My cactus has put on a growth spurt over the last few weeks – following a distubing ‘drooping’ episode where its two branches changed angle significantly over the period of about a month.
In early July the cactus was showing early signs of growing its smallest arm:
6 weeks later, its growth has been well beyond my expections, the smaller arm is now almost as large as the other one having grown at least another two areoles along, almost doubling its length:
I have enjoyed watching how it grows, the areoles appearing at the tip and unfurling outwards as the arm pushes them out of the way. For the small arm, the process has been surprisingly violent, with the unfurling of the arm from the end almost seeming to rip the ridges apart, and leaving a ‘tide mark’ of newer stem.
The spines grow much like small branches (though I believe they are modified leaves), shrinking, drying and changing colour once each section is about a week old. The wetting qualities of the cactus have changed too, the older stem seems to be the most hydrophobic, followed by the newer stems (a slightly lighter green if that is perceptible), followed by the older spines and the newer spines. The areoles, especially the quite furry new areoles, are positively hydrophilic, the water clinging to them across the whole surface. Reading Superhydrophobic and superhydrophilic plant surfaces: an inspiration for biomimetic materials it seems most super-hydrophobic plant materials are found in wetlands, keeping water away from the plants helps to prevent bacterial attach and help stop accummulation of dirt on the leaves. In the case of the cactus, I suspect the hydrophobic nature of the stems helps increase the proportion of any rain reaching the root system, and helps to prevent burning of the plant caused by lensing through attached water droplets after rain.
The order of growth of the stems seems to be dominated by the availability of space at the end of each stem. Looking down above the main stem, it is clear that the 7 ridges alternate somehow in growing a new areole and bump, though as there are 7 ridges, it is not clear quite how this order is determined.