[Taxacom] leaves facing the same direction

[S.R.Edwards]

This is maybe not exactly what you want, but how about heterotropic as in 
Ulmus, as opposed to homotropic? This whole business is interesting because 
it involves different adaptations to maximising light capture. Maybe a 
better example is in the Marantaceae, e.g. Ctenanthe burle-marxii vs 
Calathea leopardina.

In both these genera (as in Ulmus) the leaves are laterally asymmetrical, 
but in Calathea the leaves are all similarly handed (homotropic) and so 
appear spirally arranged. Whilst in Ctenanthe they are alternately leaf- and 
right-handed (heterotropic) and the shoots are flattened as in Ulmus, and 
the petioles must be twisted through up to 90 degrees. The leaves also move 
upwards at night because of the active pulvinus at the top of the petiole, 
hence "prayer plants". In the daytime, the leaves flatten out and their 
adaxial sides indeed face the same direction, and the shoots could at least 
loosely be called complanate and distichous.

I have illustrations to show the distinction between heterotropic and 
homotropic, and can email them to you if you like.

Many leafy liverworts exhibit the condition that you describe, and the angle 
of the insertion is variously twisted from the transverse. These leaves are 
variously heterotropic, at least with regard to their insertion.

In the moss genus Fissidens which has complanate distichous shoots, the 
leaves all tend to be either left- or right-handed on one shoot (homotropic) 
but both sorts of shoot are found even in one gathering. Thus the back of 
the shoot is the same as the front. But in some species such as Fissidens 
serrulatus you will find not only either left- or right-handed shoots, but 
also shoots as you describe where the leaves on either side are mirror 
images of the leaves on the other side, i.e. heterotropic and all 'facing' 
the same direction. The origin of the Fissidens vertically flattened 'leaf' 
is not through twisting, and they have no 'petiole'.

Although these bryophyte shoots are described as distichous and complanate, 
neither term describes which way the leaves face.

So heterotropic is not the word you want, because twisted leaves all facing 
the same direction is not describing their symmetry. But in almost(?) all 
cases of this condition the leaves are at least to some extent heterotropic, 
and all(?) heterotropic leaves show this condition.

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As an afterthought, maybe heterotropic should really be bitropic, unless 
anybody knows of laterally asymmetric leaves with more than two 
orientations? Or maybe the bitropic condition describes species that can be 
either heterotropic or homotropic, as in Fissidens serrulatus?

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Sean Edwards, Vine Cottage, The Street, Thursley, Surrey GU8 6QF, UK
sean.r.edwards at btinternet.com
tel: 01252-702-890 cell: 07768-706-295