The Biosphere Blog

What is a Fruit?

A fruit, botanically speaking, is the seed-bearing structure which develops from the ovary of a flowering plant. Last week was a brief introduction to the diverse categories of fruits that exist — with maybe a few surprising revelations of berries which aren’t really berries, nuts which are really seeds, and seeds which are really fruits!

This post is specifically about the jujube fruit.

Jujube Fruit Classification

The jujube fruit arises from a single ovary, is fleshy with a thin fleshy skin, and contains a hard central ’stone’, or ‘pit’ (the botanical term for this stone is pyrene). This pyrene is not the seed, but a hard structure containing the seeds, and in the case of jujubes will have two seeds. The fruit is indehiscent, in that it doesn’t split open on maturity to release the seeds.

Al these characteristics classify a jujube fruit as a drupe.

The Flower Ovary

This overview of a typical flower’s anatomy along with this Encyclopaedia Britannica diagram on the plant ovary specifically will hopefully make the following easier to follow.

The photo below is from Photo Journal: Anatomy of a Jujube Flower, and shows the location of the single ovary in a jujube flower:

The jujube ovary is a superior ovary, in that it sits above the receptacle (the thickened part of a stem from which a flower grows). (Fun fact: it is the receptacle of a strawberry that develops into the edible ‘fruit’ part — the little ’seeds’ on its exterior are the real fruits, each an ovary containing a real seed!)

The following stylised diagram illustrates this superior position, along with the two other ovary types of half-inferior and inferior:

Drupes typically develop from flowers with a superior ovary.

The photo below is of three Chico fruits at different stages of development, as shown by the enlargening ovary when moving anti-clockwise from the youngest at the top left to the oldest immediately to its right:

Here is a very young Shanxi-Li fruit dissected to reveal a cross-section of the maturing ovary:

Let’s explore this anatomy further!

Anatomy of a Fruit in General, and a Drupe Specifically

All fruits are comprised of two main parts, the seed(s) and the pericarp.

The seed is made of a seed coat, which contains the embryo (fertilised egg) and a food source (the endosperm) to sustain the embryo during germination.

The pericarp develops from the ovary wall, and is the surrounding tissue which protects the seed(s). It is made of three layers: the inner endocarp, the middle mesocarp, and the outer exocarp (also called the epicarp). These layers are not distinguishable in dry fruits, but are very clear in the fleshy fruits.

How the pericarp develops in the fleshy fruits depends on the fruit type, and in a drupe, the endocarp forms the hard seed-containing stone, the mesocarp becomes the fleshy edible ‘fruit’ part, and the exocarp develops into a thin edible skin.

The illustration below is of a peach — also a drupe — and shows the typical anatomy and distinct layers of the pericarp in this fruit type:

Anatomy of a Jujube

The photo below is the same jujube fruit as above, but with its anatomical components labelled:

 

With time the two embryos will develop into two seeds — or one or none if either or both fail. The endocarp will continue to harden and thicken into a very tough stone regardless of the embryos’ development, and it is not unusual to crack open a jujube stone (with difficulty!) to find anything from nothing, to two plump seeds, with all variations in between! The stone is very similar to an olive’s, in size, colour, and with the same distinct pointy-edged ovoid shape.

The mesocarp, green in the above photo, will continue to enlarge and lighten, then gradually sweeten and change colour from a fresh apple-like whitish texture to a more chewy red-brown date-like texture as it continues to ripen, lose moisture and dries. The surrounding exocarp will lighten to a yellow-green colour as the fruit ripens to fresh-eating stage, then takes on a red-brown date-like texture as it ripens further to the dried-eating stage.

Fruits

Bananas, peaches, apples, watermelons, and yes, jujubes are all immediately known and recognised as ‘fruits’, and if asked what makes a fruit a fruit, one may perhaps use words such as ‘food’, ‘juicy’, ‘delicious’, and ’sweet’.

But pumpkins and tomatoes — commonly called ‘vegetables’ (not a botanical term) — are also fruits, albeit savoury ones. As are cumin and caraway ’seeds’ (really the dried fruits containing the true seeds). And hazelnuts and chestnuts are also fruits! (’Nut’ has a specific meaning in botany though it is used in common speech to include the culinary ‘nuts’ such as pistachios, almonds, cashews and walnuts, which are all seeds.)

So what exactly is a ‘fruit’?

‘Fruit’, Botanically Speaking

A ‘fruit’, to a botanist, is simply the seed-bearing structure which forms from a flower’s ovary. And only flowering plants, the Angiospermae, produce ‘fruits’.

Going further, the word ‘fruit’ commonly implies ‘food’, but this meaning does not apply in botany — Camellia spp. flowers produce fruits, as do potato flowers, but neither of these feature in our diets. Potato fruits are actually toxic and should never be consumed!

And my, how such a simple definition belies an incredibly diverse range of seed-bearing structures! Let’s explore…

Fruit Types

There are three broad categories of fruit types: simple fruits; aggregate fruits; and multiple fruits.

Simple fruits arise from a single ovary in a flower, and examples include jujube, peach and pea.
Aggregate fruits arise from multiple ovaries within the one flower, and examples include strawberry, raspberry and blackberry.
Multiple fruits, also called false or composite fruits arise from all the flowers within an inflorescence (flower cluster), and examples include pineapple, fig and mulberry.

Fruits can be further categorised as dry (eg cumin and caraway) or fleshy (eg jujube, apricot, tomato).
Fruits can also be categorised into dehiscent (split open on maturity to release the seeds, eg legumes) and indehiscent (don’t split open on maturity, eg chestnut).

Dry fruits can be categorised further into several categories based on very specific botanical criteria way beyond the scope of this post to describe in detail. Two recognisable names are legume (eg pea) and nut (eg hazelnut). More obscure ones include capsule (eg Brazil nut), samara (winged seed, eg maple), silique (eg cabbage) and achene (eg strawberry — yes, the strawberry is a dry fruit! The fleshy part is not the ovaries, but the receptacle. The ovaries are the ’seeds’, or achenes, on the outside, which contain the seeds).

There are more categories still: follicle, caryopsis, cypsela, schizocarp, …

Fleshy fruits also can be categorised further into a few categories based on very specific botanical criteria. Such as berry (eg grape), drupe (or stonefruit, eg jujube) and pome (all of the family Rosaceae, containing a ‘core’ surrounded by a tough membrane, with seeds inside the core and the fleshy ‘fruit’ external to the core, eg apple).

Aside: If you ever fancy daily headspins, please do consider studying botany! Did you know that strawberries and raspberries are not botanical berries? A botanical berry is a fleshy fruit without a stone, from a single flower with a single ovary. Meaning bananas, tomatoes, grapes, cucumbers and many other fruits are … berries!

Fruit Development

Please do take the time to review the excellent Encyclopaedia Britannica diagram on the plant ovary, which will make the following easier to understand.

Each plant ovary contains an ovule(s) — an unfertilised egg essentially. A pollen grain landing on a flower’s stigma germinates and forms a pollen tube, which joins with one ovule. Two sperm (which were in the pollen grain) enter the tube — one enters and merges with the ovule’s nucleus to form a zygote (a fertilised egg), and the other merges with two polar nuclei to form the endosperm (food source for the future-germinating embryo). (Any geneticists reading this will recognise immediately that the endosperm is triploid — and this is normal! More botanical headspinning stuff!)

The ovule is now a fertile seed, and the ovary, depending on the species, will mature into either a dry or fleshy fruit which encloses those seeds.

The ripened ovary wall is called the pericarp.

In fleshy fruits, the pericarp becomes the edible tissue surrounding the seeds, and has three distinct layers: the endocarp, mesocarp, and exocarp (also known as the epicarp).

In (botanical!) berries and drupes, the endocarp directly surrounds the seeds, the mesocarp is the fleshy middle layer, and the exocarp is the outermost skin.

The fleshy fruit of Citrus spp. is a modified berry type called a hesperidium, in which the mesocarp and exocarp form a distinct, separable peel. Another modified berry type, the pepo, has an inseparable rind, and cucumber fruit is an example of this.

The endocarp of pomes is of a cartilaginous appearance, and the layers within the pericarp of dry fruits are not distinguishable.

---

I hope you enjoyed that little crash course in fruit types! Next week we shall cover their fleshy drupe type in a bit more detail — this is a blog about jujubes after all!

Around 7pm the other evening I was perusing my trees, and — as I often do, gently pulled down a fruiting branchlet above to look more closely with my loupe at the flowers and developing fruits along it. To my absolute horror it came away in my hand! I reconciled myself by thinking that maybe it was structurally weak and always destined to fall off, and decided to make good of the situation by writing about it!

So here it is, the anatomy of a Shanxi-Li fruiting branchlet!

This was a large branchlet, about 385 mm along a straight line, but closer to 405 mm long, as measured by following every bend of the stem with a piece of string and then measuring the string. It was so noticeably large (most in my experience tend to be under 300 mm long) that I really do wonder if it was structurally weak and would have snapped off eventually as the developing fruit on it grew larger and heavier?

As with all new jujube branch growth during a season, the oldest part of this branchlet (which was closest to the fruiting mother branch) was turning red while the youngest part of the shoot was still the bright green of new growth.

The proximal end (of anything) is that closest to the point of origin or attachment. Here, the point of attachment of this fruiting branchlet was the fruiting mother branch. Conversely, the distal end (of anything) is that furthest away from the point of origin or attachment:

Here is where the point of attachment was:

And here is the proximal end of the branchlet, with a transverse (crosswise, cross-sectional) view of the point of detachment:

A node is the point along a branch from which leaves and other branches grow. An internode is the interval between two nodes.

The closest node (and leaf) to the fruiting mother branch was 15 mm away. The largest internode, and the third along from the fruiting mother branch, was 40 mm long. The internodes were then spaced at 30 mm intervals, then 25 mm intervals, and finally the last internode was just 3 mm long, but with still developing leaves, and would have lengthened by season’s end. The penultimate internode was 10 mm long.

The leaves are alternate, meaning there is a single leaf at each node. The leaves alternate sides along the branch, because the nodes alternate sides, hence the name. (The two other leaf arrangements defined in botany are opposite and whorled.)The largest leaf on this branchlet was 90 mm long and 55 mm across the widest part. The smallest leaf was 35 mm long and 15 mm across its widest part. The very small, still developing leaves at the distal end were just 5 mm long.

The flower arrangement (inflorescence) at each node is a simple cyme. A cyme is a group of flowers in which the oldest flower occupies the end of the peduncle (the main supporting stalk, or main axis), and newer shoots come from the sides of that stalk.

As the flowers differ in age within a cyme, so too do the fruits which develop from those flowers:

And as flowers (and fruits) differ in age within a cyme, so too do they differ in age along a branchlet. Those at the fruiting mother branch (proximal) end are oldest and those at the (distal) tip are youngest. Here, these cymes which were closest to the fruiting mother branch have no flowers anymore, but do have the largest fruits. Note too the change in colour of the branchlet when moving from the proximal end towards the distal end:

The cymes at the very tip of the branchlet have the youngest flowers of all — minute buds which have only just begun development:

The cymes between these extremes occupy a gradient of mostly fruit and some flowers, to some fruit and mostly flowers, to mostly flowers and some buds, to mostly buds and some flowers. Note too the colour change along the branchlet. You can tell that the banchlet segment in the top photo below is closer to the fruiting mother branch than the segment in the lower photo below, as it is more red at the proximal end:

Fruiting branchlets are the only branches on a jujube tree to produce flowers and fruits — so writing about one allowed me to slip more botany in than I otherwise could have with another branch type!

To go further and discuss the flowers and fruits on a fruiting branchlet really require their own posts to do those topics justice. I covered Photo Journal: Anatomy of a Jujube Flower earlier, and fully intend to do one on fruit later, but here is a good place to wrap up this post with, I guess, a teaser for what will come!

The following photos are of the largest fruit on the branchlet discussed here. This fruit was the most proximal and 8 mm long.

Do revisit the flower anatomy post — this page may help too — and see if you can work out what the two little brown dots on this fruit’s distal end are. (Distal in this context refers to the end of the fruit furthest away from its point of attachment, and not to be confused with the distal end of the branchlet discussed above.)

What about now?

Yes, those are the two stigmata and the branched style of the original flower! The green fruit you see is the maturing ovary of that flower. But more on that to come later!

The proximal end of the same fruit reveals the sepal remnants:

Let’s (almost) bisect it. I say ‘almost’, as a real bisection would have had two equal halves, each with that delicate little peduncle cleaved cleanly in two along its length. Sorry but that was never going to happen!

Just looking at this you may well be able to make out which parts will become the seeds, the stone that contains the seeds, and the fruit itself? This, and what happens as the fruit grows, is definitely the topic of a future post!