study – Lili Barras-Hargan

The vascular systems of a stem

It was another day of college today and my laptop decided to fail me, just in time for online learning! Learning is never easy outside of the classroom and not being able to access the documents we’re working on makes it even harder. Still, we covered a lot and I only got slightly left behind!

Here are some of the topics we focused on today:

Health and Safety Legislation
Vascular system of a plant’s stem

I’m going to focus on the second topic, for obvious reasons (i.e. posting about legislation an interesting blog does not make). I would argue that the stem of a plant is one of the least appreciated parts of a plant. When looking at a plant’s distinguishing features, you often think of things like the foliage shape and texture, the showy flowers or the berries and fruit. However, the stem is the literal backbone of the plant. It acts as support in adverse weather (with the help of the roots), it transports nutrients around the plant and the stem is one way you can clone your plant to produce a genetically identical replica, for free!

We dived into botany today, looking at plant cells up close and discovering the different processes that happen beneath the surface and just how important they are. To kick off the learning, let’s talk about monocotyledons and dicotyledons:

Monocotyledon (or monocots): this refers to flowering plants that have scattered vascular bundles in their stems and are characterised by their lack of cambium (see below) between the xylem (also see below) and phloem (just see below for all, please). There is also no distinction between the cortex and pith and no annual rings (tree rings) are formed.
Dicotyledons (or dicots): unlike monocots, these have a limited number of vascular bundles arranged in a ring. Cambium does exist in dicots and and the cortex and pith are distinguishable. Secondary thickening can occur (I’ll get into this later) and annual rings form as a result of this.

Distinguishing between dicotyledons and monocotyledons is important, as it tells us a lot about what the internal structures are going to look like. The best way to differentiate between a monocot and a dicot is seeing what its first leaves look like. If a pair of kidney bean-shaped leaves appear, that’s a dicot, whereas if one grass-like leaf grows out of the soil, that’s a monocot.

Let’s look at the structure of a stem. Most dicots have a similar structure to this and may only show slight variation or modified bits and pieces. Here’s a diagram I put together at the beginning of the year:

Terminal (or apical) bud: this is the topmost bud and the one responsible for terminal growth. Due to the auxin hormone, most of the energy goes into growth from this terminal bud and growth is inhibited in the lateral buds. This is why gardeners ‘pinch out’ plants that they want to grow bushier or to put on more lateral growth – the energy stops being solely directed to the terminal bud and lateral buds get a chance to grow.

Lateral (or axillary) bud: These are the aforementioned buds that are further down the stems. They are produced in the leaf axils and are responsible for the lateral shoots from the main stem.

Flower bud: These develop into flowers and are often larger in size than the buds that produce vegetative growth.

Leaf scar: This is the scar from from the place where a leaf was joined onto the stem. It is also called the abscission scar.

Node: This is the point on the stem where a leaf used to be. The angle between the petiole (leaf stalk) and the stem is known as the leaf axil.

Internode: This is the length along the stem between two nodes.

(Not mentioned in the diagram:)
Lenticel: These are pores in the stem through which gasses may be exchanged. The relative size and shape of the lenticels can be a distinguishable feature when identifying plants.

Growth rings: This is sometimes called the girdle scar and indicates where the growth stopped after the end of the growth period the year prior. Therefore the length of the stem between two girdle scars or the terminal bud and the previous girdle scar will advise how much the plant grew the previous growing season.

And now, let’s look even closer – with a microscope! Inside any plant’s stem, there are three main players: the xylem, the phloem and the cambium. These form the vascular system of the plants, transporting food, water and minerals around the plant and offers support.

Vascular cambium (F): this is known as a meristem, meaning it is made up of undifferentiated cells and is capable of cell division. These cells have the ability to develop into any of the other tissues and organs in the plant. It is located between the xylem and the phloem inside the bark of the stem. Its cell division and growth make it responsible for increasing the girth of a stem.

Xylem (D): this conducts water and dissolves minerals and nutrients upwards from the roots to all the other parts of the plant. The xylem vessels’ walls are thickened with secondary deposits of cellulose and this causes secondary thickening. In older plants, the xylem stops helping with transporting water and nutrients and serves to give support to the growing trunk. As such, wood is xylem and when counting the annual rings of the tree, you are counting the rings of xylem.

Phloem (located under the bundle cap – E): this is produced towards the outside of stem on the other side of the cambium. Phloem transports the glucose produced through photosynthesis in the leaves and around the rest of the plant.

Jon Houseman / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Secondary thickening can happen in all dicots, however, it is more noticeable in perennials and woody dicots. Secondary thickening refers to the thickening of the stem as a result of the primary xylem and primary phloem being moved further and further apart. Annual rings will develop as a result of this and appear as alternating rings of spring and autumn wood.

There’s a lot more we can get into around this topic, however, I think modified stems a little more attention grabbing. While typical stems occur above ground and feature all the different parts listed above, modified stems can exist above and below ground and serve many more purposes than you might think.

Chlorophytum comosum variegatum (Spider Plant) produces plantlets, which are young plants that arise from modified flowering stems. These can also be called stolons.

Sempervivum tectorum (Common Houseleek) dsplay offsets, which are young plants produced from the base of the rosette forming new plants.

A crown is an area of compressed stem tissue. This is where new shoots are produced, generally found near the surface of the soil. Taraxacum officinale (dandelion) are compressed stems which produce leaves and flowers on short internodes. Runners are a kind of stolon, produced from the crown, from which new plants will grow.

Stolons are horizontal stems that are fleshy or semi-woody and lie along the ground. Stolons are specialised stems that run across the soil surface and create a new plant at one of more of its nodes. Strawberries are good examples of this!

Stolons are often confused with rhizomes, which are a different kind of specialised stem which grows horizontally just below the soil surface. They act as a storage organ and a means of propagation for some species. Some rhizomes, such as in irises, are compressed and fleshy.

Spurs are compressed fruiting branches, common on fruit trees such as Pyrus spp. (Pear trees), where they bear fruit.

Tubers are enlarged portions of an underground stem, such as potato tubers. Like any other stem, a tuber has nodes that produce buds. These are the eyes of a potato and contain clusters of buds. It is important to note that root tubers also exist.

Corms are solid, swollen stems. They are different to bulbs in that they do not contain fleshy scales. Instead, they have been reduced to a dry, leaf-like covering. New corms are formed on top of older, exhausted ones, meaning that adventitious roots develop to sink it to the correct depth.

Unlike corms, bulbs have fleshy scales and produce shortened, compressed, underground stems. The scales envelop a central bud located at the tip of the stem.

So there’s a mammoth amount of information about stems! Stems are one of the most important parts of the plant and I always see it as the HQ of the plant, sending nutrients where they’re needed and offering support. I can’t wait to do some stem cuttings at college next week so I can put all this theory into practice!

What I learnt today: preparing a bed for lawn seed and broadcasting

Today – for the first time in about seven months – I sat in a classroom with the other apprentices on my course. It was amazing. I had missed talking as a group, finding out what we had all been up to and working together. These kinds of college days remain a rarity, as we will be studying from home every week except for practical days. Today we worked on the following:

Soil cultivation, preparing for a lawn
Seed broadcasting
Calibrating a broadcast spreader
Hand scarifying with a rake
Hand aerating with a fork
Top dressing
Ident walk

We kicked off the day with some soil cultivation, preparing a bed for lawn seen broadcasting. Preparing a bed for any use is time consuming and – in some cases – back breaking. For example, if the practical test pare required a 20m² bed to be prepared for planting vegetables, good luck! This likely means you will be double digging the area or at least digging to a depth of about 30cm. Then, you will rake this over, tread it in and rake level.

Fortunately, lawn establishment only requires the top layer of the soil for the roots. As such, we worked on simple digging, which involves inserting the bottom third to two thirds of a fork into the soil and tousling it. This allows for gaseous exchange in the soil and alleviates compaction without digging too deep. After that, we roughly raked over the soil with a soil rake, combing through it and flicking away any large rocks, while also giving larger clods a good bash to break them down. Then comes the penguin walking, or treading in, which involves putting all of your weight into your heels and methodically taking very small steps across the soil. This presses the soil in and firms it up as when you cultivate, it adds more air into the mix and raises the level.

Once we made our way zig zagging back and forth over the bed once, we used an industrial rake to rake it over again. Unlike soil rakes, which have wider teeth and are primarily used to move material, industrial rakes have very fine teeth and are about 1m wide. They are used to break down clods and gently create a level without moving the soil around too much. Importantly, they also have a long flat bar when you turn it upside down, allowing you to smooth the surface of the soil and create a presentable, flat level.

Buddleja ‘Buzz Velvet’, Hand scarifying and aerating, Preparing bed for lawn planting, seed broadcasting and pretty patterns by Jordan.

Once we had finished leveling off, we began working on broadcasting grass seed. As grass seeds are so fine and accurate broadcasting is important to grow a strong, consistent sward, measurement is key. As per the box instructions, the grass seed had to be cast at the rate of 30g/m². Our prepared beds were 4m², which made it easy to make out these 1m² boxes, as all we had to go was place a stake in the middle of the original square.

To provide even distribution, the broadcasting is done in two passes (i.e.: from North to South, then from East to West). This means that the volume of seed recommended per m² needs to be halved. In our case, this means that 15g of seed will be broadcast per metre squared. When sowing seed, it is important to factor in a certain loss of seed to animal feeding. As such, it is common practice to add an extra 10% on top ‘for the birds’. This made our total seed volume 33g/m² and 16.5g per pass.

Broadcasting seed is best done with broad passes, using your hand or a cup/ container. Try to keep each pass as even as possible and fill in any gaps when making the final pass. When all the grass seed had been broadcast, gently rake over the soil to lightly cover the seeds. The last step is watering, very lightly, to avoid puddling, seeds pooling in one are and the soil level being disturbed. Use a rose adapter on your hose or watering can to diffuse the water and pass over it a few times with a spray, as opposed to a drench.

I had a wonderful day working with my classmates again and having a laugh while we learned some new skills. I can’t wait for our next practical day next month!

The brassica ‘triangle of U’

Today was our first day back at college since we broke up for summer. And by ‘back’ I mean back behind a computer studying from home. I have been missing the learning aspect of college, as well as the day of respite it offers from the working week. Nonetheless, I do miss our classes in person, when we could chat during breaks, wander around the college grounds and learn about plants the best way I know: through touch, smell, sight, sound and, occasionally, taste. Next week, we will get to do this, as we go for a safely distanced day of practicals. I can’t wait.

Today, we covered tonnes of content, from legislation and health and safety to plant nomenclature and how much it costs to plant a hedge (a lot, if you were wondering). One thing that jumped out at me – and by that I mean: confused me – was the concept of the ‘triangle of U’ when talking about brassicas. I’m going to do my best at explaining what exactly that is and hopefully help myself figure it out along the way. Just to warn you, as much as the ‘triangle of U’ sounds like the name of an angsty indie song, it’s actually very scientific and delves into the evolution of plants. So strap in!

The Triangle of U is a theory first published in 1935 and named after the botanist Woo Jang-choon’s Japanised name “Nagaharu U”. This is a theory about the evolution of plants in the brassica family (Brassicaceae). The basis of this theory is that three ancestral brassicas, which were diploids, combined to create three common brassicas, which were tetraploids. Before we jump into the ins and outs of the theory, let’s get some terminology out of the way:

Genome: this is the genetic material in a living organism
Diploid: ‘Ploidy’ refers to the number of complete sets of chromosomes found in the nucleus of a cell. In somatic cells, the chromosomes exist in pairs. This is known as diploidy, and the cells are referred to as diploid (2n). Except for human sex cells, which are haploids (containing a single set of chromosomes), the rest of our cells are diploid, containing chromosomes from each of our parents.
Tetraploid: While diploid cells have chromosomes in pairs, polyploidy (when a normal diploid cell acquires one or more additional sets of chromosomes), means that some cells can reach up to twelve sets of chromosomes. Tetraploid cells have four sets of chromosomes.

The relationship between these brassicas is best shown through the diagram below:

While this may look intimidating, once you break it down, it becomes a lot easier to understand. The AA, BB, CC, AABB, etc. differentiate between the three diploid species (AA, BB, CC) and the remaining tetraploid species (AABB, BBCC, AACC).

After that, it is important to note the ‘n=’. This is simply referring to the number of pairs of chromosomes present. In Brassica rapa (or AA), for example, there is a total of ten pairs of chromosomes. In the tetrapolid cells of Brassica juncea (or AABB), there is a total of 18 chromosomes present, as the number of chromosomes in AA and BB have combined.

What we observe is that the diploid species, or the Brassica nigra, Brassica rapa and Brassica oleracea, are the ancestral genomes, with only set of two chromosomes. When combined, these produce Brassica napus, Brassica carinata and Brassica juncea, species with tetraploid cells.

Since this discovery, an ‘allohexaploid’ has been created, which would sit in the middle of this triangle. It combines the three different sets of chromosomes to create AABBCC.

If your brain hasn’t melted by now, here’s the horticultre element: these derived species make up the bulk of the brassicas you know and love today. Brassica oleracea has many cultivars which produce these favourites: brussel sprouts, broccoli, cauliflower, kohlrabi and more!

I can’t say I have a comfortable understanding of this just yet, but hopefully I can review this in a few months and make a little more sense of it then!