A bone marrow transplant

has to "match" the recipient. 
I don't know much about these things but I do know that much. Bone marrow transplants were just starting to occur when I was in my teens. By the time I went to the UK to university the research into how they could be done and what they could do was expanding rapidly.  
A registry of potential donors was being set up. There are now 28 million people on that register - and it isn't enough. They could do with 280 million people - and maintaining that sort of register is unfortunately expensive, very expensive. People move, people become ill, they have accidents, they die. All those things may mean they are no longer suitable or available as donors. It is the same for all medical donations. And nobody wants the sort of "big brother is watching you" approach that would allow governments to keep all citizens in mind for potential donations.
I was thinking about this yesterday because I went to visit a friend in hospital. She is elderly, close to 90. She has heart disease, diabetes, bladder cancer, kidney failure, and other issues. Last year she had a stroke from which she made an extraordinary recovery. From being unable to use her right side she has gone back to knitting - and winning prizes for it - but she is still a very sick woman.  I admire her determination and persistence.
A bone marrow transplant or any other form of transplant will not help her - nor would she want it. Despite being cheerful and delighted to have another visitor she is realistic that she "won't see a hundred". Fair enough. 
I thought about this some more this morning because there is a small paragraph in the paper talking about the death of someone for whom no suitable donor for a bone marrow transplant could be found. Her family was asking that, instead of flowers, people register to be donors.
I think that's a wonderful, practical and sensible idea. I love flowers but I love life even more. Giving life to someone else may allow them to grow flowers.