Corrugated leaves

I have the odd plant - indoor (orchid) and out (iris) that have developed corrugated leaves Horizontal pleating across the leaf - usually towards the base of the leaf. At first I thought it was something obstructing the leaf growth - but I have it appearing where there is no such obstruction.

Any idea what this is caused by? Plants are healthy, and in the case of the Iris flowered well.

I would like to know what is likely to be the problem - any ideas??

 

For others who may have the same problem, I found the answer from the RHS. It is apparently due to changes in growing conditions especially when the leaf is forming - particularly temperature.

 

Thanks Fran - thats interesting. There is a similarity with humans. Doctors look at your nails and, I understand, can tell from any horizontal banding if there has been a sudden change in your health.

 

Yes indeedy - same as growth rings on trees Puzzled me that I hadn't moved or changed the routine of any of the orchids and it only affected two - then I remembered that they were the two I got earlier this year - rather than the older ones. Made sense then.
The Iris Europa in the garden with the same thing - may have been affected by our weird April temperatures and snow - the ones in the sunny border are fine, but the one in the shady part developed the same corrugated leaf pattern.