed of thin-walled parenchymatous cells.
4. Cortex is completely sclerenchymatous in xerophytic species.
5. Generally, there is no intercellular space in the cortex.
6. Centrally located stele (or steles) is connected with the cortex with the help of many long, radially elongated cells called trabeculae.
7. In between these trabeculae there are present many air cavities or big intercellular spaces.
8. That these trabeculae are the endodermal structures is shown by the presence of many band like casparian strips in them.
9. Trabeculae are absent in species like S.adunca, S. rupestris, etc.
10. The number of stele is variable from 1 to 16 as under:
(i) S. spinulosa- one stele (monostelic).
(ii) S. kraussiana – two steles (distelic).
(iii) S. laevigata-up to sixteen steles (polystelic).
11. Structurally, a stele varies from a simple protostele to a polycylic siphonostele in different species. But typically it is protostelic, i.e., xylem is surrounded by phloem with no pith.
12. Each stele consists of pericycle, phloem and xylem.
13. Pericycle consists of many thin-walled parenchymatous cells.
14. Phloem consists of seive ceils and phloem parenchyma. There is no companion cell.
15. Xylem is exarch with one or two protoxylem groups. It consists of only tracheids.
1. Anatomically, the root (Fig. 214) consists of centrally located stele surrounded by cortex and epidermis.
2. Outermost layer is the epidermis made up of large cells.
3. From some epidermal cells arise root hairs.
4. Below the epidermis are few layers of parenchymatous cortex. But in old roots, cortex is the region of thick-walled sclerenchymatous cells.
5. Usually the endodermis is not well-developed.
6. Pericycle is one to three-layered.
7. Stele is monarch and exarch, and the structure of xylem and phloem is similar to that of the stem.
T.S. Rhizopore (Gr. rhiza-root; phora-bearer):
Anatomically, it is similar to the root except following differences:
1. A thick-walled epidermis is the outermost layer.
2. Root hairs are absent (Fig. 215).
3. Presence of a thick-walled hypodermis.
4. Presence of few thick-walled outer layers of cortex.
5. Stele is monarch and exarch like root.
1. It is bounded by an upper and a lower layer of epidermis (Fig. 216).
2. Cells of the epidermal layers contain chloroplasts.
3. On the lower epidermis are present many stomata.
4. Mesophyll is not differentiated into palisade and spongy parenchyma (Fig. 216).
5. In the centre is present a single vascular bundle surrounded by a bundle sheath, which encloses a layer of phloem and many centrally located xylem tracheids.
Study first the external features of strobilus under dissecting microscope, cut L.S., stain in safranin- fast-green combination, mount in glycerine and study. Also compare your preparations from the permanent slides of L.S. strobilus.
1. Spore-bearing organs are present on the apical part of the main axis as well as on the lateral branches. These are called sporangia (Fig. 217).
2. Sporangia are present in the axil of the leaf-like structures called sporophylls.
3. Each sporophyll has a ligule at its base and a stalked sporangium on the adaxial surface, and thus the sporangium is present in between the main axis and the ligule of the sporophyll.
4. Sporophylls are loosely and spirally arranged, usually in four rows on the axis.
5. All the sporophylls and sporangia form a four- angled loose cone called strobilus or sporangiferous spike.
6. Length of the strobilus ranges from 1/4 to 1 or 2 inches in different species.
7. Sometimes the apical portion of strobilus grows into a vegetative structure as in S. cuspidata. It does not bear any type of spore.
8. Two types of sporangia are present in the strobilus, i.e., macrosporangia and microsporangia.
9. Sporophyll containing macrosporangium is known as macrosporophyll.
10. Each macrosporangium contains only four macrospores.
11. Sporophyll containing microsporangium is known as microsporophyll.
12. Each microsporangium contains numerous microspores.
13. In majority of the cases, as in S. kraussiana, lowermost one or two sporophylls containmegasporangia and rest of the other upper ones contain microsporangia (Fig. 218A).
14. In some species all the sporophylls of one side of axis bear only microsporangia while that of the other side only macrosporangia, as in S. oregana, S. inaequalifolia (Fig. 218B), etc.
15. In S. gracilis, a strobilus bears either microsporangia or macrosporangia.
16. Both the types of sporangia are stalked structures surrounded by a sporangial wall, consisting of two outer jacket layers and an innermost layer of tapetum. Tapetum (Fig. 219) is very clear in young sporangia.
17. Outermost layer is thick and columnar, and its cells contain chloroplast. Inner layer of jacket consists of many, thin-walled, flat and elongated cells.
18. Each microsporangium is smooth, rounded or ovoid in shape and red or brown coloured body, and contains many microspores.
19. Each macrosporangium (Fig.220) is four-lobed, pale, greenish-white or orange coloured body, and contains only four macrospores.