THE LIVING WORLD
Unit Seven. Plant Life
The entire series of events that occurs between fertilization and maturity is called development. During development, cells become progressively more specialized, or differentiated. Look at the lower series of panels of figure 34.6 and you can see that the first stage in the development of a plant is active cell division to form an organized mass of cells, the embryo shown in 6. In angiosperms, the differentiation of cell types within the embryo begins almost immediately after fertilization. By the fifth day, the principal tissue systems can be detected within the embryo mass, and within another day, the root and shoot apical meristems can be detected, as shown in 8. The developing embryo is first nourished by the endosperm, and then later in some plants by the seed leaves, thick leaflike food storage structures called cotyledons.
Early in the development of an angiosperm embryo, a profoundly significant event occurs: The embryo simply stops developing and becomes dormant as a result of drying. In many plants, embryo development is arrested at the point shown in 8, soon after apical meristems and the cotyledons are differentiated. The ovule of the plant has now matured into a seed, which includes the dormant embryo and a source of stored food, both surrounded by a protective and relatively impermeable seed coat that develops from the outermost coverings of the ovule.
Figure 34.6. Development in an angiosperm embryo.
After the zygote forms, the first cell division is asymmetric 3. After another division, the basal cell, the one nearest the opening through which the pollen tube entered, undergoes a series of divisions and forms a narrow column of cells called the suspensor 4. The other three cells continue to divide and form a mass of cells arranged in layers 5. By about the fifth day of cell division, the principal tissue systems of the developing plant can be detected within this mass 7.
Once the seed coat fully develops around the embryo, most of the embryo’s metabolic activities cease; a mature seed contains only about 10% water. Under these conditions, the seed and the young plant within it are very stable.
Germination, or the resumption of metabolic activities that leads to the growth of a mature plant, cannot take place until water and oxygen reach the embryo, a process that sometimes involves cracking the seed. Seeds of some plants have been known to remain viable for hundreds, and in some cases thousands, of years. The seed will germinate when conditions are favorable for the plant’s survival.
Key Learning Outcome 34.4. A seed contains a dormant embryo and substantial food reserves, encased within a tough drought-resistant coat.