Friday, 16 March 2018

Brachythecium glareosum

Several Bryophyte guides mention the problems of distinguishing Brachythecium glareosum from Homalothecium lutescens and Brachythecium rutabulum in the field. All three species occur along forest tracks in NPT, sometimes together, and on more than one occasion I have mistaken  B. glareosum for one or other of the other two. This winter I've tried to sort this out and I thought it might be useful if I posted some comparative observations. However, there is nothing new here and I apologise if I appear to be teaching my grandmother how to suck eggs.
In its typical form, B. glareosum appears to be quite distinct with those long, slightly twisted acumens at the tips of the leaves, which form conspicuous, pointed clusters of bristles at the apex of the stems and branches. This is a major feature used by Smith to distinguish B. glareosum from all the other Brachytheciums. 

                                    Brachythecium glareosum, forest track, Glyncorrwg

                     Long, bristly stem tips of Brachythecium glareosum (note distinct pleats on leaves)

Confusion with H. lutescens arises because it too has a long fine point to its leaves and both species also have clearly pleated leaves.

                                                             Homalothecium lutescens

                                                         Homalothecium lutescens

The leaves of H. lutescens are narrower and more triangular than those of B. glareosum and the pleats on the leaves are very prominent and almost parallel. Also, H. lutescens can be distinguished from any Brachythecium if you examine a leaf under the microscope. Leaf cells are very narrow, and the basal cells of the leaf are distinctly thick walled (incrassate).

                  Part of Homalothecium lutescens leaf showing  thick-walled (porose) basal cells

'Typical' Brachythecium rutabulum is usually not a problem. Confusion occurs when you encounter specimens with long drawn out tips to their leaves, which happens frequently. Even then, however, B. rutabulum rarely exhibits the long, conspicuous, bristly stem tips that characterise B. glareosum. Most texts refer to the twisted leaf tips of B. glareosum, but this is not always a very prominent feature in my experience.

 Brachythecium rutabulum (note vaguely pleated leaves)

There are other subtle differences between B. rutabulum and B. glareosum that can be observed in the field. The leaves of B. glareosum are narrower than those of B. rutabulum, (although not as narrow as H. lutescens), and more noticeably pleated. The typical habit of B. rutabulum always seems to be plumper and more robust to me, compared to the slender, graceful and more prostrate habit of B. glareosum, reminiscent of Homalothecium. B. glareosum is a calcicole and on forest tracks usually occurs with species like Ctenidium molluscum, Ditrichum gracile and Trichostomum crispulum.
The leaves of B. glareosum and B. rutabulum look different under the microscope. The alar cells of B. rutabulum tend to be long-rectangular and form decurrent tongues (almost like auricles), and the upper leaf border is clearly toothed.

                                                  Long-rectangular alar cells of B. rutabulum

                                                        Teeth on upper part of leaf of B. rutabulum

The alar cells of B. glareosum are short rectangular and not decurrent, and the leaf has very indistinct teeth.

Short-rectangular alar cells of B. glareosum

Indistinct teeth on upper part of leaf of B. glareosum

1 comment:

  1. Some very helpful pointers and images Charles. I dread to think how much time i must have spent pondering over Brachythecium specimens!