Aquatic animals with external fertilization
  Mammals and internal fertilization
  Cleavage to larva in aquatic species
  Cleavage to fetus in mammals
  Flower structure, microspores and megaspores
  Gametophytes and double fertilization
  Embryos of flowering plants
  Germination and growth of seedlings

ACTIVITY 5. Development and growth of plant embryos

After fertilization has occurred, the fertilized egg nucleus (zygote) develops into an embryo, the endosperm grows and stores food for future use by the embryo, and the coverings of the ovule become the seed coat. Thus the ovule develops into a seed after double fertilization occurs.

ovules Each ovule within the ovary of this flower contains a zygote which will develop into an embryo. Endosperm cells within the ovule will accumulate nutrients for use by the embryo as it grows. When nutrient storage is complete, a fibrous coat will form around each ovule. The ovules will then be seeds.

Meanwhile, the surrounding ovary develops into a fruit which aids in seed dispersal. View the following animation which illustrates the development and functions of fruit.

animation - fruit development

Embryos of flowering plants

Now let's take a closer look at the embryo within a seed. The following animation and images give an overview of embryo development within a dicot seed.

animation - development of a dicot embryo

bean seed
This longitudinal section of a seed diagrams a developing embryo of those flowering plants called dicots (for the two cotyledons in the seed). This mature bean seed has been split in half. Note the large cotyledon and the embryonic root and leaves.

Study the following diagram and microscopic images which show development of a typical dicot embryo (Capsella) from a few cells to a relatively mature stage.

basal After fertilization, the diploid zygote divides to form a basal cell and a terminal cell. The basal cell undergoes a few additional divisions to create a suspensor while the terminal cell divides repeatedly to form the embryo.

Microscopic slides - Embryonic development of Capsella

The other major group of flowering plants are called monocots because (as the name implies) they have only one cotyledon. Examine the seed and embryo of the monocot plant below.

The embryo of monocots develop similarly to dicots with one important difference. The embryo absorbs only a small amount of endosperm, so when the embryo is mature the cotyledon is small and most of the seed is filled with endosperm. This diagram is a wheat seed. The main nutrient within the endosperm is starch, but proteins called gluten are also present.

Now you are ready to examine a real wheat seed by virtual microscopy. Capture an image at a magnification that shows the entire embryo. Label the cotyledon, embryonic root, embryonic leaves and shoot, and the seed coat. Submit your labeled image to WebAssign for question 15.Magnify the endosperm region until you can see the individual cells. Note the presence of starch grains within the endosperm cells, then answer question 16.

slide - longitudinal, stained section of a wheat seed viewed by virtual microscopy

Wheat is an important food source for humans worldwide. Read this short web article on the distribution of nutrients within wheat seed. It is written by proponents of "whole grain" consumption. While wheat contains important nutrients, the gluten protein causes allergic reactions in 0.5-1.0% of the human population. This problem, called coeliac disease, causes intestinal distress and mal absorption of nutrients. Thus, affected people cannot eat food containing wheat products. If you are interested in this problem, read the article on wheat gluten listed on the news page of Development of Plants.

Starch is the major nutrient within the endosperm of most seeds. A simple test can determine whether starch is present. Iodine reacts with starch to form a blue/black color. Observe the iodine test as applied to a bean seed.

iodine starch
The seed coat has been removed from this bean seed and several drops of iodine have been placed on the cotyledon. When a thin slice of stained cotyledon is examined under the microscope, the starch grains are blue in color.

When you understand the development of seeds, fruit, and embryos, answer question 17.

Germination and growth of seedlings

You have studied seed germination and emergence of the seedling in the Development of Plants topic. Take a moment to review these processes and note of the difference in early growth of dicot vs. monocot plants.

Once the seedling has emerged, the roots and shoots must elongate. This type of growth can be accomplished by two mechanisms: cell division or increase in cell size. In a growing plant, both processes occur in a coordinated fashion. Observe the root tip below by virtual microscopy. Its tip is covered by large cells that form a protection cap over the growing tip as it pushes through the soil. Just above the root cap, the cells are actively dividing (look for mitotic figures). Above the mitotic zone, the cells cease division and grow, becoming long and thin. This is called the zone of elongation.

slide - root tip viewed by virtual microscopy

Whereas the root of a plant grows down, the shoot grows up. Growth is again accomplished by a combination of mitotic cell division and increase in cell size. The tip of the shoot is called the apical meristem. This is where cells are dividing. Behind this region, the cells stop dividing and become larger (growing cells). The apical meristem also gives rise to pairs of new leaves. Meanwhile, older leaves grow larger and cells within them begin to differentiate into the xylem and phloem of veins.

Observe the shoot tip below by virtual microscopy and locate the above regions. If you need help, review this slide show of primary meristems. Capture an image of the shoot tip at appropriate magnification to show its parts. Label the apical meristem, growing cells, and new leaves (the smallest leaves in the image), and submit your labeled image to WebAssign for question 18. Then examine an older leaf and locate a region where a vein is forming. When you can identify a developing xylem vessel within the leaf (using high magnification), answer question 19. You should recognize xylem vessels from the previous assignment on specialized plant cells.

slide - shoot tip viewed by virtual microscopy

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