All the documentaries I have watched, books I have read so far on the starting of life on earth only talk about animals. The very first life form on earth was bacteria. Then evolution took us this far. But what about the trees? They must also come from that single cellular bacteria. Since when they got separated from animals? What made them to be so much different than animals? Besides, apparently their evolution is very slow, isn't it? Why are they lagging behind in the race of living organism?
This question shows so much misunderstanding in evolutionary biology (no offense) that it is hard to fully answer. As @MattDMo suggested, you will benefit from having a look to an introductory course to evolutionary biology such as Understanding Evolution.
But what about the trees? They must also come from that single cellular bacteria
The Most Recent Common Ancestor (MRCA) of animals and plants was not a bacteria but single-celled eukaryote.
Since when they got separated from animals?
The MRCA of animals and plants lived about 1.6 billion years ago.
What made them to be so much different than animals?
Time has passed and with time different lineages diverse through a number of different processes. By increasing your understanding of evolutionary biology (Understanding Evolution again), you will be able to understand these processes much better.
Note that while with time (1.6 billion years) genetic differences have accumulated between animals and plants, there are still a large number of similitudes. Some of these similitudes are also shared with other organisms such as bacteria for example and some are not shared with bacteria. Here is a short, non-exhaustive list of similitudes:
- Both use DNA to store genetic information
- Both use Adenosine Triphosphate (ATP) for transferring energy within cells
- Both use lots of water.
- Both have a cell membrane
- Both undergo cellular respiration
- Both undergo mitosis
- Both undergo meiosis
- Both have their DNA contained in a nucleus
- Both have endoplasmic reticulum
- Both have DNA organized in chromosomes
- Both contain mitochondria
- Both have histones
Note by the way, that I focused exclusively on similitudes that are caused by shared ancestry and not by convergent evolution.
Besides, apparently their evolution is very slow, isn't it?
The terms slow and fast are obviously relative. To discuss really how fast is evolution, you would need to know what evolution is and you would need to know a little more about the processes that drive evolution.
In very short and largely simplified: Evolution is a change in allele frequency (at a given locus) trough time. Such change occurs every second in a big population. However, the evolution of a large phenotypic trait that was previously completely inexistant may take much more time depending on how much genetic variance, you have to start with, depending on the mutation rate, depending on the number of mutations required, depending on the selection coefficient for each of these steps and depending on a number of different things.
So the answer to "How much time does it takes for evolution to occur" varies between a fraction of a second and infinity depending on what you mean exactly.
Why are they lagging behind in the race of living organism?
I don't understand this question. I don't know who is "they", I don't know what you mean by "behind" and I don't know what you mean by "the race of living organism".