Botany: Part II Growth & Development

Plant Growth Plants are able to grow throughout their lifetime due to the presence of undifferentiated tissues called meristems. Apical meristems add primary growth and cause the stem to increase in length. Apical meristems are located at the tips of stems and roots. Lateral meristems add secondary growth and cause the plant to increase in girth. Meristems in plants are analogous to stem cells in animals. This tissue is undifferentiated and the cells begin to divide when conditions permit. Explain to students that if you carve your initials in a tree a meter from the ground as a 15 year old teenager, then visit the tree again as a 45 year old adult you’ll find the initials are still only a meter from the ground as a result of apical growth.

Reproduction in Angiosperms Flowers – reproductive structures of angiosperm sporophytes, both male and female organs are present in some The anatomy of a flower is not on the AP exam, but rather considered “prior knowledge” thus we provide a brief review here. This idealized flower has male structures such as the stamen which is composed of both an anther and a filament. The anther is the site of pollen production. This flower also has a female reproductive structure, the carpel, which is composed of a stigma, style, and ovary.

Reproduction in Angiosperms Flowers – reproductive structures of angiosperm sporophytes, both male and female organs are present in some This simplified life cycle of an angiosperm is a great overview of the reproductive process. Fertilization occurs when the male gamete fuses with the female gamete to form the plant embryo (2n) that is found in the seed along with the endosperm. The next three slides will cover the concept of double fertilization which is unique to angiosperms. Though the details of specific reproductive cycles of plants and animals are no longer specifically covered on the AP exam, double fertilization is so crucial to the evolutionary success of angiosperms that it is included here. Another great example of the evolution of adaptations and resulting adaptive radiations made possible by them.

Double Fertilization in Angiosperms If a pollen grain germinates, a pollen tube grows down the style toward the ovary. Take note of the two sperm nuclei in the pollen tube.

Double Fertilization in Angiosperms The pollen tube discharges the two sperm into the female gametophyte (embryo sac) within an ovule.

Double Fertilization in Angiosperms One sperm fertilizes the egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo sac’s large central cell, forming a triploid cell (3n) that develops into the nutritive tissue called endosperm. These two events comprise what we call double fertilization that is unique to angiosperms.

Co-Evolution of Pollinators and Flowers Important! Make sure you take a little time to talk about co-evolution between pollinators and flowers. Butterflies see yellow thus the flowers they pollinate are yellow. Moths are out most frequently at night. They pollinate white flowers that are fragrant. Bees see yellow, blue and UV light. The flowers they pollinate are yellow or blue or have UV radiation. Humming birds need a lot of sugar and see red. They pollinate flowers that are red and usually have nectar reserves. Bats and opossums are nocturnal and pollinate flowers that are fragrant, attract insects and are white. The pollinators and flowers have co-evolved. For example, the California buckeye tree is pollinated by native California honey bees that are immune to the neurotoxin found in the pollen. Other honey bees are sensitive to the pollen. Some orchids resemble insects. Male insects will try to mate with the orchids and become vectors for pollen. Emphasize that this is a slow process. Suppose that a red flower has a mutation resulting in the production of nectar. This flower now has an advantage over red flowers that produce no nectar, thus is more likely to be pollinated by a passing humming bird resulting in those genes being passed on to offspring.

Structure of the Mature Seed A seed consists of a dormant plant embryo, surrounded by its food supply (cotyledons, endosperm, or both). The seed is surrounded by a hard, protective seed coat. Dormancy describes a period when the embryo within a seed stops growing and its metabolism nearly ceases. This is an adaptation for tough times (drought, freezing winters, etc.). Certain environmental conditions trigger the seed to break dormancy. The timing of seed germination and its reliance on both temperature and water are specifically tested on the AP exam (see Big Idea 2, section E) Teachers do not need to cover the differences between monocots and dicots.

Two Common Types of Germination Germination begins with imbibition, the uptake of water by the dry seed. Rupturing of the seed coat triggers metabolic changes that allow the plant embryo to resume growth. The timing of seed germination and its reliance on both temperature and water are specifically tested on the AP exam (see Big Idea 2, section E) Low water potential of the seed triggers imbibition. In common garden beans, straightening of a hook in the hypocotyl pulls the cotyledon from the soil. In maize and other grasses, the shoot grows straight up through the tube of the coleoptile. Breaking dormancy is a phenomenon attributed to enzymes and hormones, students need not know the names of the specific enzymes and hormones involved. The essential seed related anatomy terms are cotyledon, endosperm, and radicle. Do dry seeds respire? Yes. Well, unless they’ve suffocated in a plastic bag or become completely dehydrated!

Adaptations for Seed Dispersal Have students link each dispersal to its evolutionary importance. A seed consists of a dormant plant embryo, surrounded by its food supply (cotyledons, endosperm, or both). The seed is surrounded by a hard, protective seed coat.

Created by: Jackie Snow AP Biology Teacher and Instructional Facilitator, Belton ISD Belton, TX