Mechanisms of multicellular development developed independently in plants and animals.
The last ancestor of plants and animals was a unicellular eucaryote. Gene comparisons show there is not much homology between the genes that make up the body plan of plants and animals. Although homeobox- as well as MADS box genes existed in the last common ancestor, the MADS box gene family plays an important part in regulation of plant development, but not in animal development, where homeobox genes are important.
Animal cells are motile.
Animal tissues may be folded and moved against each other easily. At metazoan gastrulation this way a triple layered system is built (entoderm, mesoderm and ectoderm). Some animal cells may even move to other sites autonomously.
Plant cells are positionally fixed.
Plant cells are trapped in rigid cell walls made of cellulose, which prevents movement of cells and tissues. Plants form three basic tissue systems as well (dermal, ground and vascular), yet without gastrulation.
Rigidity of the body shape
The animal body plan is in most parts clearly determined.
The basic body plan of an animal during its different life stages is mostly clearly determined by its genes. If the environment changes they may react e.g. by moving to another place or changing their short and long term behaviour.
Plant development is highly regulated by the environment.
As in most cases it may not choose or change its environment, it has to adapt to it. The body plan is variable and characterised by multiple times occuring, often iterative structures. Proportions and frequency of organs may vary.
During the animal life cycle there is just one continuously multicellular stage
It is what we refer to as "the animal". Yet many animals undergo one or more transformation, when their body plan changes dramatically.
The life cycle of land plants (and many other plants) has haploid and diploid stages.
This kind of life cycle is called alternation of generations and leads to two different body plans during the life cycle of the plant (sporophyte and gametophyte).
In animals gametes are formed directly through meiosis.
There is nothing that could be compared to the gametophyte in plants.
Plants undergo no gametic meiosis, but a sporic meiosis.
In plants the meiosis produces spores and not gametes. First the gametophyte is formed by mitotic divisions, which then forms the gametes.
Many animal species set aside reproductory stem cells early in development.
This decreases mutation accumulation.
No reproductory stem cells are set aside early in development in plants.
Still some plants leave certain meristems or meristem parts more inactive till the gametophyte is to be formed.
Animals develop to a distinct, complete body shape.
During their life stages still some reorganisation may take place, yet only in seldom cases new structures will develop. Some animals develop stepwise into different shapes.
Plants go through a longer period of morphogenesis.
During their development plants do not head for a distinct body plan. Many plants just grow and develop on and on till they die.
Areas of actively dividing, undifferentiated cells, called meristems, allow for iterative growth and the formation of more and more new organs and structures during a plants life. They resemble embryonal stem cells in animals, yet they continue existing during adult life stages.
Animal cells are determinated early in development
When animal cells develop into tissues they are clearly and in most cases irreversibly determinated. While most tissues are regenerated from stem cells, the regeneration of whole organs just occurs at some animal species like Ambystoma mexicanum.
Plants show an enormous plasticity in their development
If, for example, a shoot is nibbled by a herbivore, axillary meristems often grow out to substitute for the lost part. This strategy resembles (limb) regeneration in some animals. Whole plants can even be regenerated from single cells. Furthermore, the form of a plant (including branching, height and relative portions of vegetative and reproductive structures) is strongly affected by environmental factors such as light and temperature, resulting in a great variety of morphologies from the same genotype. This amazing level of plasticity helps plants compensate for their lack of mobility.