Result and Discussions
1.Refer to Fig 1 and 2.
2.Refer to Fig 3.
3.There are two reasons to explain why the gametophyte part of bryophytes may not often carry out photosynthesis substantially for the whole plant.
First, they lack the vascular tissues xylem and phloem which are use to conduct water and food. In addition, the cell walls of the bryophyte water-conducting cells are not lignified and they only possess thin root-liked structure rhizoids which are mainly for anchorage rather than for water absorption, as water is one of the major things for carrying out photosynthesis, lack of water supply may lead to this result.
Second, they are flat plants that most often grow in moist locations in temperate and tropical forests. They grow in a way lying on the ground and do not have any vascular tissues to support themselves to a better position for absorption of sunlight. Moreover, as they grow in such a low position, most of the sunlight is already shaded by the other plants such as the large canopy of trees, they do not have enough sunlight to let them carry out photosynthesis substantially for the whole plant.
4. Psilotum is unique among living vascular plants in that it lacks both roots and leaves. Therefore they have a system of rhizomes with many rhizoids. This system helps the bryophyte to anchorage tightly into the substratum, so that it can locate at a better position to obtain the maximum amount of sunlight for carrying out photosynthesis. Moreover, these branching root-liked structures greatly increase the surface area for water and nutrients absorption under the ground.
For Lycopodium, the horizontal stems produce at frequent intervals along their length, allowing the stem to grow indefinitely along the ground. Therefore, it possesses adventitious true roots in addition to rhizoids
For Equisetum, the are greatly reduced, in of small, segments fused into nodal sheaths. Therefore, the surface area for transpiration throuhgh the lamina is greatly reduced.Water loss can be prevented and thus it can survive even in dry condition.
For ferns, they possess fronds which are highly dissected, feathery and relatively big in size compared with others. Larger leaves can greatly increase the surface area for sunlight absorption for carrying out photosynthesis and also act as and umbrella-like covering to protect the sori. At maturity, the spores are catapulted from the sporangium by snapping action. Thus, the chance for reproduction is increased with the protection by leaves.
5. Refer to Appendix A, B, C and D
6. Refer to Fig 4.
7.The mechanisms for liverwort Marchantia, horsetail Equisetum and fern Dryopteris to disperse their spores are different.
For liverwort Marchanitia, the mature sporangium contains elongate cells called elaters having helically arranged moisture-absorbing wall thickenings. The walls of elaters are sensitive to slight changes in humidity. After the capsule which contains the spores dries out and opens into a number of petal-like segments, the elaters undergo and twisting action helping disperse the spores.
For horsetail Equisetum, when the spores are mature, the sporangia contract and split along their inner surface, releasing numerous spores. They also have elaters which are observed as the thickened bands arising from the outer layer of the spore wall. However, they will coil when moist and uncoil when dry which is quite distinct from the elaters in Marchantia which are elongated cells with helically arranged wall thickenings.
For the fern Dryopteris, a sorus is a cluster of on the edge or underside of a fertile . They are protected by an umbrella-like cover called the indusium. Sori occur on the generation, the sporangia within producing meio. The sporangia are stalked, and each contains a special layer of unevenly thick-walled cells called and annulus. As the sporongia mature, the indusium shrivels so that spore release is unimpeded. The sporangia then dry out, and the contraction of the annulus causes tearing in the middle of the capsule. The sudden explosion and snapping back of the annulus to its original position then result in a catapultlike discharge of the spores.
8.Table showing the comparison of the inter-relationship between the gametophytic and sporophytic generations in moss, Psilotum and fern:
Conclusion
After studying the representative species, we can have a clearer concept among their morphological characteristics, reproductive features, and different adaptations to their mode of life as well as understanding more about their life cycle.
Reference
http://en.wikipedia.org/wiki/Sori
Biology of plant, by W.H. Freeman and company Worth Publisher
Biology, by Peter H. Raven, George B. Johnson, Jonathan B. Losos and Susan R. Singer
