My friend Sue categorises some plants as ‘Botanist’s plants’ i.e. ones which only a serious botany geek could find interesting or appealing, or possibly even notice at all.  She puts Sanicle, Sanicula europaea, into this category, a little harshly, perhaps.  However I have to agree that one of the plants I noticed in flower for the first time in the Lake District this week, Shoreweed, definitely falls into it! Ennerdale Water has much less water in it than usual, at the moment, so the Shoreweed is more visible.  It’s not much to look at – a turf of fleshy, grass-like leaves, which put it in the plantain family.  Long stamens protruding from the tiny male flowers are the only the only hint the plant is flowering – another feature of many other plantains. When submerged, this plant reproduces using vegetative runners, only flowering when it is exposed, out of the water.  As for many such plants, flowering is a high stakes strategy for desperate times; conditions are such that the plant is at risk of drying out altogether and dying, so it makes a last ditch attempt to preserve its genetic material in the form of seeds, some of which may end up in a more suitable growing environment than the parent plant.

My other lakeside find was much more attractive, if only a little more conspicuous.  When we stopped for a picnic lunch on the beach at the eastern end of the lake I spotted some small spikes of pale lilac flowers, also growing from rosettes of fleshy leaves.  Water lobelia, Lobelia dortmanna, is another species which would normally be found growing in, rather than beside, the water. 

The Lobelia’s attractive bell-shaped flowers reveal it to be a member of the Campanulaceae, if an atypical aquatic one.  It’s likely the Lobelia, too, is flowering more vigorously than normal now that it has been left high and dry on the lake shore.  Normally the leaves would remain submerged and only the flower spike would appear above the water surface for pollination to occur.  The exposed leaves have a giveaway red tinge; they have resorted to producing pigments such as carotenoids to help them cope with the much higher levels of light to which plants are exposed on land.

Both Water lobelia and Shoreweed are unusual amongst aquatic plants in being unable to obtain the carbon-dioxide they need for photosynthesis by taking up bicarbonate ions from the surrounding water – instead they have to depend on CO₂ dissolved in lake sediments. This makes them members of a small group of unrelated aquatic plants known as the isoetids, after the quillworts (Isoetes sp) in which this growth strategy was first noticed.  Both plants have stiff, waxy leaves and a better developed root system than most aquatic plants and they use this root system to take up CO₂ from the small gaps between sediment particles where it is released by bacterial action. I guess this partly explains why, as aquatic plants, they are relatively able to live on land.

The fleshy leaves look a little like those on succulent plants, from hot, dry environments, which use a special form of photosynthesis known as Crassulacean Acid Metabolism (CAM); carbon dioxide is taken into the leaves at night through stomatal pores when there is less danger of the plant losing too much water at the same time. The plants convert the CO₂ to organic acids which are stored in the leaf vacuoles overnight.  The CO₂ is then re-released during the day, when light energy is available to convert it to sugars and starch.   It turns out that the similarity doesn’t stop with the leaves’ appearance; isoetids also store CO₂ as organic acids and the waxy leaves, instead of helping retain water, help retain CO₂ which might otherwise diffuse out. 

These plants can only live in ‘soft’ (i.e. more acidic) water and don’t thrive in eutrophic conditions, so are also indicators of more pristine water bodies, often in mountainous areas. Shoreweed is less fussy and is found in acidic waters throughout western parts of the UK but Water lobelia is restricted, in England, to water bodies in the Lake District, which mostly lie on top of ancient volcanic rocks. Both are fairly small, slow-growing plants, largely due a lack of nitrogen and phosphorous in the water where they live but, even then, they are well adapted to compete.  Oxygen which leaks from their respiring roots binds up the limited amounts of Nitrogen and Phosphorous present in the lake bed sediments in forms largely unavailable to higher plants; the isoetids, like many land plants, but few aquatic ones, have associations with mycorrhizal fungi which help boost their mineral uptake. Their unusually well-developed root system makes this easier.

I must have seen both these species before but, without their flowers, even the most nerdy botanist could be forgiven for not noticing them. How much of interest we easily miss!

Much more on aquatic plants in the New Naturalist volume Lakes, Lochs and Loughs by Brian Moss (2015)