Having studied and examined numerous cacti over decades I’ve made numerous observations about cactus biology and anatomy that appear to be missed or ignored by the experts today. In fact, several misconceptions about cacti are found in circulation. I’m going to address a number of them here.
First, let us start with the most obvious aspect of cacti, the leaf sheath. Cacti did not lose their leaves, they fused their leaves into a sheath-like structure surrounding and covering the inner stem. More on this in a bit, but first lets address the stem of cacti.
In woody plants, of which cacti are an example, the vascular tissues which conduct water, called xylem are rather similar to those of other plants. The other vascular tissue, known as phloem, is also similar but it has a major difference, rather than this tissue being just underneath the outer surface of the stem as it is in most plants in cacti it is found deep inside the plant as a ring. Extending from this ring are individual tracts of vascular tissue that reach out to the ribs and areoles of the plants. These little veins, so to speak, are often called vascular traces. There is a concept that cacti evolved this unlike other plants which lack them, this is incorrect. All woody plants have such vascular tissues, in their leaves as leaf veins. Cacti have not developed radically new tissues, rather they have the same tissues as other plants and they are just slightly modified by evolution.
As mentioned previously, cacti did not lose their leaves, rather they fused their leaves. This resulted in a sheath like structure that covers the stem, leaving the primary vascular tissues deep inside the plant. Extending from this tissue are vascular traces of leaf veins. The leaves themselves have become flattened and wide and fused together. What we see in terms of green tissue, in ribs and other structures like tubercles. These cover the stem and so are easily confused with stem, but they are in fact leaf structures formed by abaxial tissue. In fact cacti, like many other woody plants, have stomata on their abaxial surface, the abaxial surface is what is normally considered to be the bottom of a leaf. The adaxial surface on the other hand is what is typically considered to be the top of the leaf.
In cacti the adaxial tissue or the upper surface of the leaf is not lost, rather it has become highly specialized into structures called areoles. These lack stomata and chlorophyll and are densely covered with various types of trichomes. In cacti several types of specialized trichomes exist, much as they do in other plant families, only in this case the tissue that bears them has become concentrated even as many of the trichomes in many cactus species became enlarged and pronounced. Among these diverse structures include long soft thread-like hairs, fuzzy felt-like smaller hairs, cotton like tufts of hairs and needle like trichomes called spines.
Many trichomes in many plants are secretory trichomes which essentially secrete sugars or other molecules. In cacti this is well known and spines often contain and express these sugars, which in this case are known as non-structural sugars. Those familiar with botany are aware that all cellulose is actually made of sugar, but these bonded sugars are considered structural molecules rather than as sources of energy. Spines are diverse in form, but they are not unique, rather than cacti evolving something entirely new they modified something that many woody plants have, in many species of cacti the trichomes became enlarged and pronounced but are still trichomes of the adaxial surface. In cacti this adaxial surface has tissue called the vascular trace. that supply it with nutrition from the phloem, just like in every other leafy plant, only we call them leaf veins in other species. Neither spines, nor areoles nor the vascular traces are radical inventions of cacti, rather they are slightly modified versions of structures found in numerous other woody plants.
In cacti the leaf structure is hidden in plain sight.
What are often confused as leaves in cacti are in fact mucro, pointed projections that many plants bear at the tips of their leaves. In cacti these tips are often too small to be seen with the naked eye, but in some cases are easily visible. These are not actually leaves but are leaf tips. They often demarcate the former boundary between the adaxial or upper surface and the abaxial or lower surfaces of the leaves in cacti. As mentioned the areole is itself the adaxial surface. This is easily seen when one examines the meristems of cacti and in many cases can also be seen in young growing branches of species that have large mature forms. See attached images below for examples of the latter.
Spines are often confused as being a type of modified leaf and areoles are often confused as a modified abscission layer. Yet in no plant family are leaves found that grow in the manner of spines, nor that have the fibrous cellulose structure of spines. In numerous families on the other hand are found trichomes that correspond to spines in terms of structure, location and growth, only in those families the trichomes are just smaller. Cacti have enormous trichomes, and why wouldn’t they? In some cases cactus meristems are literally many thousands of times larger than those in other families. Yet in all families apical meristems make leaves but do not make trichomes, which grow from meristems at their base, as do spines. In cacti the spines are also not formed by the primary meristem, but grow as they do in cacti, from specialized cells that produce them after the leaf has formed. Rather than leaves undergoing a radical series of evolutionary changes to become spines, cactus just evolved to have, in many cases, enormous trichomes. In fact, rather than being novel original structures formed from modified leaves the structures of the areole, be they spiny, woolly, felt-like or hairy are all forms of trichome with equivalents in other plant families
The leaf unit of cacti varies to some degree, but they are formed the same way by the meristem.
The increment of a leaf in cacti still has the basic parts that they do in other plants, including an adaxial surface, an abaxial surface and in many cases a pointed tip or mucro. In many families as well, trichomes are multi-function and include needle like structures that can protect the plant mechanically, as well as thread and felt like covering that shade, insulate and shelter the plant Additionally the trichomes can be glandular or secretory and form close evolutionary relationships with endophytic organisms as well as higher life forms like insects. Ant and wasps are commonly known to feed on sugars from large and developed secretory trichomes in many plant families. Cactus spines don’t just have the same way of growing and the same structure as trichomes in other families, in many cases the cactus trichomes aka spines perform the same types of functions that smaller trichomes do in other families. They can secrete sugars, shade the plant, provide mechanical protection and more!
The anatomy of a cactus reveals that the areole is an adaxial surface both in terms of microscopic features of the newly formed cells at the meristem as well as in terms of the shape and structure of larger tissues produced by the meristem.
Because areoles are the adaxial surface of the leaves of cacti, but have been reduced and concentrated into dense structures by evolution, they are adjacent to the dormant meristems where plants branch, thus cacti appear to branch at their areoles, yet upon further and closer examination the growth points of the lateral buds are not the areoles but are adjacent to them, just above and behind them. Thus when a new branch begins it emerges near the areole from inside the plant it appears to be bursting forth from inside the plant rather than growing out of the areole as a projection the way leaf buds do in nearly every other woody plant. I am aware of no other plants have new branches bursting forth like this. Likewise, no woody plants bears bud scales or abscission layers that produces leaves that are differentiated into central and radial forms, nor does any woody plant produce leaves from basal meristems at the abscission layer, rather they form a new lateral meristem where leaves are formed in the normal way and then enlarged. It makes very little sense to consider cactus spines a type of modified leaf or to consider cactus areoles a type of modified bud scale or abscission layer.
I am including a number of images illustrating the various anatomical facets of my observations.
In the first image A represents the mucro, here plainly visible. This is the leaf tip. Above it is the adaxial surface or top of the leaf, bearing the specialized trichomes or spines. In the image B represents an entire leaf structure or increment.
In other images one can see that areoles are the tops of leaves, formed in the same location and way that the adaxial surface is in other families. One can see that the mucro or leaf tips are part of the rest of the increment and the areole has adaxial identity.
