How do plants develop?
Every organism begins life as a single cell. You started off as a fertilised egg floating down a fallopian tube and a plant begins life as a fertilised egg nestled inside an ovary, inside a carpel, inside a flower. Out of one though, comes many (to invert a famous US motto), because in both cases this single cell and its descendents divide millions of times to produce the final form of the organism.
Uncontrolled and without coordination, division like this would simply lead to a large group of similar-looking cells all behaving the same way. But just as we are not homogeneous clusters of cells, nor are plants: a tree is obviously more complex than a large green blob. Indeed, all multicellular organisms have a specific form - a definite shape with definite types of cells arranged in definite places. Organisms get from this initial one-celled state to the integrated end result by controlling their cell divisions and growth in a process of development.
Embryogenesis - a common base
As you can imagine, there are only so many effective ways to go from one spherical cell to many and in a complex shape, so organisms as diverse as animals and plants share some common ground in development. They both begin development from the original cell by cell divisions and growth to produce a basic form (fig 1). This period is known as embryogenesis, and in it the basic body plan is laid down. In plants this body plan is later added to in post embryotic development, whereas in animals this initial body plan is final - this is why when you see a baby animal you know what adult it will grow into. All of the limbs and major organs are already present; it's just a matter of growth to create the final adult form.
Figure 1: Plant (green) and animal (red) development compared. Both start off as a single cell and progress through embryogenesis until birth or germination (darker shade). After this they diverge. Animals simply grow to adult size where as plants both grow and develop new organs to produce their adult shape (lighter shade).
This is especially evident in animals such as ourselves which have only a single stage in our life cycles, but it is also true in creatures with more complex life cycles with larval forms such as frogs, butterflies and crabs. They undergo an initial period of embryogenesis to produce the larval form. This may then continue to develop gradually, just as a tadpole turns into a frog, sprouting legs and losing a tail to reach its fixed adult form.
Alternatively, development may happen in a series of fits and starts. Crustaceans, for instance, go through several larval stages, moulting between each, and insects go through a single mighty metamorphosis. Nevertheless, after each burst of development the form of the larva is fixed, making development almost a string of consecutive embryogeneses. For all animals, once the adult form has been reached no further development can happen. Adult form is final.
Plants and post-embryogenesis
If then, the hatching of an egg or emergence from a cocoon is the end of animal development, the germination of a seed is just the beginning for plants (figure 1). In plants the products of embryogenesis are seedlings; with two leaves, a short stem and a root they definitely are not simply miniature versions of the adult plant we recognise. Without developing more organs, a seedling, no matter how big, would still most definitely look like a seedling. For plants embryogenesis serves a different role; it sets up a basis for further growth.
This future (post embryonic) development continues throughout the life of the plant and occurs from populations of stem cells in the tips of the shoot and root which were originally placed there during embryogenesis. These are known as meristems. Leaves, shoots and flowers develop from shoot apical meristems (the groups of stem cells at the tip of stems) and roots are produced at root apical meristems at the tip of roots. By growing from meristems, a plant not only increases in size, but also, as it is rooted in one place, investigates its environment and increases its ability to harvest energy and nutrients (by producing more leaves and roots). This continuous development is vital to the way plants live. They cannot move to escape predators or shelter from inhospitable weather, so they must be able to alter their development and growth effectively to their changing environment and re-grow any parts eaten by predators.
Environmental interactions - keep it constant or embrace the change?
That plants can (and do) alter their development in response to their environment should not be surprising, however strange it seems to us animals so hung up on embryogenesis. Development is controlled by the interaction of the genes in an organism and the environment surrounding them. Genes produce proteins and these proteins are simply chemicals which will behave differently in different environments - for instance, molecules move faster in higher temperatures, and protein structures can change when different chemicals bind to them.
Indeed, the reason why embryogenesis is sheltered in a womb or an egg in animals, or an ovary in plants, is to maintain a constant environment to ensure a consistent result every time. This is vital for both animals and plants. If the end product of embryogenesis is wrong, the rest of the life cycle doesn't stand much chance of being right.
However, even in animals which are stuck with embryogenesis as their only means of development, environmental influences may still have important effects on developmental progress; the sex of crocodiles is determined by the surrounding temperature with males developing at around 31.6 °C and females at slightly higher or lower temperatures. The African Gaudy Commodore butterfly develops into grossly different summer and winter forms depending on the temperature of larval development. These important exceptions nonwithstanding, most organisms, even plants, generally try to keep the environmental conditions of embryogenesis constant to produce a reliable result. It is in their post embryonic development that plants excel in responding to environmental signals.
Indeed, plants are masters of the environment. They have, in their post embryonic development, embraced the ability of development to respond to the environment and live their lives by deciding what to grow and when to grow it. For instance, many aquatic plants have two types of leaves: a frond-like, divided kind for the underwater world, and one large and flat kind for the part of the plant that grows above the waterline. But in order to know when to grow each, they must sense the environment and alter development accordingly.
Even reproduction is governed by environmental decisions. A light-sensitive protein in plant leaves measures the length of the day (or more truthfully, the length of the night) and only when a certain threshold is reached will the plant alter its activity in order to produce flowers and reproduce. These are just two of many examples. Every part of the plant's life is affected by developmental decisions based on its environment. When to germinate, when to flower, how to most effectively thicken the trunk of a tree in response to a prevailing wind are all developmental decisions and involve reacting to an environmental cue.
Thinking like a plant
Plants and animals then, despite sharing a common base of embryogenesis where both endeavour to keep environmental conditions constant and ensure repeatable development, could hardly be more different. For animals this is the end of development, and the rest of their lives are a story of growing to full size and moving in response to stimuli. For plants however, embryogenesis is just the beginning and it is via post-embryonic development and growth in response to a varying environment that they perform the manifold tasks we take for granted.
Their unique development is what makes plants special. In this context it is interesting to think, for a moment, on brains. Our brain is our defining feature. It makes us human and able to think and learn. A brain is, however, not fully developed at birth. All the basic parts are there, but there are far less neurons and connections between them than in an adult brain. Learning, that most human of processes, is development. Every time we learn a new skill or fact new neurons grow and create new connections in response to these stimuli - our external environment.
So perhaps, in this most important of ways, we are more like plants than we realise...