From Cell to Organ-1   

The Invasion of the Land is really the
Invasion of the Atmosphere!!!

The Protoplasm of Individual Plant Cells is surrounded by a Cellulose Wall. While Cellulose is strong and prevents mechanical damage to the cell contents, it is extremely hydrophilic and readily absorbs water. However, Cellulose easily loses water via evaporation. Cellulose is like a sponge. If you drop a sponge in water, it saturates instantaneously. A wet sponge readily loses water when it is placed on a dry substrate. In order for an isolated plant cell, like a unicellular alga, to survive, it must be in constant contact with water.

In order to withstand periodic dry spells, plant cells needed a water protective coating.

One of the most important plant adaptations is the Cuticle.
It is a waxy material that is secreted to the outside of the plasma membrane. It fills in the spaces between cellulose fibrils and forms a continuous external waxy layer to the outside of the cell wall. This makes the cell watertight!

This cell can be called an "all purpose" cell because it

Regulates its Water Balance and performs Photosynthesis. 

The Cuticle keeps water inside but it also prevents water uptake. The Cuticle is usually thicker on the side of the cell facing the light. Consequently, water could enter the bottom of the cell where the cuticle is thin and where water is more abundant, and be retained within the cell by the thick cuticle on its upper side. This could lead to the formation of colonies. The first multicellular forms could be filaments. These might be followed by flat sheets.

The Chlorophyta (Green Algae) is algal group which probably gave rise to land plants. The genus Coleochaete is regarded as the closest relative for early land dwellers. It is a disk-like organism that is several cells thick in the center and one cell thick at its margins. It has marginal growth rather than apical growth This means that cells along the perimeter of he disk are mitotic and growth spreads around the edge of the organism.

Top view of Coleochaete
This series of images tries to illustrate the results of Marginal Growth. The cells along the perimeter (Red Areas) are meristematic and produce an organism that has a circular outline and radial symmetry.