Angiosperms constitute the upper Plant Taxa. Whereas the Gymnosperms are naked-seed bearers, the Angiosperms have coated seeds. The Angiosperms consist of Dicotyledonae and Monocotyledonae, so named for their cotyledons (‘cotyls’ or ‘cots’) or leaf life organs that are formed as they split from the seed coat and open to form the first source for chlorophyll-induced photosynthetic energy.
In general, we are taught that the Angiosperms bear two or one cotyledon, but in fact, some of the most primitive Angiosperms are polycotyledon, bearing 3, 4, or perhaps more in their most primitive families. In view of the form and nature of the Pinopsida seedling, this makes sense. The Pinopsida seedling in polycotyledonous with a dozen to several dozen ‘cotyls’ formed to serve a photosynthetic purpose.
One common teleological question often asked is why this reductionist approach to developing “cotyls”? Why produce a lot haphazardly, never the same from one seed to the next even for the same species at times, only to later decide to reduce this production in order to make much fewer structures of this form? The possible answer (proposed guess) to this question is the plant did this to enable a larger homogenous form of central space to exist in each “cotyl”. This gave each cotyl more space to begin to evolve their chemical reactions and pathways needed for improved survival. This enabled plants to store more nutrient in terms of volume. When you have a lot of cotyls with a lot of surface area, you have a lot of phytosynthetic capability but lose the intracotyl workspace that is needed to experiment with life’s possible chemistry, so to speak. In addition, a lot more energy goes into just making and maintaining the surface tissues needed to keep the living system together for each cotyledon. When you compress these numerous cotyls down to just a few, two or one, you reduce the energy use in making surface protection material and allow for more energy to be spent at the intercotyl level. (All of this is of course just a humanistic guess.)
The following makes up the gross chemistry and ethnobotany of the Upper taxa — Dicots and Monocots — with the other previously mentioned lower taxa groups provided for comparisons.
In this flowchart, the Monocots are displayed as offshoots or a branch of the Dicots. Now some of you are going to ask ‘is that correct?’
This is meant to display the chemical complexity changes, in ascending order, not necessarily the most acceptable taxonomic ranking of these groups.
There are two theories out there about the Dicto-Monocot relationship. The first states that the monocots are an offshoot of the dicots, which is grossly supported by the ranking of “cot counts” from “polycots” (Pinidae) to “oligocots” (very early Angiospermae) to dicots to monocots. The other is that the monocots broke off fairly early, about the same time the oligocots formed the dicots, meaning the two major cot groups went in different directions from the beginning, very early in angiosperm development.
The latter line of reason very much fits the scenario we have in terms of complex chemistry. A complex chemical evaluation of dicots versus monocots demonstrates that very few of the specialized secondary product pathways are shared by the two, in the classical way of defining these secondary paths. My definition of secondary (environmentally induced or developed to meet environmental need) and tertiary (developed to meet ecological need throughout all evolutionary periods and series of stressors) pathways demonstrate this particular feature even more. Monocots appear to evolved some secondary products along pathways similar to those found in dicots, but for the most part parallel evolution has occured when a monocot evolves a digitalis glycoside (i.e. Liliidae, as an isolated species or small closely-related taxonomic group) versus a dicot (Digitalis sp.).
Digging into this line of reasoning a bit further, we’d expect some fairly common and basic pathways to appear in the monocots that are identical in terms of certain products with their parallel plant group the dicots. This exists more for certain basic chemical and ecological needs, but not so much with the more complex tertiary (Level 3) product needs. All primary life-sustaining pathways are shared, only some secondary (Level 2) environmentally selected pathways are shared, and even fewer pathways related to ecological need. For example, the monocots opted to avoid lignin production in large amounts, for the most part. The Palmidae, the most archaic monocot group) and perhaps the Zingiberidae, have some tendency to produce fairly sizeable wood-like plants, more cellulose rich though than combined lignin-cellulose rich. Flavonoids appear to be shared by both dicots and monocots. When we look at certain alkaloids that are shared, as well as selectively toxic steroids, we find very different pathways for synthesis involved. So at some levels, there is a shared ancestor that produced flavonoids, and perhaps was non woody, one that is undecided in its cot production, that could represent a link between the dicot and monocot. Such a species of course is not known.
Both dicots and monocots also rely heavily upon aromatic oils or oxygenated terpenoid production.
When we compare the two major groups ecologically, we find there to be an interesting tendency for monocots to have evolved towards becoming highly symbiotic, and very interactive with natural organisms when it comes to pollination. It is possible to think of the dicots as a fairly widespreading series of plants developing all sorts of chemical pathways, engaged in Levels 2 and 3 throughout the various stages of their chemical development. The monocots on the other hand work more from the Level 3 end, spending more time and energy developing relationships that are unique to their specific genotypes. The are able to parallel some of the evolution of chemicals that took place in the dicots, but not as willing to reproduce these same pathways along numerous avenues. This means that when it comes to uniqueness with regard to chemistry and in particular ecological chemistry, we must look to the monocots for new sources of unique chemical associations between different plants, between plants and animals, and perhaps even between plants and man. In turn this means that when it comes to chemical diversity and the possible discovery of new and innovative interactions between chemical paths, we need to focus on the dicots to improve out chances for finding these new pathways.
When it comes to finding plant uses other than related to wood/fiber, food, and medicine, we look to the dicots due to their inherent chemical diversity. [The pie charts illustrate this–notice the much smaller wedges for non-wood/fiber/food/medicine options for chemical application.] When it comes to better understanding uses that often lack a need for chemical diversity, the following conclusions are inferred by these findings:
- chemical diversity make plants more interactive with the environment in a active-passive fashion
- chemical diversity makes plant more variable in their environment and less likely to become extinct when changes happen
- chemical diversity can make plants or their parts less palatable
- chemical diversity is more likely to make plants toxic and not consumable
- chemical diversity increases selective toxicity, levels and ranges of toxicity (and therefore the potential uses for plants by man)
The lack of chemical diversity sets up the ecological stages for the following
- interaction and even symbiosis with certain organisms
- less likelihood of being toxic to desired organisms, and at time toxic enough for the competitors
- the production of more effective end products when chemical evolution is required as a part of the survival activities
Brian Altonen, MPH, MS 