Seed plants survive on land by virtue of having overcome two hurdles. The first are a set of vegetative adaptations that seed plants share with non seed vascular plants (that is ferns and friends)
C. Vegetative Adaptations- 1st major adaptations to life on land
a. Desiccation is a major problem, so the plant "internalizes" an aqueous environment by surrounding itself with a waxy cuticle derived from waste products.
b. But the cuticle is also impermeable to gas, and C02 and O2 are essential in photosynthesis. So, plants develop holes, called stoma (stomata, plural) in the cuticle.
2. Rhizoids and roots
a. In water, if plant is cut from a substrate, it can float along and generally land someplace that will be essentially the same it was before (except for the beach).
b. On land, if you blew with the winds or gravity you could end up someplace "bad" (teleology).
c. The initial anchor was provided rhizoids, or cellular extensions. Later evolution created roots, multicellular organs composed of several tissue types. (Footnote - a rhizome is NOT a rhizoid, it is an organ, in this case a horizontal stem faking a root.)
d. Both roots & rhizoids also serve to pick up moisture and nutrients from the soil.
3. # (1) and (2) involve the absolute bare necessities for living on land. But, eventually overcrowding and competition will become an issue, and to compete, a plant will need to grow higher in order to get lighter.
a. Getting higher requires two things: 1) support (against gravity) and 2) conduction- as you get higher; you need a mechanism for bringing water and nutrients up from the soil
4. Conducting cells (xylem and phloem)
a. These cell types form vascular tissue, which provides support and conducts fluids and nutrients throughout the plant body.
- Xylem conducts water and nutrients up from the soil.
- Phloem conducts photosynthetic products throughout the plant.
b. Wood is a further extension - masses of xylem can form a resistant trunk allowing great height.5. Waste Storage.
a. The cells of the Xylem are impregnated with a resistant substance called lignin - a plant waste product (yes, plants poop!). This solves some aspects of waste management. (E.g. the heart wood of cedars, which are rich in metabolic waste products.)
b. Lignin also has a couple of other characters - it is very resistant to compression, and thus lignin in vascular tissue forms the "hard parts" of plants.
- This is a reason why the fossil record of land plants is dominated by woody plant lineages
c. It also is hydrophobic & helps the transport of water as the inner faces of xylem conducting cells are coated in lignin.
BUT these adaptations do NOT address the ability of sexual reproduction to survive on land.
The weak link- sexual reproduction
A. Life cycles
1. In ocean:
a. In ancestral green algae, both the sperm and the egg were released into the water, and the sperm swam & accomplished fertilization, guided by specific chemicals released by the egg.
b. Upon fertilization, the zygote floated off to establish a new generation.
c. In some algae, the egg retained in the archegonium, but the sperm was still "free swimming" and attracted to the archegonium.d. The latter (the archegoniates) are the sister group to land plants.
2. On Land
a. Free-swimming sperm can swim to the retained egg only work in the presence of "free water", otherwise the sperm is likely to dry out.
b. However, this is precisely the reproductive strategy that the vegetative adaptations "dragged" on shore with them.
c. This is also the reproductive syndrome that characterizes Pteridophytes, and explains their restriction (for the main part) to wet areas.
3. So, how do we solve this problem?
a. One stop gap solution is vegetative reproduction. You will remember from the previous lecture that plants can generally reproduce themselves clonally, making new individuals that are of the same genetic makeup as the parent. Thus, one "uncommon" sexual reproductive event can create a population of vegetatively reproductive survivors. This is commonly observed in the Devonian - we see a locality dominated by one plant type and then ten feet away, a second area dominated by a second plant type. We interpret as two clones.
b. However, ultimately evolution needs to bite the bullet & deal with the real problem - free-swimming gametes (eek, more teleology). The result is the evolution of the "seed habit" an advance comparable to that of the coelom in metazoa in terms of its influence on the ability of plants to invade land. In order to understand the solution, it is necessary to first comprehend the often confusing alternation of generations.
c. BUT.. While the Alternation of generations is confusing, the basic principle of what are about to discuss is really quite simple - it is the plant version
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