|
In Laboratory6 you were introduced to the vascular plant level of organization. It was noted that the sporophyte generation is the dominant and persistent phase of the life cycle and that this phase consists of a complex plant body composed of roots, stems and leaves. In today's laboratory we will examine the growth of these organs and the structure of roots and stems. Primary emphasis will be placed on the seed plants and, in particular, the angiosperms. You should realize, however, that many of the concepts developed here apply equally to the growth, structure and function of the seedless vascular plant sporophyte.
The growth of multicellular plant organs is the composite result of the growth of their individual cells. You have previously examined the process of mitosis and should now realize that these cells are formed as a result of nuclear and cytoplasmic divisions. These cells grow larger because of water absorption and an increase in the volume of protoplasm. Finally they become modified in various ways during the complex process of cell differentiation.
Plants exhibiting the thalloid and vascular plant levels of cellular organization have segregated the cells responsible for new cell formation into special areas called meristems. At the apex of the moss thallus and at the root and stem apex of the seedless vascular plants, the meristem consists of a single apical cell. On the other hand, the apical meristem of seed plants consists of groups of apical initials rather than single apical cells. The apical meristems produce all the primary tissues of the plant body. Also, in some of the seed plants, the girth of the plant body is increased as a result of cell divisions in lateral meristems, the vascular cambium and the cork cambium. These two meristems produce the secondary tissues of the plant body.
In Laboratory 7 we examined in some detail the reproductive features of the seed plants. It was noted that the mature seed consists of a seed coat, a source of nutritive tissue and an embryo (the new sporophyte plant).
Examine a previously soaked corn grain cut in median longitudinal section:
Identify the endosperm tissue and embryo of the grain consisting of a single cotyledon and two growing points, the root apex and the shoot apex. The tetrazolium test will indicate the parts of the embryo that are actively undergoing cellular respiration. The initially colourless tetrazolium is reduced by the active respiration enzymes of the embryo to a bright pink form.
