A start into Ferns. Bipinnate or Tripinnate.

A start into Ferns. Bipinnate and tripinnate ?

I started looking at ferns in the autumn of 2020 in West Cork as my normal summer trip had been delayed due to COVID and flowering plants were over their best.

As a complete beginner I made some progress helped by the arrival of the new fern book, 'Britain's Ferns' by James Merryweather which arrived towards the end of my holiday. This new book contains a lot of information and many photographs and is well worth the price of £16.  It does lack some of the detail found in 'The Ferns of Britain and Ireland' by Chris Page which was published in 1982 and has more on hybrids etc. No photos though.

This blog looks at Buckler ferns, as I made basic mistakes in 2020 and missed identifying Broad Buckler- fern ( Dryopteris dilitata).  This was corrected in 2021.

The first reason for going wrong was not understanding the distinction between bi-pinnate and tri-pinnate, which is used a a basic key differentiator in 'Britain's Ferns'. Second reason was not fully appreciating the difference in the size of the pinnule teeth of the Shield ferns vs. the Buckler ferns. 

As a complete beginner, the sheer density of information in 'Britain's Ferns' key was a problem.  Not fully understanding the definition of bipinnate and tripinnate definitely puts you at a disadvantage, along with the fact that this is not a clear distinction in practice.

The above diagram is based on the textbook definition of bi and tri pinnate (Ref.  The Kew Plant Glossary) and is not to scale.  Nor does the shape in the textbook diagram come close to a real fern but it does show the principle.

 If each division were stalked as shown, by the blue Pinnule and purple Pinnulets, it would be fairly straight forward but in real life ferns have a much more complex shape.  In a real fern, the divisions might start off being stalked but become merged, so the distinction between bi and tri has to be based on looking at the lowest pinna rather than the tips. This applies to the pinna division as well as the further pinnule and pinnulet divisions.  An example of these problems is shown in the green highlighted pinnules which show that the pinnules are clearly stalked at the base of the pinna but become merged at the tip. Also the pinnule are segmented into pinnulets at the base of the pinnule but become merged towards the tip.

 The amount of segmentation that forms the pinnulets can cause problems which will be illustrated later using photos.

One suggestion might be that it pays to be methodical. Start with the main frond stem (stipe) and moving up from the base, determine what is a pinna, the first division.  Then move up that pinna and check if the pinnule nearest the stipe is divided into pinnulets, i.e.  separated into distinct sections. This is where it can difficult, as this third division is not always clear cut and this gets worse as you travel up and out on the frond. Also as I will demonstrate later, not all plants conform to the segmentation being complete, even at the base.

Buckler Ferns.

Buckler ferns are tripinnate in 'Britain's Ferns' initial Key. The pinnulets have pointed tips but not as strong as those shown on the bipinnate Shield Ferns. Broad Buckler-fern has the classic shuttlecock form. Separation of the different species of Buckler-fern is difficult but the Broad Buckler-fern normally  has stipe scales that have a dark line through the middle. Separating Buckler ferns species is not the goal of this blog, but it was these dark strips that first alerted me to the correct identification. I had been seeing a Buckler-fern not a Shield-fern. As a beginner with no experience, you can make this type of mistake.

Broad Buckler-fern stipe near the base showing dark centre line.  23Sept21

  Back to the frond detail.

Detailed photo of the pinnule of a Broad Buckler-fern. 23Sept21

I might be breaking my rule to look at the lowest pinna but in this example even the mid pinna show a pinnule (shown outlined in red) has at the base pinnulets, which are segmented right down to the stem of the pinnule (outlined in white). Although the pinnulet base is quite wide and gets wider as you go up the pinnule the segmentation remains 100% until you get close to the tip. This is clearly a tripinnate frond. 

Broad Buckler-fern. Part of pinna near top of frond.

The above photo from the same frond but further up, shows that the pinnule is segmented but only to about 60%.  If you only looked at this part of the frond, the plant might be regarded as bipinnate.  

My first error using the Key was thinking Broad Buckler-fern was bi-pinnate because I was looking too high up the frond.

I wondered if I had found a Shield-fern, never having seen a Shield-fern?

Photo showing pinnulet tips follows.

Broad Buckler-fern showing tips to pinnulets.

Where it starts to get difficult, is that on some plants only the very lowest pinna and the closest pinnule to the stipe has anything like a full segmentation, all the other pinnules have partial segmentation.

There seems to be a lot of variation in this feature. Some fronds are clearly tripinnate for at least the lowest four pinnae and only above that become bipinnate.

Narrow Buckler-fern ( Dryopteris carthusiana) showing lowest pinnule lacking segmentation into pinnulets. 

The photo above shows the lowest pinna base coming off the stipe. The pinnule is segmented but only the very lowest pinnule next to the stipe is fully segmented right down to near the stem of the pinnule.

Photo of the lowest pinna base with the pinnule closest to the stipe. It shows the first pinnulet

is segmented back almost to the stem, the second segmented to about 90% and the third only to about 80%.  I have highlighted the pinnulet margin in black to show this. The next potential pinnulet out is far less segmented. I do not care if the pinnulet is attached on a broad base only its division from the next.

 I think the implication of this is that Broad Buckler-fern and probably all the Buckler ferns are variably bipinnate to tripinnate. 

Buckler-fern frond with tripinnate pinnulets highlighted in yellow, all the rest is bipinnate.

Conclusion. It is good to understand the principles behind the terms bipinnate and tripinnate but also be aware that in real life, ferns require a more complex description. It is interesting that Poland and Clements in their 'Vegetative Key to the British Flora' do not use the term bipinnate and tripinnate but use terms like 'Group BH - Lvs 2-3(4)-pinnate for all the Buckler-ferns, Shield ferns and even Bracken.  

I take from this that the Buckler-ferns can be bipinnate through to tripinnate and is therefore not a good fundamental separator to use in a key, as used in 'Britain's Ferns'. This applies if you are a complete beginner.  The experts know all this anyway.

To finish-  a few shots of Soft Shield Fern to show the different frond structure and the stiff terminal spines to the tips of the pinnules.

Soft Shield-fern, taken West Cork, 25Sept21

Soft Shield-fern. Base of pinna with pinnules with a thumb. 

Soft Shield-fern. Stalked pinnule with thumb and stiff terminal spines.

The first comment on Soft Shield-fern is that the pinnule is only slightly segmented so this is a bipinnate  frond. The shape is very different to that of a Buckler-fern with a much less symmetrical pinnule, unbalanced by the thumb which has a margin that runs very close to the stem.  The tips have terminal spines which are longer than those on the Buckler-ferns, but this is a variable feature.   I saw my first Shield-fern after returning from Cork in 2020, in late November in a ditch in Cambridge.

References:-

Beentje, H. 2020 The Kew Plant Glossary. (2nd Edn.) Kew Publishing.

Merryweather, J. 2020   Britain's Ferns. Wild Guides

Page, C.N. 1982 The Ferns of Britain and Ireland. Cambridge University Press.

Poland. J & Clements. E.J. 2020 The Vegetative Key to the British Flora.(2nd Edn.) John Poland.

 

Peter Leonard

Rampton

November 2021

Other terms not used in this blog:-

 The segmentation of the pinnules is partial and this is called pinnatifid where the depth is not specified, pinnatipartite where segmented to about half way and pinnatisect where divided to almost the midrib.

Narrow Buckler-fern  D. carthusiana.

 Yellow areas are tripinnate pinnulets which are separated to more than 90% of pinnulet length. Rest of frond is bipinnate.

Broad Buckler-fern D. dilatata 

Yellow areas are tripinnate pinnulets which are separated to more than 90% of pinnulet length. Rest of frond is bipinnate. There is a lot of variation about the extent of the frond being tripinnate.

These two fronds were collected in the Kings Forrest in February 22.