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Figure 0.41. The
diploid zygote forms after the pollen tube has finished forming so that the male generative nucleus (sperm) can fuse with the female egg.
0.59 Without a megasporangium, an egg would not form; without a microsporangium, pollen would not form.
- The sporangium of plants protects the spores from drying out. Apical meristems ensure that a plant is able to grow in the two directions required to acquire water and nutrients:
up toward sunlight and down into the soil. The multicellular embryo is an important adaptation that improves survival of the developing plant in dry environments. The development of molecules
that gave plants structural strength allowed them to grow higher on land and obtain more sunlight. A waxy cuticle prevents water loss from aerial surfaces.
- The bryophytes are divided into three divisions: the liverworts or Marchantiophyta, the hornworts or Anthocerotophyta, and the mosses or true Bryophyta.
- It became possible to transport water and nutrients through the plant and not be limited by rates of diffusion. Vascularization allowed the development of leaves, which
increased efficiency of photosynthesis and provided more energy for plant growth.
- The four modern-day groups of gymnosperms are Coniferophyta, Cycadophyta, Gingkophyta, and Gnetophyta.
- The resemblance between cycads and palm trees is only superficial. Cycads are gymnosperms and do not bear flowers or fruit. Unlike palms, cycads produce cones; large, female
cones that produce naked seeds, and smaller male cones on separate plants.
- Angiosperms are successful because of flowers and fruit. These structures protect reproduction from variability in the environment.