Am just looking at a collection of Pemphigus spyrothecae Spiral Galls on Lombardy Poplar from Newbury, Berks. They all have a single twist – ie only one crack opens up when you gently uncurl very slightly. In the AIDGAP galls book (Redfern et al) it is described as having 2-3 twists. Think the concept of “twists” is misleading. If it had 0 twists it would still have a single thickened layer, 1 twist would give 2 layers, so 2 twists gives 3 layers. Fig 618 shows a specimen with 2 twists (ie 3 layers – or maybe just one twist – the original line of the petiole often gives a groove which looks like a join – the drawn section [incorrectly] shows the tissues continuous – ie no twists). Suspect the drawing might be more often interpreted as 3 twists. But am not sure how better to phrase it! Perhaps use the no of cracks that open up when you uncurl it slightly.

Maybe this pedantry is a long term after-effect of a PhD project that involved counting the no of whorls on several hundred snail shells… ;o)

HTH

Malcolm

www.bioimages.org.uk