Biology 360 Botany Lab

When a pollen grain fertilizes an ovule inside of the ovary, development of the embryo begins. Embryonic development, occurring within the growing seed, creates a new individual from a fertilized egg. The embryo goes through several increasingly complex stages as its mitotic divisions continue. These stages include the early proembryo, the globular stage, the heart stage (cotyledons begin to emerge), the torpedo stage (differentiation of the vascular tissues begin), and finally the mature embryo.

The endosperm of most developing seeds is at first non-cellular or liquid. In both corn and coconuts, a portion of the endosperm remains liquid after the outer part of the endosperm has become cellular. After the endosperm has become cellular, it begins to convert available nutrients into insoluble reserves such as starch, proteins, and vegetable oils, storing up these products for the future use of the embryo during seed germination.

The final important feature of embryo development within the seed is the cessation of growth when the embryo attains full size.This is commonly called embryo dormancy.This is a temporary, physiologically imposed dormancy.If it fails, the embryo continues to grow and the seed germinates within the fruit, the seed becoming useless as a means of reproduction.

B.Seed Viability

Once the embryo is mature, a seed is formed.A seed is a mature ovule.Seed viability is the ability of a seed to germinate.Viability is often variable; some seeds from a plant are not capable of germination.Instead of just trying to germinate a seed, we will test viability by applying tetrazolium to a split seed.Tetrazolium will turn red when reduced.It will be reduced by dehydrogenases that are involved in cellular respiration inside the dormant seed.If there is no respiration occurring, then the stained seed will not turn red.

The jars on the table at the front of lab contain a mixture of viable and inviable seeds.Select one of each for a tetrazolium test.Split the seed open with a razor blade (PLEASE be careful!) and add one or two drops of dye.Wait about 30 minutes and observe any color change.

4.Seed coat - a structure derived from the wall of the ovule to protect the inner parts.

See if you can identify these structures by looking at the seeds you dyed to test for viability.

There are two types of seedling development: epigean and hypogean.In epigenous development, the cotyledons emerge from the soil.In hypogenous development, the cotyledons remain in the soil. Look at the corn and pea seedlings and see if you can determine which kind of development these plants undergo.

Plants are composed of three tissue types: parenchyma, collenchyma, and sclerenchyma.These tissue types are located throughout the plant body and have a variety of functions.

Parenchyma cells are the biochemistry machines of the plant. They are alive at maturity and are specialized in any number of structural and biochemical ways. NOT including support functions, this cell type is the basis for all plant structure and function. Parenchyma cells have thin primary walls, and highly functional cytoplasm. The cells are alive at maturity and are responsible for a wide range of biochemical function.For example, other than xylem in vascular bundles, the leaf is composed of parenchyma cells. Some, as in the epidermis, are specialized for light penetration, regulating gas exchange, or have anti-herbivory functions. Other cells, as in the mesophyll, are specialized for photosynthesis or phloem loading.

Collenchyma cells are also alive at maturity and have only a primary wall. These cells mature from meristem derivatives. They pass briefly through a stage resembling parenchyma, but quickly differentiate into collenchyma, a phenomenon that is apparent from the very earliest stages. ER and Golgi apparatus assist in the accumulation of additional primary wall. This is laid down where three or more cells come into contact.Areas of wall where only two cells come in contact remain as thin as those of parenchyma cells. Their design and function is to build and maintain the special unevenly thick primary cell wall. The cells are also typically quite elongate. The role of this cell type is to support the plant in areas still growing in length. The primary wall lacks lignin that would make it brittle, so this cell type provides what could be called plastic support that can hold a young stem or petiole into the air, but they can be stretched as the cells around them elongate. Stretchable support (without elastic snap-back) is a good way to describe what collenchyma does. Parts of the strings in celery are collenchyma.

Sclerenchyma cells are hard and brittle. These cells develop an extensive secondary cell wall (laid down on the inside of the primary wall). This wall is invested with lignin, making it extremely hard. Lignin, plus suberin and/or cutin make the wall waterproof as well. Thus, these cells cannot survive for long as they cannot exchange materials well enough for active (or even maintaining) metabolism. They are typically dead at functional maturity...the cytoplasm is missing by the time the cell can begin to carry out its function. Some functions include support (the wood in a tree trunk, fibers in large herbs) and conduction (hollow cells lined end-to-end in xylem with cytoplasm and end walls missing).Sclerenchyma also includes the fibers used for making thread and fabric...particularly the fibers from flax that are spun and woven into linen.

Examine the various Helianthus and Aristolochia and Pyrus slides for these tissues.