Today's adventure was a tour through the Gifford Arboretum, where we took a look at sporophytes from the following phyla:

Phylum Psilophyta (Whisk ferns; epiphytes on the palms in the arboretum)

Phylum Cycadophyta (The Sago palms, some of which are dioecious)

Phylum Coniferophyta (Our sole conifer in Dade is Pinus elliotii, the Slash Pine)

Phylum Anthophyta (All the flowering plants)

We took samples of flowers, leaves and stems into the lab and had a look at them under the microscope to identify their important structures. See what you missed if you didn't come to class?

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One way to help link the similarities (and note the differences) between the plant phyla is to start at the haploid spore (always produced in the sporangium of a diploid sporophyte via meiosis), and work our way around the life cycle. Let's begin with the Bryophytes:

The SPORE in mosses and liverworts germinates to become the haploid gametophyte. A spore may be male (and grow into a male gametophyte) or female (and grow into a female gametophyte).

The gametophytes produce either sperm or eggs. Sperm swim to the female's archegonium (within which lies the egg) and fertilize it. The ZYGOTE so produced becomes the sporophyte.

The sporophyte never leaves the mama gametophyte's loving "arms." It grows to its full size, produces a sporangium inside which spores are produced via meiosis. Voila! The sporangium releases the mature spores and the sporophyte dies, leaving the gametophyte to live another year.

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And now onto the Tracheophytes:

The SPORE in tracheophytes is produced in the sporophyte's sporangium via meiosis. It, too, grows into a gametophyte (male or female or bisexual) which produces gametes (sperm or eggs or both). These fuse to form the zygote, which grows into a new sporophyte! In the process, the growing sporophyte crushes the female gametophyte out of existence. In tracheophytes, it's the SPOROPHYTE that lives longer and is the dominant, obvious generation.

So how do the tracheophytes differ with respect to their spore development?

Seedless Tracheophytes (Psilophyta, Lycophyta, Sphenophyta and Pterophyta):

• Spores are produced in sporangia on the sporophylls.
• Spores are released into the environment where they germinate.
• The gametophyte that grows from the spore is free-living and independent of its sporophyte parent.

Gymnosperms (Cycadophyta, Ginkgophyta, Gnetophyta, Coniferophyta):

• Microspores (male) are produced in microsporangia on microsporophylls
• Microspores are never released into the environment!
• Each microspore grows into a male gametophyte (pollen grain) inside the microsporangium.
• When mature, each pollen contains two sperm. It is released to the wind to find a willing female!

• Megaspores (female) are produced in megasporangia on megasporophylls
• megaspores are never released into the environment!
• Each megaspore grows into a female gametophyte inside the megasporangium, also known as the OVULE.
• At maturity, the Ovule contains one or more mature female gametophytes, each with one or more archegonia containing an ovum.

• Pollen landing on the female cone will grow a pollen tube down to the ovules, and release their sperm. Sperm fuse with ova in the archegonia. ONLY ONE OVUM PER OVULE WILL DEVELOP: Only one embryo is contained in a normal seed. In cases of multiple fertilization, one embryo out-competes the others, which die.
• The female gametophyte's tissues contribute to the nutritive NUCELLUS, which serves as a food source for the growing embryo.
• The ovule, once it contains a zygote, is called a SEED!

Angiosperms (Anthophyta)

• The male cycle is exactly like that of the gymnosperms.

• Megaspores (female) are produced in megasporangia on megasporophylls (carpels), which are part of the flower.
• megaspores are never released into the environment!
• Each megaspore grows into a female gametophyte inside the megasporangium, also known as the OVULE.
• At maturity, the ovule contains one mature female gametophyte which consists of a mass of cytoplasm and EIGHT NUCLEI in specific positions.

• Pollen landing on the female's stigma will grow a pollen tube down to the ovules, and release their sperm.
• One sperm fuses with the ovum to form the diploid ZYGOTE. One sperm fuses with the two polar nuclei to make TRIPLOID ENDOSPERM, which then multiplies to form a 3n mass that serves the same nutritive function as the nucellus of the gymnosperm seed. This fertilization by two different sperm is called DOUBLE FERTILIZATION, and is unique to the flowering plants!
• The ovule, once it contains a zygote, is called a SEED!
• The seeds are connected to the insides of the megasporophylls (carpels), which form the walls of the fruit.

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