A common differentiation for woody plants is: does it bloom on old wood (when flowers are initiated the season before they bloom) or does it bloom on new wood (when flowers are initiated the same season as they bloom).
When viewing all these wonderful photos of cacti, I see the distinctness of flowers originating from the sides of the plant, or flowers emerging from the tops. Is this necessarily due to the same phenomenon? In other words, are flowers on the sides because they are initiated the previous season, and flowers at the top because it is the current season's growth?
And, are cactus flowers really initiated as I hypothesize, or do flower always initiate in the current season, irregardless of the age of the stem of origin?
From my meager observations.
As a general rule, I know that when I root a mature Opunia pad it will often flower when it starts to put out new growth. However after it has rooted it will not (well almost never, we are talking about Opuntias , and they have a mind of their own some times) :rolleyes: flower again but produces only pads. These second generation pads when mature will, in the next growing season, put on new buds (some pad-buds, some flower-buds). Each pad only producing flowers it's first mature season. No matter how old the pads get they can continue to produce new pads.
Now for one exception to the rule. Opuntia Basilaris can produce flowers on older mature pads over multiple seasons.
As you point out some types of cactus shoot flowers from their sides on mature growth such as the Echinocereus cacti while others from the top as in Sclerocacti. They all are setting their buds on mature growth even those that flower on what appears to be new top growth. The buds circle the growing point but are on last years growth. I would say your hypothesis in correct flowers are initiated, on mature growth. :)
Now I question, why do some species of Ehinocereus cacti set blossoms on mature growth two years old, while other species of Ehinocereus can set flowers on growth only one year old? :-\
Plants (Cacti) flowering when they strive to root is a stress symptom. Stressed plants try to flower and seed in case they die. This is controlled by hormones.
Likewise is flowering on new or older wood hormone controlled and adaptions to different growing conditions and/or different strategies. It is often not obvious why plants do this or that - many strategies pay off, not only one.
The spine/glochid clusters are in fact produced on very compressed shoots that sometimes continue to grow (they possess apical meristem) although the pad itself doesn't grow more. So the flowers can be produced on new growth even on a two years or older pad!