Chapter 27, Problem 42A

Summary Introduction

To write:

About the steps involved in the pollen grain and egg production in anthophytes

Introduction:

Anthophytes are the most diverse and widespread group of plants. They are unique because they have flowers. Anthophytes have distinctive life cycles and, like all plants, exhibit an alteration of generations. Like conifers, the sporophyte generation of anthophytes is dominant and supports the gametophyte generation. However, there are many variations of the anthophytes reproductive process.

Explanation of Solution

Once pollination occurs, the pollen grain can form a pollen tube. Usually, the pollen tube grows down through the style to the ovary and the two nuclei travel in the pollen tube toward the ovule. When a compatible pollen grain lands on a stigma, the pollen grain absorbs substances from the stigma and a pollen tube starts to form. The length of a pollen tube depends on the length of the pistil, and can vary from a few centimeters or less to over 50 cm in some corn plants. As the pollen tube grows, the generative nucleus undergoes mitosis, forming two non flagellated sperm nuclei. The pollen grain is now a mature male gametophyte. When the pollen tube reaches the ovule, it grows through the micropyle and releases the two sperm nuclei. One sperm nucleus fuses with the egg, forming the zygote. The other sperm nucleus and the two polar nuclei in the center of the ovule fuse, forming a triploid or 3n cell. Because two fertilizations occur in an anthophytes egg, this is called double fertilization. After fertilization, the ovule and the ovary begin to develop into the seed and fruit.

Conclusion

Double fertilization takes place only in anthophytes because in anthophytes two fertilizations occur in an anthophytes egg.