Aroid Anatomy 101

Pursuant to a discussion that Steve (PhotoPro) and I have been having, here's a visual primer on the anatomy of Aroids. I'm using Anthuriums in most of my examples, because I've got detailed anatomical shots of them, and because the one I have many such shots for produces lovely huge leaves that give me lots of room to caption over. Anthuriums present their own special challenges, and I'll detail what's going on that's special when it comes up.

Part 1 - Leaf Anatomy
(Many Aroids, particularly Spathiphyllums, have simpler leaves than this plant; however, in more complex leaves these are the general terms. The tuberous dormant genera, like Amorphophallus, deserve their own treatment, and I'll let Steve explain those because I don't grow them.)

This first pair of photos shows only the Abaxial (top) surface of a leaf. The leaf lobes are only applicable to genera that produce saggitate (arrow-shaped) and cordate (heart-shaped) leaves; in simpler leaves (such as the second pictured), there is only one lobe.


This second photo, which deals with the Adaxial (bottom) surface of a leaf, details a structure unique to Anthuriums - the geniculum. This allows the plant to swivel its leaves towards the sunlight. Everything else shown in this photo is common to other Aroids.


This third photo deals with stem anatomy - namely the point at which the petioles attach to the plant. The Anthurium pictured is hemiepiphytic (grows in soil but attaches itself to trees, and subsists in both states on what it can pull out of the air.) and therefore shows Air Roots. Fully terrestrial Aroids, like Spathiphyllum, don't have these.

Beth, thank you so much for starting this thread. For many years I have felt if people would learn to recognize the basic parts of their plants they could learn so much more about how the plant exists and survives in Nature. I promise not to run away with technical terminology but there are a few things I would like to follow up on.

First, take a look at Beth's photo of the veins of the leaf. There are many vein types in this photo and all have a particular purpose. Up in the upper lobes you will find thicker veins known as the basal veins that mechanically support the leaf. In other words, they support the blade and help to give it the beautiful shape we admire.

All the veins are just bundles of vascular organs that form a branching framework for the purpose of supporting the leaf as well as conducting the transfer of fluids.

In addition to the midrib which is also a vein you should be able to see the primary lateral leaf veins (the larger veins that extend from the midrib to the leaf edge (margin), interprimary leaf veins which are the secondary veins, tertiary veins which are smaller and those that are even smaller. These can be very important to determining the species.

Look closely at the Collective vein in photo two. The collective vein is a submarginal vein (one that is found near the leaf edge) that lies parallel to and near the leaf margin. The primary lateral veins run into the collective vein. The collective vein may be a continuation of a primary lateral leaf vein and is often the lowermost or the uppermost basal vein. The collective vein may encircle or partially encircle the leaf. Collective veins are observed in Anthurium, Syngonium, Alocasia, Colocasia, Xanthosoma some Amorphophallus and other genera. You will never see a collective vein in a Philodendron.

Now look closer at the petiole. The petiole is a part of a leaf. It is not seperate from the leaf. The petiole is the stalk that is attached to both the leaf blade and to the stem. Once again: any single leaf is actually composed of two parts, the leaf blade and the petiole. The petiole is often incorrectly called the "stem" but those are two very different parts of the plant that serve totally different purposes. I would bet Beth will introduce you to the true stem at some part in this thread.

The structure of the petiole is important to the identification of a species since it may be terete (round), square, quadrangular or have other shapes as well as possess lines known as striations, single or multiple canals known as sulci (singular of sulcus, then said to be sulcate) as well as other unique features. A petiole is often incorrectly called a “stem” by plant collectors but the petiole and stem are completely different since the stem is the central axis of the plant and serves a totally different purpose.

Please, if any of this is not clear to anyone, ask! Beth is an excellent teacher and as I suspect she will will eventually also explain how some of these terms get their names. She is highly skilled in the use of Latin and many words used in botany are based in Latin.

One comment on the cataphyll. A cataphyll is a bract-like modified leaf which surrounds any newly developing leaf to protect an emerging blade as it develops. If you grow aroids look at a newly emerging leaf and if their is a leaf-looking bract surrounding that new leaf you are observing,that is a cataphyll. A cataphyll may be unribbed as well as single or double ribbed and in some genera have important characteristics used to determine the species. The cataphylls of some species just dry up and remain on the plant, others drop from the leaf and some (as in Beth's photo) dry to form fibrous filaments. How the cataphyll remains or drops from the he plant is important to the determination of a species.

One last comment and I'm done. Thanks so much Beth for introducing the terms abaxial and adaxial. they simply mean the top and bottom of the leaf but the structural formulation of each can be very different and each can help to determine the species. I understand most won't have any desire to learn or use these terms but they help immensely when anyone is asking a question, especially one that is seeking an identification.

Bravo Beth. Thanks for starting this one. None of this needs to be complicated but any grower will find all of these terms useful. Promise. Besides, your friends will think you have a PhD if you learn to use them correctly. (Well, my kids just think I'm an old geek.)

Steve

Part 2 - "Flower" Anatomy

Now that you've got a handle on how leaves go together, let's look at what makes an Aroid an Aroid - its inflorescence. Most gardeners refer to this as the plant's "flower" - but this is a bit misleading.... Aroid inflorescences are made up of two parts - the spathe and the spadix. These are supported by a special petiole called the peduncle. The size, shape, and arrangement of these three parts is one of the main ways that species are differentiated. Shown here is an inflorescence from a large Anthurium.



And here's a Philodendron. In most Anthuriums, the spathe detatches completely from the spadix, however in Philodendrons (and also in Caladium, Xanthosoma, Colocasia, Alocasia, Dieffenbachia, and a host of others), it remains partially attached and covers the female florets. In this particular plant, thermogenesis is likely occurring, since the spadix has moved forward in relationship to the spathe. (See below under "thermogenesis" for a better explanation.) The covered arrangement of the spathe and spadix around the female flowers and nectaries serves to trap pollen-covered beetles in the area where they'll do the most pollination, ensuring that a good number of berries is set on the plant.



The spathe is a modified leaf that covers all or part of the spadix; in most aroids it serves like a reflector dish, pushing the scent of the inflorescence in one direction, and in some cases also focussing heat (Philodendrons in particular; Steve has some great photos of thermogenesis, the process by which large Philos attract their pollinator beetles.) Many Aroids are pollinated by specific beetles, and the spathe evolved to help the plants attract these insects more easily and effectively. In some genera, particularly Calla, Zantedeschia, Spathiphyllum, and Anthurium (below), the spathe is quite flashy - this has led to the cultivation of the aforementioned plants specifically for their blooms.



The spadix is the main reproductive organ; it is a dense grouping of florets which may be male, female, or bisexual (depending on species.) In most Aroids, these florets are grouped on the spadix according to their gender - hence, in bisexual inflorescences, male florets (pollen producers) normally appear towards the tip of the spadix, whereas female florets (pollen receptors, which eventually become the berries of the plant) normally appear towards the base. There may also be buffer zones of neutral florets (particularly on Philodendrons) - these generally produce attractant nectar or pheromones or both and are purely there to bring in the pollinators.



When the inflorescence has developed sufficiently, it enters the stage of anthesis - basically, it becomes fertile and begins to produce and receive pollen. In some genera, only the male flowers will be active during the first stages of anthesis, and these will become inactive before the female flowers open - this prevents self-pollination. Other genera and species are not so picky. Above is the inflorescence of the same large Anthurium pictured in the general anatomy section, at anthesis - the pollen grains are clearly visible, as are some of the open male florets.

In Philodendrons in particular, a second process accompanies anthesis - thermogenesis. The spadix during this process produces up to 10 degrees Celsius of extra heat, over and above the ambient temperature. To insects, many of which see the infrared spectrum, this lights the inflorescence up like neon. The really neat thing is that during this process, the spadix actually moves, generally away from the spathe, in order to properly focus the heat it's broadcasting.

Personal note? It's pretty cool to watch this process happen, especially if you live in the native range of the plants. Here's an example. When I lived in Puyo, on the edges of the Amazon, my neighbours had a huge cultivated Meconostigma (tree) Philodendron that they had collected from the jungle about 10 km away, and which was of sufficient maturity to produce inflorescences. When it came into anthesis, our house was literally blanketed in these green beetles - they were fighting for a chance to get into the blossoms. I don't have a picture of the plant (their dog hated me) but I do have a picture of the pollinators; they'd hang around for several days on my back steps, completely drunk on Philodendron pheremones. These beetles are about the size of a quarter.

Terrific!

You're giving me one of those botany lessons I wanted! Say - does the vocabulary you and Steve are using generalize to all flowers/plants (I mean, obviously, excepting words like spathe and spadex, which only apply to plants that have them)? Inflorescence, for example?

Part 3 - Habits

Now that you've got a basic idea of the parts of the plant, let's talk about habits - specifically, how these plants grow. There are 6 basic habits for Aroids, which is considerably more than most other families, largely because of the extreme adaptability of these plants.

Terrestrials
Terrestrial Aroids grow out of the soil. Period, the end. Genera that have terrestrial members include (but are not limited to): Anthurium, Caladium, Xanthosoma, Colocasia, Alocasia, Dieffenbachia, Spathiphyllum, Zamiioculcas, Zantedeschia, Calla, Arum, and the tuberous dormant-period temperate Aroids like Amorphophallus, Draconitum, and Dracunculus.

Shown below is Xanthosoma pichinchense, a huge terrestrial Aroid native to northern Ecuador. On the photo, I've clearly labeled the stem (the structure from which the petioles arise) as well as the petioles and leaves. This species forms small trees; this is a younger (and hence shorter) example of it. This plant is from Pichincha province, on the West slope of the Andes.



Epiphytes
Epiphytic Aroids grow on trees or similar supports, without connection to the soil. The word itself literally means "Air Plants." Genera with epiphytic members include (but are not limited to): Anthurium, Philodendron, Monstera, Pothos, and Epipremnium. Epiphytes do not harm their host trees - they're just using them for support to get closer to the sunshine. Most epiphytic members of the Araceae are referred to as scandent (vining) plants.

Shown below is a smaller, unidentified Philodendron. This one is unique for having winged petioles - which, I imagine, are one of the identifying features for the plant. However, I look at so many Philos on any given hike that their names rarely stick in my head for very long. The stem of this plant is barely visible, as it is covered in moss, but it is present, and is held to the tree by means of its aerial roots. This is from Pastaza province, on the East slope of the Andes.



Hemiepiphytes
Hemiepiphytic Aroids are a transitional group - they maintain roots in soil but also share the growth habit of fully epiphytic plants; ie - they climb trees. Genera with hemiepiphytic members include: Anthurium, Philodendron, Monstera, Pothos, and Epipremnium. The hemiepiphytes include both scandent and truncicolous (trunked) plants - Philodendron hederaceum being a good example of a scandent plant, and Philodendron bipinnifitidum being a good example of a truncicolous one.

Shown below is an unidentified hemiepiphytic Anthurium which grows in companionship with Tropical Walnut trees (Juglans neotropica). This photo is particularly good in that it shows a feature common to Epiphytes and Hemiepiphytes - the holdfasts. These are a rootlike structure whose only purpose is to anchor the plant firmly to its host tree. This plant is from Pichincha province, in an interandean valley.



Lithophytes
Lithophytic Aroids grow directly on rock. Included within this category are Anthuriums and Philodendrons; this habit is fairly rare. The word itself means "Rock Plants." Like the epiphytes and their trees, the lithophytes don't harm the rocks they grow on. Most lithophytic Aroids grow in companionship with mosses, particularly Sphagnums.

Shown below is a very small, unidentified Aroid that was found on an Andesite boulder. It's growing in companionship with Sphagnum moss. This plant is from the Pichincha province, in an interandean valley.



Aquatics
As the name suggests, aquatic Aroids grow with their roots fully submerged in water. The most common of these genera are Lemna, Wolffia, and Spirodella (the duckweeds), and Anubias and Cryptocoryne, although arguments have also been made for Spathiphyllum and several species of Anthurium - I prefer not to include them as true aquatics, however - see "Riparians" for more information about the unique growth habit of these plants. Anubias and Cryptocoryne are common plants in the Aquarium hobby, and are notable for being unpalatable to most fish. The Duckweeds are common nuisance plants in slow waterways worldwide.

Shown below is Anubias, with an inflorescence (thank you to Hardjono Harjadi for the photo).



Riparians
Riparian Aroids grow in seasonally flooded soils or along the banks of running water. Included in this category are several species of Anthurium, the Spathiphyllums, some Dieffenbachias, and some members of Anubias and Cryptocoryne.

Shown below is a riparian Anthurium, which I am currently working with Dr. Croat to resolve. It may be an example of his 100,000th collection, or it may be a new species entirely. This plant grows with its roots seasonally submerged in the Rio Nambillo; it's from Pichincha Province on the West slope of the Andes.

The only plants I can recall other than Aroids that produce a spathe and spadix are the Peppers and Peperomias....

Inflorescence applies to plants that produce multiple flowers on a single "stem" (in this case, the correct term is rachis) - Delphiniums are a good example, as are Bananas and a good many other plants as well.

The leaf anatomy stuff definitely pertains to other plants, as do the terms describing habit. And when I get around to the terms that describe stem structure and leaf shape, these are also usable with other plants. For example, many Anthuriums have cordate leaves (heart-shaped), and this is a completely applicable term for plants like Morning Glories.

Generally yes Ann. Many species have specific terminology but in most cases the terms, "petiole", "leaf blade", "stem" etc are universal. Where the term "peduncle" describes the support of the inflorescence of an aroid, with a Bromeliad that support is known as a "scape".

Beth, another great post on pollination. I will post later on part three.

Rather than add any comments I will simply give a link that elaborates on the items you have explained. This was written largely by my close friend Julius Boos who is now near death with pancreatic cancer. Once Julius completed the basic work he asked that I add additional input which I gladly did but I prefer the ownership of the article remain with him.

For any that are members of the International Aroid Society, our oldest living founding member, Tricia Frank, died two nights ago of the same condition. I have been answering email from around the country and globe today about Julius' health and will be calling him once again in about one hour.

Here is the link one the photos of an aroid at thermogenesis are included. If anything is unclear I'm sure Beth or I will be able to explain. How Mother Nature has chosen to keep all the aroid species pure as well as help to prevent cross pollination in the wild is a fascinating subject I have been studying for over two years.

http://www.exoticrainforest.com/Natural and artificial pollination in aroids.html

By the way Beth, you still owe me seeds from the incredible plant with the purplish spadix!


Steve

Oh no! I didn't even know that Trisha was ill! How terrible.

Steve, be assured that as soon as I find one with seeds, I will definitely be sending them off to you! The last time I found one with an infrutescence, it wasn't mature, and by the time I got back something had eaten it. (I suspect Brocket Deer.)

And while we're on the subject of inflorescence supports, here are a few more family-specific terms.

Agaves - Mast
Orchids - Rachis
Bananas - Rachis or sometimes also Peduncle (rachis is used on species that produce pendant bunches, and peduncle on species that produce erect bunches).

Beth, both Julius and Tricia developed pancreatic cancer about the same time. Tricia was very quiet about her condition and we literally had to "pull teeth" to find out what was happening. She instructed everyone not to talk about it.

You are more adept with orchids even though I have studied them for 20 years. What is the scientific term for the spike that supports the inflorescence? Is that what you described as the rachis? I am editing a book and the writer included some orchid species and since he is French and English is his 6th language (he uses all of them in his text) I've been trying to learn the correct name for the spike even though he always calls it the "stem" which I know is not correct.

Steve

Steve, with mutliple-blooming orchids, it's a rachis. On single-bloomers, like the Paphs, it's a peduncle. How's that for obscure terminology?!?

Certainly is, but it answers what I need to know. All the orchids in the book only include plants with a rachis. I'll change all of that in the editing tomorrow.

Thanks!

Steve

Once again Beth, a great job of illustrating terms that growers sometimes read but largely pass over because they sound too complicated.

Just to expand briefly on Beth’s great photos and post.

A stem is the base or central axis as well as the main support of a plant and is normally divided into nodes and internodes. The nodes often produce a leaf in the same axils from which they produce roots as well as hold buds which may grow into shoots of various forms. The stem's roots anchor the plant either to the ground, a tree or to a rock. A stem may even spread as a repent rhizome creeping across the soil) but may either grow above ground, underground or partially above the soil. The stem may also be a tuber and grow completely underground. Despite common posts on a variety of websites, no aroid grows from a corm or bulb, only a tuber. A tuber is little more than an underground storage unit containing starch that also stores moisture.

The “winged petioles” are simply petiolar sheaths. A petiolar sheath appears to be “wings” or extensions at the sides of the petiole that may envelop the newly developing leaf blade of the successive leaf. In most cases the petiolar sheath drops from the petiole once the next leaf emerges. You can often see these on young Monstera species but also on Philodendron species to some extent.


Hemiepiphytes are divided into to forms. A primary hemiepiphyte begin as seeds in the droppings of a rain forest inhabitant that germinate on the trunk or limbs of a tree as does an epiphyte. They then develop long roots that eventually reach the ground and in this way they attain height in the forest as quickly as possible in order to reach brighter light. Monster species are almost always hemiepiphytes.

Secondary hemiepiphytes start their lives in the soil or on a tree trunk near the ground with roots to the ground to gather additional nutrients. They then climb the host where they morph into the adult form and sometimes completely lose their connection to the ground. Again becoming an epiphyte. As Beth said, epiphytes never have a connection to the soil.

There are a couple of forms of semi-aquatic aroids: rheophytes and riparians.
Rheophytes include semi-aquatic as well as aquatic plant species which live attached to stones in fast moving currents. The force of the fast moving current creates oxygen-rich water so the rheophytes are able of living in this environment due to their streamlined leaves. These streamlined leaves produce little resistance to the flow. They possess extremely strong wide spreading root systems that steadfastly hold the plant in position. Rheophytic plants include some Anthurium species. Riparians are plants that grow semi-aquatically in the region separating land and a stream. Araceae that grow as riparians include Arisaema, Xanthosoma, Philodendron, a small group of Anthurium species, Spathiphyllum and others.

Beth, drop me a private note about Tom's 100,000th collected plant.


Steve

Beth and Steve -

Thank you both so much for this amazing thread! It's better than a classroom! (High praise from a former teacher!) Hope I get to see all the rest of it, and more! OMG - and DUCKWEED?!? Aroids are really incredible; it's no wonder you find them fascinating!

Well, Duckweeds have been under review for some time now; they were formerly grouped in their own genus (Lemnaceae), but microscopic studies of the plants in bloom as well as genetic assessments have moved them back into the Araceae, albeit in their own special section. Under APS II, they're Aroids. This means that the plants with the world's largest inflorescences (Amorphophallus titanum) and the smallest-bloomed ones as well (Wolffia spp.) belong to the same family. Ain't nature grand?

Two months ago I was walking (really rolling) through the gigantic Missouri Botanical Garden Climatron with Dr. Croat and 30 other IAS members attending a conference at MOBOT. We were talking about inflorescences and at that moment were right beside a small pond. Tom's assistant leaned over and plucked a miniscule inflorescence from a pond filled with water lettuce. Tom challenged me to photograph it so I sat there for 20 minutes to see if I could do it. The entire inflorescence was 1 inch tall.

I'm certain I will never photograph the inflorescence of Duckweed but this one is pretty small all by itself. This is Pistia stratiotes from two different angles.

Steve

Part 4 - Stems

Here's a couple of photos illustrating the basic structures that Steve has detailled for us. The plant shown is a specimen of Dieffenbachia tarabitense in my own collection. (Ignore the other plant; it's a volunteer Chusquea nigra bamboo - there must have been a bit of inactive rhizome in my planting mix.) I think this plant is a particularly good example of what we're talking about, largely because the petioles are strikingly different from the stem.

Hi Beth and Steve,
Is the point at which you are refering to as the leaf axis also the auxillary bud? As I understand it the leaf axis is the node with auxillary buds forming just above the node.

Thanks for this thread!

Scott