In September 2000 I obtained a mature Copiapoa labelled 'Copiapoa bridgesii - Jumanery 1975'. Presumably the previous owner originally obtained the plant from Tom Jenkins' Jumanery Nursery in 1975.

The lower part of the stem was badly marked / corked and distracted from the beauty of the top half, so I decided to make a cutting of the top half of the plant. I left the lower part of the plant in its pot in the hope that it would produce some offsets.

The top part rooted without problems and is growing well as a solitary stem.

The lower part produced a dozen or so offsets that look completely different from the parent plant.  They are tubercled instead of ribbed like the parent plant.

Offsets on my Copiapoa bridgesii in 2001
The tubercles are beginning to align themselves into ribs

Since then, I have acquired a number of mature Copiapoa plants, some with offsets, but in these plants the growth of the offset is clearly similar to that of the parent plant. In these cases the apex is still in place.

What causes the phenomenon observed in my Copiapoa bridgesii?
Is it a freak occurrence or something that can be explained easily?

I found most hobbyists understandably reluctant to take the top half out of a perfectly healthy 25 year old mature plant to repeat the experiment and the offsets would take too long to form on plants in habitat, so that I would not be able to record these results. I have however seen similar offsets in habitat.

It may be that we are observing the significance of apical dominance - the hormones that trigger the formation of offsets (or the amount of hormone produced)  may be different when the plant produces these spontaneously with its apex in tact, to  those (or the amount) produced when the apex has been removed. If this should be the case, that what ever hormone is responsible for this phenomenon, it raises the question what other triggers might cause a change in production of this hormone and what other hormones (and their triggers) might affect the appearance of Copiapoa?

The offsets on my C. bridgesii bear uncanny superficial similarities with plants of the C. humilis complex: soft bodied, tubercled, with eventually the tubercles lining up into tubercled ribs and weak spination compared to the parent plant.

In 2001, I took one of these offsets from the mother plant and have rooted it successfully.  I can now baffle visitors with my 'theory' that the subgenus Copiapoa consists of just one highly variable species, with their appearance variability due to growing conditions. Even experienced Copiapoa growers find it difficult to believe that the rooted offset and the top cutting of the mother plant are the same species. They are however convinced once I show them the lower part of the parent plant's stem.

E. W. Bentley makes similar observations in The Chileans 12(40):21:

I have come to the conclusion that many Copiapoas have not only one juvenile form which can differ from the adult form, but two. One of these is in seedling plants and the other is seen when adult plants are offsetting. The offsets now breaking out of the C. longistaminea look as though they belong at the coquimbana / vallenarensis end of the shelf! An elongated Copiapoa with a yellowish-green body and slim, curved, ginger-brown spines has produced a number of offsets that I have removed and rooted up; these offsets have remained flattened globular in shape and have themselves put out further similar-looking offsets, so I now have a clump of flattened globular heads that has grown into a much larger pot than the original columnar parent.