Inside the seed, we can see the embryo. When it grows, part of it becomes the root (radicle) and the other becomes the shoot (epicotyl). But in order for it to grow, of course it needs food! That is where the cotyledons come in. Some plants have one cotyledon and are called monocots, while others have two and are called dicots! Take note, though, eudicots are not called that way just because of their cotyledons.

The first stage is the zygotic stage. Here, we have a single-celled zygote        which       divides asymmetrically  into  two:  a  small top  (apical)  cell   and   a  slightly elongated bottom (basal) cell.  The second stage is the globular stage. As   the   name   suggests,   it’s spherical  in  shape  that  is radially symmetrical.   It  also  consists  of eight   cells   and   already   has  a protoderm.

At  the  third  stage  (heart stage), cells  at  two  certain  points  on either  side  of  the  future  apical meristem divide rapidly to form two cotyledons. This gives it a heart shape   and   is   now   bilaterally symmetrical.    These    outward projections eventually come closer together to point at the top: at this point,  it enters the torpedo stage. Cell  elongation  will  have occurred at this point.

Finally, at the end of development, the  entire  seed  loses  water.  It enters a period of dormancy, which is almost like a hibernation, but for plants.  At  this  point,  the  seed is already  prepared  to  venture  out into the world to be carried by the wind or by many other animals who will bring it somewhere it can grow.

Monocots  are  a   little   different, though.  The  first  two  stages are the same,  but  at  the third stage, monocots   enter   the   coleoptile stage  which  has  root  and  shoot apical  meristems   as  well  as   a radicle.   The  fourth  stage  is  the juvenile  vegetative  stage  where a number   of   baby   leaves   are initiated. However, once it matures, it’ll be just like eudicots.

Still, this doesn’t answer the question: how do the seeds know which way to go? An experiment with mutant plants allowed scientists to learn that at least four genes are responsible for this! GURKE gene allows the plant to create acetyl-CoA carboxylase, this is important for the plant to create the top portion of the embryo.

The gene FACKEL is important for creating a C14 sterol reductase which is critical for the formation and patterning of the central part of the embryo. MONOPTEROS encodes an auxin response transcription factor and allows the embryo to form the basal part, while GNOM encodes a guanine nucleotide exchange factor to create the polar distribution of auxin efflux carriers.

Endogenous dormancy is caused by the embryo itself. It is usually chemical or genetic in nature, and it might be because the embryo is still too young (immature) or is undergoing after-ripening. Sometimes it may be because of too much abscisic acid or phenolic acid. We can break this seed dormancy by exposing the seed to acids or high heat.

Exogenous dormancies, on the other hand, are caused by factors other than the embryo. This includes impermeable seed coats that prevent water from entering, or extremely sturdy seed coats that do not allow the expanding embryo to break out. These can easily be germinated by mechanical scarification.

Germination can be divided into three stages. First, the dry seed (no longer in dormancy) absorbs water through the process of imbibition. At stage two, the water is already enough, so the intake slows down while the metabolic process starts. The embryo will expand and break the seed coat by protruding the radicle outside. At this point, germination has ended! Finally, water uptake will start again and the food inside the cotyledons are consumed by the growing seedling.

As we have mentioned earlier, seeds have stored food reserves in the cotyledons. These are used in the process of germination. At the subcellular level, starch is kept in amyloplasts. Two types of amylases are used to break down these food stores and consume them. Protein is also kept in storage inside vacuoles called phytins. Phytase is the enzyme used to break open this storage to release all the proteins inside. Lastly, lipids in the form of oil are also consumed.

Once the seed starts growing, it is important for the seed to know which way is up or down. Can you imagine a plant whose roots grew upwards and leaves grew underground? That plant will definitely die. For this, the plant first uses gravitropism.

At the end of the root cap, there are very small structures called statoliths. These are filled with starch that is, of course, pulled down by gravity. When this happens, the plant sends signals to its growing roots to recognize the gravity: after all, the statoliths have already been pulled down, so there’s no mistaking where down is.

Another important tropism is called phototropism. This is the phenomenon where the plants grow towards or away from light, depending on its needs. At the molecular level, this can be explained by the reaction of auxin to sunlight. It becomes dissociated and degraded in the presence of sunlight, and therefore growth there is limited. This is different at the other end of the plant where sunlight is scarce. There, auxin is plenty and growth continues rapidly and asymmetrically towards the sunlight.