Waves have gripped me over the last fortnight.
Not those which batter our nearby coasts, crashing on rocky cliffs, or transporting drifting fleets of jelly fish, as seen recently near Cwmtydu.But those that transform our hay meadow above the house at this time of the year, as the Sweet Vernal grass stems reach for the sky. When the winds pick up, it transforms a flat scene of subtle colour variations, into an active rippling floral plateau. The inflorescences of millions of grass plants in a show of spectacular synchronised dancing, with the gusting, pulsing wind ripping up over contoured hills and warmed from the South, as the master choreographer.
How do I describe the colours of this scene? It depends on the light. And the angle of view. And the time of day. And how developed the grass “flowers” are. Early on, and it’s all preparatory greens, then the anthers develop, extend, and let it all hang out and add a grey, or is that a purple, or purple brown, or rust, or white. or yellow colour to the mix? I read that Sweet Vernal Grass, Anthoxanthum odoratum, (yellow scented anthers?) above, isn’t that palatable for grazing stock, (click here for more information on upland native plant grazing preferences) but our native Tor ddu sheep seem to manage it, and since it’s now the predominant grass in this meadow, post Yellow Rattle introduction, that’s just as well.
And what about the appropriate technical language for describing grasses? I found an excellent simple guide by Jean Turner which you can access by clicking here. This gives much of the terminology used to describe grass morphology – quite different to many of the words familiar to descriptions of our garden flowers. So there are culms, stolons, and tillers to describe the leafy parts of the plant…
And Lemma, Palea, Glumes, Lodicules and Spikelets, as words completely novel to me, to describe many of the features of the individual grass flowers. After all, grasses being wind pollinated have no need for showy coloured petals or sepals to attract pollinating insects, they just have to hang out their anthers and let that same choreographing wind disseminate. And waft those clouds of dust like pollen grains from one bending plant to another.
I’d never thought much about grass stem, or more correctly culm, rigidity, before these latest displays. Then I happened to grab a couple of grass culms growing just outside the back door, on the steep bank. And looked a bit closer at their physical structure.
There were the obvious nodes, which usually contain a central cross wall (or septum) at right angles to the culm, and internode sections, which seemed to vary in length as the grass reached the flowering spikelets. The diameter of the stem also seemed to vary considerably, tapering at its tip but being essentially circular, at least in Sweet Vernal grass, throughout its length. However the thickened node is clearly a key component with considerable extra strength which plays a role in giving the grass flexibility to withstand wind damage, in its springtime growth spurt.
Little research seems to have been carried out on the mechanical strength of hollow grass culms, and their septate nodes. However studies by Karl Linklas demonstrated that hollow stems can grow about 25 % taller than solid stems, for a given biomass. But a problem arises as internode (stem) length increases – the hollow stems become much more prone to permanent damage causing the stem to fall over when stressed with side forces such as strong winds (known as lodging). All such circular hollow structures begin to ovalise, as they bend under such forces, and once a critical point is reached, then irreparable damage and crimping occurs. It seems that the nodes with their cross walls tolerate much greater forces, and can flex in strong winds, allowing sufficient give in taller grass culm heights.
In one of Nicklas’ papers, he even demonstrates that perforating the integrity of the cross wall of the node, with a tiny needle, diminishes its strength by about 35 %. He proposes that the nodes act as springs, storing energy when the stem is flexed, and releasing it once the bending strain dissipates. (Click here for more.). A great example of hugely sophisticated adaptive design in something as simple as a meadow grass trying to survive and reproduce in a demanding environment.
What controls the development of the grass as it grows into these specialised structures is something I’ll return to shortly, but firstly I must record that a meadow full of flowering grasses becomes a haven for a huge invertebrate population. Best appreciated as the sun sets, and with a hat to shield me from the still strong direct light, bending low and looking uphill, one can appreciate the thousands of insects caught, contra-jour, above the shimmering meadow. Pinpricks of mobile transluscence, shifting above the swaying stems. A bat and swallow haven.
Once in a while I hear a discussion on the radio that makes me stop, sit down, and listen, whatever I was about to do. This happened recently when “A Life Scientific” on BBC R4, featured Jim Al-Khalili interviewing the plant biologist Ottoline Leyser. A name I’d never heard before, but this programme is a must listen to for anyone interested in how plants work. In part because of the subject matter, and in part because Ottoline is a supremely clear and articulate communicator. Click here for the programme. And here for more on this talented and hard working scientist.
Moreover this clarity of thought and ability to communicate it, carries on in her written work. I found a really useful review article she’d written in 2011, which explains how plants control their growth and development using a mixture of plant hormones. Much of this review article is again very useful background science for anyone interested in gardening, since it puts into context why many gardening techniques such as pruning work in the way that they do.
Ottoline has spent much of her work studying hormonal levers which control plant development and growth whilst working on Arabidopsis thaliana. Commonly known as Thale Cress, Ottoline explained that it had become the plant researcher’s equivalent of the fruit fly, Drosophila melanogaster which was favoured by early geneticists studying genetic variations and their impact on development in animals, because of the Drosophila’s small genome, and very short life cycles.
Arabidopsis has similar traits, and having a very short life cycle makes it an ideal candidate for exploring the outcome of genetic changes on the progeny, without having to wait ages for the next generation to develop. In this regard I notice that Arabidopsis is very closely related to one of our bete noire weeds- Hairy Bittercress, Cardamine hirsuta.
In our wet maritime climate this is the weed which dictates the frequency of our hand weeding regime. Pretty much all through the year it can grow from tiny seedling through flowering, and producing viable seed capsules in less than 6 weeks. And the numbers of seed produced per plant can be huge. So miss a single generation, and discover a plant where the slender pods have already split and dispersed their seed widely and you know you have years more weeding ahead of you in that part of the garden. Vigilance really can’t be relaxed, and unfortunately as the year progresses and other taller plants merge, this plant is even capable of going through seed production whilst being hidden beneath the taller canopy of surrounding foliage.
But back to some of Ottoline’s research, which she has brilliantly summarised in a review article, ( click here), in “Current Biology” . She draws the interesting comparison between multi-cellular animals, where embryogenesis lays down the basic animal structures and organs which are tweaked during the adult animal’s growth, and plants where although the basic axes of plant structure are laid down early on, continuous development is possible in later life in response to environmental and other stimuli.
The critical tissues with plant growth are the meristem cells, which at the tip of shoot, and root, grow the plant by producing new cells, at least in most dicotyledenous plants. But secondary meristem tissues can also be allowed to develop which can lead to a very branched and complex structure eventually forming, the whole being influenced by environmental parameters. This development is under the control of plant hormones transported around the plant in the xylem and phloem – the two systems of channels designed to move fluids and other nutrients around the whole of the plant. The fact that no one shoot or root is critical to survival of the whole plant confers a degree of flexibility of development which she likens to a colonial structure.
In the xylem channels, fluids and nutrients can only move one way, upwards, from the roots to the leaves, in what is called the transpiration stream, driven by the constant evaporation of liquid from the leaves’ pores, or stomata. The xylem channels themselves consist of open empty tubes, of essentially dead cells, rather like drinking straws, lined with woody lignin. A number of hormones – cytokinins, which produce shoot branching; strigoloactones which inhibit shoot branching, and abscisic acid which has a role in shutting down leaf pores in response to drought conditions at root level, are produced in root tissues in response to environmental conditions there and then move up the tubes to influence cellular behaviour in the leaves and shoots above.
The phloem consists of a separate system of channels made up of still living cells, with cell walls, which principally transports sucrose, the sugar produced from photosynthesis in leaves, either up or down the plant to storage organs, or other active tissues like the shoot meristem regions where growth is occurring. The phloem also serves as the transport route for auxin, one of the other major plant hormones, which moves around the plant linked to a number of specific plant transporter chemicals. There are 3 useful diagrams in this review paper, which I’m grateful to be able to include to illustrate these principles. For much more detail though, do have a look at the link above.
Cells in the primary shoot apical meristem divide to produce a stem below it and leaves around its circumference. Secondary shoot apical meristems are established in the axils of leaves (axillary meristems) with the same developmental potential as the primary shoot apex, but they may enter a dormant state and never fulfil this potential. Signals modulating the growth and development of the shoot are transported in the transpiration stream in the xylem, driven by evaporation of water from the leaves pulling water up from the roots, bringing along dissolved nutrients and signals. Signals can move bi-directionally in the phloem, which transports fixed carbon in the form of sucrose from source tissues, such as mature photosynthetically active leaves, to sink tissues, such as the roots or the growing shoot tip.
The chemical structure of indole-3-acetic acid, the most common natural auxin, is shown top right. Auxin synthesised in young expanding leaves is transported towards the roots in files of xylem parenchyma cells running parallel to the xylem vessels.
Top row, solitary buds on isolated nodal stem segments: (A) Activation of untreated bud to produce a branch. (B) Inhibition of bud by apical auxin application. (C) No effect on bud of basal strigolactone application. (D) Super-inhibition of bud (red asterisk) by simultaneous application of basal strigolactone and apical auxin. Bottom row, stem segments with two buds: (E) Activation of both buds when untreated. (F) Activation of only one bud when treated with basal strigolactone.
Having now completed our list of favourite plants constituting the main palette which we have used to “paint” our garden ( see the separate Gelli Uchaf plant palette web pages), it only remains for me to fine tune it.
This year I took a long look at plants in early May and opted to replace the very pretty Dodecatheon meadia alba with another flower of American origin, the white flowered form of Limnanthes douglasii subsp.nivea “Meringue”, which we use more widely in the garden at the same time of year.
Known more commonly to gardeners as the Poached Egg Plant,(above) L. douglasii “Meringue” has much in common visually with L. alba, known in the U.S.A. as Meadowfoam. Native to Western states of Oregon and California, it is a vigorous annual which thrives in wet, nutrient poor, and poorly drained locations along rivers and streams. When it comes into flower early in the season, it’s not difficult to see how it came to get its common name.
We’ve always found it a very successful low growing ground cover in the retyred matrix garden, where it self seeds and often germinates in the autumn, so has a head start in spring and begins flowering in late April. It’s a fantastic nectar and pollen source, particularly for honeybees early in the year, but I also discovered that it’s being grown commercially in areas of the U.S.A. for its seeds which contain a large percentage of an extremely high quality mix of oils, the majority of which are very stable long chain fatty acids which have found widespread uses within the cosmetic industry. (Is there a coincidence in the “nivea” subspecies name tag, though the derivation simply means “snow white”)There is sufficient excitement about this oil’s special properties that it’s generated interest amongst plant GM specialists in the U.S.A. who are looking to breed this oil producing capacity into soya beans, which clearly already have a very well developed agri-business associated with their cultivation.
I’m wondering whether it could be a a perfect alternative crop for growing in wet upland Britain, given its basic growing preferences. However there’s something of a debate going on here as to whether I should scatter a few seeds into our hay meadow? I’m in favour, Fiona is against, on the basis that it isn’t native and might become too dominant. The jury’s still out, but as an interim strategy I might try scattering a few seeds peripherally to see if it can cope with slug predation which is a huge issue for any meadow plants trying to establish as seedlings here. Click here for more on the properties of Meadowfoam oils.
Maybe one day Meadowfoam oils from the hills of Wales will pamper the faces of the world’s most glamorous ladies, and the hills around us will be white with flowers in spring, blowing in the wind, and alive, not with the sound of music, but of happy humming honeybees.
Somehow Meadowfoam creates a more alluring mental image than the the current lovely white froth of flowers, gracing our meadow’s shaded banks.