From Cell to Organ-2  
201 Syllabus

It is inefficient for one cell to perform a multitude of functions. Consequently, cell specialization evolved. The most important basic functions for the survival of land plants are

acquiring water,
retaining water,
translocating water,
performing photosynthesis and
transporting photosynthate (sucrose dissolved in water).

All of these involve water!

The next level of complexity which arose  could have been a multilayered mat (Thallus) of cells. The cells on the bottom could have been specialized for water absorption. They might have had a thin Cuticle and could have hair-like projections called Rhizoids.

Rhizoids are like root hairs and they are specialized for the absorption of water. They also anchor the thallus to the substrate. This would constitute a Tissue called Epidermis.

 The next step could be the formation of a separate photosynthetic layer composed of Photosynthetic Tissue called Chlorenchyma (a.k.a. Photosynthetic Parenchyma).

The upper layer could loose its photosynthetic abilities and become more specialized for water retention. It would constitute another layer of Epidermal Tissue.

The Chlorenchyma is thus embedded within the two specialized Epidermal layers.

The next major advance in plant evolution was the Stomata! This is one of the most important plant adaptations.It allows plants to regulate gas exchange with the atmosphere. A solid Epidermis with a thick cuticle would hamper gas exchange because Oxygen and Carbon Dioxide can not readily pass through the Cuticle. These gasses are not readily soluble in water and would not be able to enter the lower Epidermis efficiently.

Photosynthetic cells need atmospheric Carbon Dioxide which is present in very low concentrations outside the leaf. Consequently, holes in the Epidermis would allow more Carbon Dioxide to enter the thallus. However, water vapor would leave  simultaneously at a high rate. This would desiccate and damage the internal tissues. However, physiological control over the opening and closing of the stomata allows the plant to balance these two conflicting processes (CO2 uptake and H2O loss).

The formation of stomata was accompanied by the formation of of air spaces in the Chlorenchyma. This kind of tissue is called Aerenchyma. These internal air spaces create gas reservoirs where Carbon Dioxide can accumulate so that photosynthesis can proceed when the stomata are closed.

Further specialization might have led to the formation of many layers and different cell shapes of Chlorenchyma to maximize photosynthetic capacity.

Our theoretical Organism has developed TISSUES. Tissues are groups of cells which are structurally/functionally distinct.