Thursday, January 12, 2012

Our Common toothy leaved Polytrichums

Working on Polytrichum drawings today (including Polytrichastrum).
The following four species are our most common that have toothy leaves.
P. commune is the largest and if fruiting is easily told from all the others by it's shorter capsule with a large 'knob' at the base (see diagram). Polytrichastrum alpinum has a dark brown cylindrical capsule (separating it from  the other Polytrichums which have angled capsules. If P. alpinum is not fruiting, then you have to section the leaf and look at the lamellae. In this species the terminal or top-most cell is papillose, meaning it has little bumps all over it. The other Polytrichums in this group have smooth terminal cells. The other two species  are more difficult to tell apart. P. pallidisetum seems to prefer a moister woodland, whereas P. ohioense tolerates drier ground. P. pallidisetum is often quite large and is the typical woodland Polytrichum found in the hardwood forests. P. ohioense tends to be smaller and darker and replaces P. pallidisetum as you head toward the coast and get into oak-pine woods. Although they can often be found growing together. They have a similar long, angled capsule. P. pallidisetum does have a paler seta, especially towards the top, although young P. ohioense also has a pale seta. The only reliable way to tell the two apart is by doing sections of the leaf.  I cut off the leaf base and tips (so they lie nice and flat on the slide) and cut a lot of cross sections about mid leaf. Then i scrape the lamellae off so they lie flat on the slide - this is the best way to tell these two apart.

Lamellae x-section of Polytrichum pallidisetum

Lamellae x-section of Polytrichastrum alpinum - note papillose (bumpy) topmost cell

Polytrichastrum alpinum - dark brown, cylindrical capsule

Polytrichum commune
(J. Jenkins photo)

Polytrichum pallidisetum

Polytrichum ohioense

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