Cactus? Look again!

Aloe erinacea superficially resembles a 
cactus, but closer examination reveals 
that the plant consists of closely-spaced,
spirally-arranged succulent leaves with
spines along the edges. In cacti, leaves 
are done away with altogether, or adapted
as spines.

 The picture at the right is a member of the genus Aloe. We all know the most common member of this genus, Aloe vera,  grown as a garden ornamental, as a source of skin ointment, or for its edible leaves. The species pictured, Aloe erinacea, with its compact, rounded overall shape and prominent spines, superficially resembles a small barrel cactus. Aloes and cacti are both succulent plants adapted to survive arid conditions by storing water in their tissues. In cacti, it is the stems that are modified to store water, while in aloes, it is their leaves. A careful look at this plant reveals that it does in fact consist of a compact series of spirally arranged leaves.  

When unrelated organisms come to resemble each other, it is an example of convergent evolution - similarity due to common adaptation, but from very different ancestors.

I've spoken of this a number of times on this site, as succulents in general are a spectacular example of convergent evolution and the all-important process of adaptation. I've been retired for a number of years, but if were to go back into the biology classroom again today, I would walk up to the front and write ADAPTATION on the board (or powerpoint screen!). I would then proceed to show how everything we see in organisms is a result of this process, which unites genetics, ecology, evolution, and systematics.

Cacti and aloes have very different evolutionary histories leading to their convergence. In this post, I want to emphasize the historical dimension of adaptation. The current functional features of a plant have histories of gradual change, sometimes adding or improving functionality, sometimes changing the function altogether. Leaves themselves underwent extensive series of adaptations in early plants just to become the flat, efficient light-gathering antennae that we know today, and similar series of adaptations happened multiple times in different groups of plants. the very leafiness of leaves itself is convergent in true mosses, clubmosses, ferns, different groups of seed plants.

 In various lineages of plants, leaves were further modified into the parts of gymnosperm cones, the parts of the flower and other reproductive structures. Leaves have also been modified through adaptation into grasping tendrils, sticky insect-catching traps, and other specialized structures. In cacti, leaves disappeared, or were converted into spines, as the stems adapted simultaneously for photosynthesis and water-storage. In aloes, leaves retained their photosynthetic function, while adding water storage.

Both cacti and aloes thus came from "normal" plants adapted for less arid conditions. Among the nearest non-succulent relatives of cacti are carnations, and of aloes they are daylilies, asparagus, and amaryllis. Cacti are eudicots, which typically have prominent stem systems and relatively small leaves. Aloes are monocots, which typically have condensed, inconspicuous, and mostly underground stems, but prominent, elongate leaves. It was "easier" for cacti to adapt their already exposed stems for water storage, but for aloes it was easier to add that function to their leaves, than to redesign their underground stems.  So in adapting to new conditions, organisms modify what they already have, in the simplest way ("along the lines of least resistance").

So in considering the process of adaptation, we must keep in mind that organisms adapt to new or changing conditions by modifying pre-existing structures. Developing new organs from scratch happens rarely, if ever. In plants, stems and leaves have been the most plastic of organs, forming a wide variety of distinctive adaptive organs.

For some zoological examples, take the wings of birds. These highly specialized flight organs evolved from the front legs of their non-flying ancestors, radically changing their function.  It happened separately in flying pterosaurs and in bats, using arm and hand bones differently. It did not end there with the birds, for wings went through another transformation in penguins, from flying organs to swimming organs. In snakes,  legs disappeared altogether or remained as a set of tiny useless bones buried within their muscles, what we refer to as vestigial organs. The same thing happened in whales, where the front legs were modified into flippers, while the hind legs were reduced to buried vestigial structures. So one possible endpoint for a history of adaptations is to disappear!

Studying the history of adaptations in particular animals, or particular organs, is one of the most fascinating areas of biology, helping us understand the strange bedfellows resulting in modern classification (carnations and cacti, asparagus and aloes!), as well as the process of adaptation and the ecology of organisms. Proposed evolutionary scenarios must always include a plausible evolutionary (i.e. adaptive) history of how they came to be. 

The aloes and some closely related genera, incidentally provide another opportunity for a theme and variations expedition like I did with the Amaryllis family a little while ago. These leafy succulents are native primarily to Africa, and here are some photos from my collection:

Close-up of an unidentified Aloe
showing the emergence of new
leaves in the center.

Aloe (or Kumara) plicatilis is
unusual in having its leaves
arranged in a single plane.

Aloe (or Gonialoe) variegata

Aloe pictifolia

Aloe dichotoma is a rare example of a monocot that becomes a tree 
through an unusual type of secondary growth.

The difference between blackberries and mulberries and why it matters

Blackberries grow on prickly vines or brambles, 
and are members of the Rose Family (Rosaceae).

 As I was picking mulberries from a tree in my back yard the other day, I was reminded of the similarity between blackberries and mulberries. They are strikingly similar in appearance. 

Like most dark fruits, they are both rich in nutrients and protective phytochemicals. For the consumer, the primary differences are the somewhat milder, less sweet flavor, and the annoying little green stems of of mulberries. Depending on the climate, one or the other may be easier to grow, and fresh blackberries are generally more widely available in stores. Dried mulberries, however, are becoming increasingly popular and more available. Beyond all that, it's a matter of taste.

Mulberries grow on trees, and are 
members of the Mulberry Family
(Moraceae).

In terms of the teaching of botany and evolution, however, the similarities and differences between the two berries tell a powerful story. Though they function the same way in natural fruit dispersal, they are not related at all. Blackberries are members of the the very fruitful family, Rosaceae, which includes raspberries, strawberries, plums, cherries, peaches, apricots, apples, pears, rose hips and many more.  Mulberries, on the other hand are members of the Moraceae, which includes figs, breadfruit, and rubber trees. 

The structures of the two fruits are quite different. Blackberries are aggregate fruits, which means that the cluster of drupelets derive from a cone of separate carpels belonging to a single flower. Mulberries on the other hand are technically infructescences, or compound fruits, similar to pineapples. Each drupelet forms from its own tiny flower. So there are fundamental developmental differences that lead to the similar looking fruits.

This might seem like a geeky bit of botanical trivia that would quickly make dinner guests fall asleep, but in the classroom, however, it illustrates some of the most central phenomena of evolution: adaptation, adaptive radiation, and convergent evolution.

These fruits, first of all, are adapted for dispersal by animals, primarily birds, though I have had to keep an eye out for hungry black bears as well while picking berries along roadsides in Washington State. Both go through green and red phases before turning black at ripening. This primes the animals for the coming feast. The berries are sweet, juicy, and flavorful. The animals gobble down the fruits, and the digestive process strips away the juicy tissues, leaving the tiny seeds to pass through the alimentary canal. The animals tend to move about after feeding, leaving seeds in their feces. (See also "What is an Adaptation?)

The different kinds of fruit to be found in the Rose Family are an example of adaptive radiation - the evolution  of a variety of descendant species from a common ancestor. As the descendants of the common ancestor  began spreading into new habitats and new geographic areas, they adapted to local conditions, including local fruit dispersers. 

As other families went through their own adaptive radiations, some descendants encountered the same opportunity for dispersal, and developed similar physical characteristics, but with tell-tale differences in underlying structure. This is convergent evolution - the development of very similar adaptations from unrelated ancestors. The ancestors of the Rose Family happened to have flowers with multiple separate carpels, and so easily evolved into aggregate fruits, while the ancestors of the Mulberry Family had tiny flowers with just one carpel in each, so a similar fruit was most easily developed by grouping the fruits of many flowers together.  I have posted earlier about evolution of cactus-like members in  unrelated families, along with numerous examples of convergent evolution in animals. (See "Of cacti and humans – are certain life forms inevitable?"

What is an adaptation?

What do we mean by adaptation?  We can use that word  both as a process and as the observable result of that process.   Adaptation is the process of evolutionary change under the guidance of natural selection.  It is the process in which populations become genetically modified to function more efficiently in their specific environment, to respond to changes in the environment, or to move into new environments.  The result of that process is new or altered characteristics that we refer to as adaptations.

An important working assumption, or hypothesis, in biology is that every observable characteristic or trait of an organism has some adaptive significance, or at least had adaptive significance sometime during the ancestry of the organism.  A related assumption is that the total set of adaptations (and hence the total set of observable characteristics) is unique for each species, and defines a unique ecological niche.  That in turn means that each species "fits" into the biosphere in a different way from every other species. Discovering the adaptive meaning of everything from leaf shape to flower color is to me the most exciting part of botany, or biology in general.

Let's just take one example: the shape of cactus stems.  First, of course, cactus stems are succulent, i.e. filled with water-storage tissue.  They gather water during the brief and infrequent rain storms, store it, and utilize it sparingly during the long dry spells.  It allows cacti to continue to function, even to bloom at predictable times, rather than become dormant during those dry periods. That is the signature adaptation made by early members of the cactus family.

Cactus stems are also, in the absence of leaves, photosynthetic.  The two major functions of cactus stems requires some interesting compromises. They need to gather light, but exposure to the intense sunlight and heat of the desert environment can potentially result in overheating and tissue damage.   Imagine leaving a plastic jug of water out in the full sun, with surrounding air temperatures over 100 degrees F.

The approximately 1500 species of the cactus family have evolved a variety of mechanisms to cope with this heating problem.  The evolution of many species from a single common ancestor is called adaptive radiation.

Many cacti are round but narrow,optimizing water storage while reducing
exposure along the sides at mid-day sun and optimizing exposure in
the early morning or late afternoon, Photo by RC Designer t-w-m-c _stockarch.com.

Most cacti have adaptations that minimize exposure during the hottest hours of the mid-day.  One strategy is to take on an erect and narrow shape.  This allows full exposure to early morning and late afternoon sun, when temperatures are somewhat cooler.  In the middle of the day, however, only the small tip of the stem faces directly into the sun, and the sides receive light obliquely.

Beavertail cacti (genus Opuntia) take that strategy a step further. Their stems develop as flattened segments, which expose even less surface to the noon-time sun, and even more direct exposure  early and late in the day.

The flattened segments of a beavertail cactus (Opuntia) gather
light optimally when the sun is low in the sky, and provide
minimal exposure in mid-day.  Photo by Stan Sherm, Wikipedia.

A spherical or barrel-shaped stem would seem to be all wrong - exposed maximally at high noon.   It is however the most efficient way to store water.  The round shape provides the minimum ratio of evaporative surface area to water storage volume, but it does potentially provide the greatest proportion of its surface facing directly to the noonday sun.  To compensate, barrel cacti often have large curved spines, or numerous long hair-like spines that provide protection from the intense sun.

Cactus spines, which are modified leaves, are particularly well-developed in
broadly rounded cacti, and serve both for protection against herbivores and for
shading from mid-day sun.  Photo by t-w-m-c _stockarch.com.

Most barrel cacti are ribbed, allowing expansion of the water-storage tissues, and
 also decreasing exposure of the surface tissues to direct sunlight.
Photo by F. B. Essig

Barrel cacti are often fluted, or corrugated, as well - their surfaces consist of accordion-like ridges and valleys.  This further reduces the amount of surface exposed to intense sunlight.   This fluting has a second function as well, allowing the stems to shrink or expand neatly as their internal water stores fluctuate.  The vascular tissues in these stems are concentrated into a series of parallel ribs, so as to allow the expansion of tissues between them.

Another aspect of adaptation is how they are chained together over time, one leading to another to arrive at the characteristic features of a current organism.  We can say that adaptive change is canalized, (see G. L.Stebbins and the process of adaptive modification),  and develops momentum in a particular direction.  Certain kinds of change come naturally based on what has come before; others are extremely unlikely.  I have referred to this as "adaptive parsimony" in some of my other essays (see Were the first monocots syncarpous?) A flying elephant is unlikely, but the evolution of flight is quite possible in lightweight animals that already leap around in trees (e.g. the ancestors of bats and flying squirrels). 

Another thing lightweight arboreal mammals can become is human.  When I taught introductory biology, I had the students do a thought experiment dealing with human evolution: could humans (or equally sentient beings) have evolved from some other starting point than primates adapted to life in the trees?  Could they have evolved from grazing ungulates or dog-like carnivores?  Could they have evolved from octopi or cuttlefish? Or from insects? The evolution of stereoscopic color vision, grasping hands with opposable thumbs, and rotatable arms in arboreal primates pre-adapted some of their descendants to walk upright and use their hands to craft and utilize tools and weapons - an essential ability for developing technology.  What other path to humanity could have occurred?  We also applied this logic to fictional aliens: how might Wookies or Huts have evolved (especially the huts!)?  Well, that's another story altogether.

Returning to plants, a number of my previous postings (including the ones mentioned above) have centered around logical chains of adaptations.  In the case of cacti, the original adaptations for storing water within the stems led to modifications of the stem to avoid overheating.  In other succulent plants, leaves were modified for water storage instead of the stems (aloes, sedums, etc.).  An aloe is not likely to abandon its water-filled leaves and transfer that function to its stem, just as a cactus is highly unlikely to sprout leaves and transfer water storage to them.  The modification of stems or leaves for water storage is an either/or situation, constrained by their separate canalized adaptive trends.

Stem segments of some epiphytic cacti, such as this Schlumbergera, have
become thin and leaf-like. Photo by Peter Coxhead, Wikipedia.

When some species of cactus adapted to life as epiphytes in tropical rain forests, overheating was not as big a problem, but they needed to absorb more of the light that came to them, They did not sprout leaves again, but instead developed flattened, leaf-like stem segments.  It was a simpler adaptive path.  They also ditched the stem fluting and heavy spines so as to expose more of their light-gathering surface.

Focusing on adaptation can be a highly useful way to teach botany.  It allows one to tell engaging stories that combine systematics (the differences among plants), ecology, anatomy, and physiology.