Help an amateur understand succulence

As I understand there are basically 3 of ways to be succulent, and I am trying to figure out the range so I will be mixing in some caucidiform and pachycaul and maybe some non-succulents to understand this tribe.

Stem succulents- columnar cacti, Baobab trees, Aeonium.

Leaf succulents- Schlumbergera, Opuntia, Hoya, Harworthia, Mesembs.

Geophytes- Underground succulents- Several species of Opuntia, Diascoria, Leuchenbergia principis, Sinningia coum.

Many plants are a combination of more that one of these forms, but they are the basic bunch. And in this group, several plants are succulent in more than one way.

But I wonder about groups such as Ledebouria, Haemanthus and other bulbs which are listed often on succulent lists. Why not tulips, Iris, and Lilies? I mean these are just geophytes? Right? Or is something else going on here that defines succulence in geophytes, water holding during dry season, use during blooming/growth season?

Rob

 

All the major botanical dictionaries describe succulent as fleshy, juicy or a fleshy plant. Succulents such as cacti store water in parenchyma cells and in water swollen stems and leaves. Parenchyma is the primary tissue of most plants which is composed of the thin cells which form the bulk of leaves, roots and other plant parts. Obviously, that leaves the "definition" wide open since it basically means a succulent stores water and many plants store water in parts including bulbs, corms, tubers, rhizomes, stems, petioles and leaves.

So what is a bulb? A bulb is an underground storage structure which stores starches and water that is a condensed stem usually with a basal plate and fleshy storage leaves surrounding the bud that will form the next plant. It is composed of thick modified leaves arranged in layers that are used for food storage.

If you want to see the inside of a bulb slice open an onion since an onion is a perfect example of a bulb. A bulb is a type of stem and this stem type occurs in many plant families

The stem of a plant is the central axis or base of the plant and not the stalk that supports any single leaf. The stalk that supports a single leaf is known as a petiole and is a part of the leaf. A stem is normally divided into nodes and internodes. A node is the point on the stem where buds produce leaves, cataphylls, and usually roots. The intervening regions between the nodes are called internodes. The stem's roots anchor the plant either to the ground, a tree or to a rock. A stem may also be known as a corm, tuber or rhizome which is a stem that runs either along (repent) or just beneath the surface of the soil.

A corm is an underground stem to which the above-ground parts of the plant may die back in the dormant season. It often stores starch and when it regrows foliage will come from the top and roots from the base like a typical stem. My friend and scientist Christopher Rogers wrote the following to further describe a corm: "a corm is composed entirely of stem tissue. It is literally just an underground stem. It has an epidermal layer, a vascular cylinder with phloem and xylem and central pith. A corm can also be a starch storage organ, but it still has true stem tissue. This is why a corm has the new foliage growth coming from the top and the roots coming from the base. Corm examples are Crocus, Cyclamen and Gladiolus.”

A tuber is a strongly condensed stem as well as an underground structure which is almost entirely a starch storage organ. The buds for future growth and the roots all develop at the apex (top) when the tuber sometimes forms as the tip of a stolon. Stolons are stem runners or stem shoots that run atop or just under the ground from a plant. Stolons possess the ability to produce new plants from the buds along its length or at the apex (tip).

Christopher explains further, “A tuber is just parenchyma (with some vascular tissue). It has an epidermal layer with some subdermal vascular tissue, and all the rest is parenchyma.”

He continues with his explanation of a tuber, “It is almost entirely a starch storage organ. This is why the foliage and the roots all come from the top. Most plants with tubers have them borne on stolons, but that is not necessary. In Amorphophallus, Arum and Typhonium for example, the stem tissue is all encased in the small bud at the top of the tuber. That bud grows upward into a leaf or two, and outward into roots, with the tuber beneath. Other tuber examples are potatoes and Sinningia." Xylem is a network of hollow cells found in a plant's vascular system that transports water and soluble nutrients collected by the roots.
Pacific Bulb has a forum and there is a brief discussion regarding Ledebouria on their website. Some of the scientific sites say Ledebouria is in the family Liliaceae which is different than is stated on Pacific bulb. Bulbs are technically different than a corm or a tuber.

I would suggest if you don't find a good answer here you join their forum and ask your question there since I know of several Pacific Bulb members that are highly qualified.

Since I'm not familiar with the plant families you mentioned I can't comment as to the precise answer you are seeking but part of the answer would lie in the exact type of stem each species possesses. You may also want to consult the Cactus and Succulent Society of America website in addition to Pacific Bulbs to learn more.


Good luck finding the answers you need.

 

It is likely convention that defines what some people may or may not include as succulents, geophytes, etc. I'm sure if we dug back into the literature, someone, somewhere along the way declared such and such aren't proper succulents because of such and such a reason. Do note that while terms such as succulent and geophyte appear in biology textbooks, they are descriptive -- but not definitive or wholly objective. There will always be species that can or cannot be included within these groupings depending on one's interpretation of the terms.

As you noted,

People use terms such as succulent and geophyte as symbols, or boxes, if you will. Slotting the width and breadth of biological diversity (in this case, of growth habits) into only a few boxes causes some of these conundrums. If you were to expand the number of terms (increase the numbers of boxes), you may eventually come up with a system that is all-encompassing -- but perhaps will have a tenth? quarter? half? as many terms as there are species to slot them in. Given the number of species, the system would be unwieldy and possibly useless.

So, the terms are useful tools -- most of the time, broadly speaking.

 

I'll also add -- it looks like you've read this article by L. Maynard Moe: What is a Succulent? via The Bakersfield Cactus & Succulent Society...

 

A quick response as a result of Daniel's post.

My apologies, after going and reading Mr. Moe's article in full I realized I had misunderstood some of Daniel's quotes so the bulk of this post has been removed.

There are however at least a couple of members of the aroid family that may appear to differ from some of Mr. Moe's conclusions. One would be Zamioculcus zamiifolia which is considered a succulent in aroid botany. That species produces bulblets or tubercles at the juncture of the petiole and stem which fall to the ground during the dry season to eventually produce a tuber. In the text The Genera of Araceae by botanists Simon Mayo, Peter Boyce and Josef Bogner the plant is referred to several times as a succulent. That species can survive months without water due to its ability to store water long term.

Overall I would say Daniel's observation is quite accurate. The differences are often perceptions by growers, not science.

 

And that, sir, was exactly the article I was looking for.

 

Thanks for the explanation of bulbs and corms, especially. It was very interesting. I admit to being ever so slightly obsessed with crocuses...