Rules of thumb for understanding Nature
Number one, drop the word "know " when talking about plants.
In all living things growth and reproduction run parallel.
There are many instances of twists an turns and overlaps but for clarity think of two paths.
There are two sorts of cell - germ cells and soma cells, there must have been a lot of research done since I picked up that information, it clouds the issue, think of germ cells as motor cells and soma cells as the rest. The motor cells are the ones with the energy for reproduction, they are responsible for the growth of the plant in the growth channel. As they are replaced the old cells become "cast off motor cells".
Of the two systems the reproductive side has first refusal on any cast off motor cells, and in a time of dearth will use energy cells from any source whatever. In fact I think plants incorporate motor cells from any source all the time.
The DNA blueprint of the motor cells can be simplified to the latent energy contained within that structure. There is no energy to expand beyond the weaker of the two dna blueprints if they are different.
Here it helps to think of a knitting pattern and a shortage of wool, some things will be missing. Maybe even the other way round, you have all the wool you want but only the energy to knit, say something the size of a bee in the case of your orchids.
So all in all it`s been too many dead bee cells in their diet that created the similarity originally and has been passed on unchanged as they turned out to be survivors.
Has this been of any help with your question - It`s a very simplified rationalisation, there are too many experts to split hairs on this subject, feed their opinions into the appropriate paragraphs of the above bullet list and carry on regardless whether its wide of the mark or not.