The Green Algae - Chlorophyta
Photosynthetic aquatic organisms that do not have vascular tissues are commonly called algae.
Review: In plants, conducting tissues and associated supportive fibres are called the vascular tissues. Xylem tissue transports water and dissolved minerals to the leaves, and phloem tissue conducts food from the leaves to all parts of the plant.

Not all algae are related to terrestrial plants.
Only green plants and the chlorophyte algae have chlorophyll a and b and store carbohydrates as a starch. Other algae (e.g., kelps, diatoms, etc.)have chlorophyll a and c (except red algae which only have chlorophyll a) and store carbohydrates as lipid.

Also
Green algae, like land plants have

1. Life cycles with a gametophyte and sporophyte generations:

1. The first group are the unicellular and colonial algae. Colonial forms are intermediate between unicellular and multicellular organisms (such as Volvox).

3. The third group, the charophytes, are multicellular with thick, branching filaments. They also have a cell plate (just as is seen in higher plants) during mitosis.

In the fossil record, charophytes grew in wide flat mats in shallow water or on mud flats. Fossil spores show that at least some of these plants were beginning to make adaptations of life out of water for they had resistant coats which would have enabled them to be dispersed by wind without drying out. This may have been similar to the ancestor to the land plants.

First there is the problem of desiccation. In an aquatic environment, dessication is not an issue. However, if you have been to the ocean you have probably seen some green algae or other seaweed left on the beach at high tide - it dies out very quickly. If a plant is going to live on dry land, it must be able to prevent this dessication. Cuticle is the answer. Cuticle is a waxy covering that can be found on essentially all exposed surfaces: leaves, stems, flowers, fruits but not roots. This waxy surface inhibits the loss of water. As stems grow, corky bark tissue replaces cuticle in function. Why do roots not have a cuticle coating?

Cuticle controls water loss. However, this causes another problem: gas exchange. All plants require CO2 for photosynthesis. In an aquatic environment with no cuticle, diffusion works fairly well for exchange of these gases (so long as the tissues are not too thick and dense). But land plants have now sealed off their outer surfaces with cuticle and this will block the exchange of gases. Therefore, there must be pores in the cuticle covering through which gas exchange can take place. These pores are called Stoma. These are not simple openings - if they were, then the plant would dry out. Instead the stoma have a pair of guard cells, which control whether the pore is open or closed.

 

When a stoma opens, the walls of the two guard cells that are closest to the pore opening move apart. This is caused by two aspects of the specialised anatomy of the guard cells:

1 .  The inner wall of the guard cell which surrounds the pore is thicker than the outer walls.
2 .  Cellulose microfibrils which make up the cell wall of the guard cell, radiate out around their circumference.

As water moves into the vacuoles of the guard cells, their content is increased and so is the pressure of  their cytoplasm against their cell walls. The cell walls begin to stretch. The arrangement of the cellulose microfibrils and the difference in thickness of the wall causes the outer wall to stretch more than the inner. The outer walls thus pulls the inner walls away from each other causing the pore to open.

In an aquatic environment, water provides a substantial amount of support. In some of the larger algae (like kelps), we see gas bladders for additional buoyancy.

But on land, plants need a different solution. We usually think of wood, or secondary xylem, as the means that plants evolved to achieve structural support, but this only applies to woody plants. Herbaceous (nonwoody) plants need a different solution. For example, think about what happens when you forget to water a plant you have in your dorm room or apartment. It flops over in a major wilt. Why does this happen? The plant is using turgor pressure to make the stems rigid. The plant "pumps" water up the stems into the parenchyma tissue to keep them turgid (rigid) and when conditions get dry, the water is lost and the plant wilts. (By the way - you should remember from introductory biology what happens to stomata when turgor pressure is lost).

How does the plant move water from the roots up into the stems to maintain this turgor pressure? If a plant is small (like an individual moss plant), capillary action is enough to pull water up the stem. However, if the plant is bigger it must have a vascular tissue (specifically xylem).

If we have a plant that is big enough to need xylem to transport water up from the ground to the leaves and stems, then we need need a transport system that is going to take food back down to the roots (remember, roots do not photosynthesize to produce their own food). Phloem is living vascular tissue that transports photosynthetic products to the roots and other tissues where it is needed or stored.

4. Protective packaging for gametes and embryos. Gametes and embryos need defense against dehydration and damage. This protection has been achieved through the evolution of various strucutres:-

(a) Multicellular gametangia - gamete producing structures that surround the reproductive cells and developing embryos with water trapping layers of cells.-

(b) Pollen grains - encapsulate male gametes in watertight packages that free these plants from the need to use water for transferring the sperm to the egg during fertilization.-

(c) Seeds - serve as protective, drought-resistant enclosures form plant embryos, enabling the offspring of seed-producing plants to be dispersed to new localities by water, wind, or animals.-

(d) Fruits - further clothe the seeds of flowering plants in additonal protective layers, enhancing embryo survival and dispersal.

Life Cycle - A second major trend in plant evolution, is a shift in the life cycle that predominately gametophytic to one that is sporophytic. This shift probably occurred because diploidy offers an important advantage over the haploid state for complex multicellular organisms: since haploid cells contain only one allele for each gene, the effects of a mutant allele cannot be masked by a dominate allele as it can in a diploid organism.

Plant Diversity
The adaptations for living on land did not appear all at once. Let's briefly review plant evolution and the appearance of these adaptations.

Bryophytes - The first land plants following the algae that lived on the edges of ponds and streams may have been bryophytes.
Bryophytes have stoma and a waxy cuticle on their body that helps protect them from dessication. A gametangia (layer of protective cells) surrounds the gametes and the embryo may be packaged in a waterproof spore that begins to grow when it encounters water.

But still restricted to moist habitats:

1. No vascular tissues
2. Sperm requires water to get to egg.

1. Conducting tissues are present, but not true xylem and phloem. So there is no skeletal system for supporting large body size on land.
2. Anchoring rhizoids but no true roots for absorbing moisture from soil.

1. Hornworts- with pointed "horn-shaped" sporophytes embedded in the gametophyte.