Could it be that the cuttings know how old they really are? i.e how old the original plant was? Either that or maybe they have picked up a virus that reduces vigor.
Racehorse breeding requires the fusion of the egg and sperm to produce a zygote, and individual organism that carries both maternal and paternal genes.
Propagation by cuttings on the other hand is basic generic cloning and produce genetically identical offsprings. Running out of steam would be purely caused by environmental factors (stress, disease, nutrition, temp etc).
But then we have to consider the possibility of genetic mutation on part of the plant which you took as cuttings. Generic mutation is common in plants resulting many beautiful variegated strands, also weaker strands as in this case.
It is impossible to tell for sure but I am leaning towards environmental factors purely based on probability.
Not exactly on topic as your question is more about individual plants, but I have been working my way through some old books on roses (fascinating, actually), and they often talk about certain varieties having lost their vigour. I don't know why this would occur and have been wondering.
(I bet Marlorena knows but I don't want to tag her in a non-rose thread.)
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(I bet Marlorena knows but I don't want to tag her in a non-rose thread.)
In the sticks near Peterborough