I will add my agreement to it being heat. Mine did similar things until I got them into cooler temps. I notice in one of the pictures a thermometer reading 73F. Mine get that an over on hot days but are more commonly around 60F and in the winter go down to an average of about 45F.
I grow mine in porous clay with moss and kept always moist. It cools the root zone and helps them cope with the hotter days. I find that when I buy new ones in plastic/bark in the summer they start looking this way until I can get them into a clay pot with moss.
I've also been interested in what Ray says about "Stretching Growing Conditions" here.
"Stretch" Your Growing Conditions
See the idea at the bottom of the page. I haven't done exactly that but have tried to find ways of having more lecca/clay area for evaporation/cooling to occur from.
Linear Mode
