I. What is a Plant?
A. General concepts, dating back to Linnaeus but still used by some today:
Animals: organisms that moved and ate other organisms and whose bodies grew to a certain size and stopped growing
Plants: all living things that did not move or eat and that grew indefinitely.
The definition of plant in the past included fungi, algae, and even prokaryotes like cyanobacteria ("blue-green algae")
A common item to include in a definition is photosynthesis: the ability to manufacture food using light energy, and green pigments present in the chloroplast of photosynthetic cells:
CO2 + H2O --> O2 + carbohydrate
An interesting perspective is by Halle (Fig. 99) from his wonderful book "In Praise of Plants" where he shows that plants concentrate energy whereas animals disperse energy
B. Current Taxonomic Definition. On the Tree of Life, we see that green plants are eukaryotes (i.e. they have a nucleus). They are related to red algae (Rhodophyta). This part of the Tree of Life shows that not all photosynthetic organisms are considered plants. Here's a eukaryotic phylogenetic tree (generated from SSU rDNA sequences) that shows that photosynthesis occurs in a number of protozoans that are not closely related to "typical" plants.
C. Clicking further on the Tree of Life, we get to the clade referred to as Green Plants. Most of these groups will be unfamiliar to you (many "algae"), but for the purposes of this class, we will be concerned with the land plants, otherwise known as Embryophytes, and within that clade, Spermatophytes (seed plants) which includes the gymnosperms and angiosperms.
II. What Features Characterize Plants?
A. Photosynthesis, to a point. As shown above, not all things that photosynthesize are plants, and not all plants photosynthesize! Some of the plants that I research are parasitic and have lost the ability to photosynthesize (in all cases, their ancestors were photosynthetic). See Parasitic Plant Connection for lots of info. on these fascinating plants.
B. Continuous growth throughout the lifetime of the plant. Although there are some very large plants (e.g. Sequoiadendron), each plant species does have an upper size limit.
C. Growth in localized zones. Plants have meristems, and we will spend time in this course looking at the anatomy of these important regions.
D. Cell walls made of cellulose.
E. Movement? Many students often say "plants don't move". Indeed, plants are rooted in place and do not move from place to place like animals. But plants DO move! Some can move at rates we can see e.g. Venus flytrap (Dionaea muscipula), and the sensitive plant (Mimosa pudica). Others move too slowly to see. The fact that plants are fixed in place has greatly influenced their structure, genetics, and biochemistry.
III. Plants and Human Psychology
A. Children are innately interested in animals. We tend to gravitate towards things that are more familiar, and plants are in some ways so different from us that they might as well be extraterrestrial! Some people even doubt that plants are alive.
B. Biological thinking has been shaped by an animal bias, e.g. terminology. Is the term "egg" equivalent in humans and plants? Plant life cycles are more complicated, thus more unfamiliar and difficult to learn (Halle Fig. 55).
C. Do you feel the same about chopping up a plant for a salad as you do when you butcher an animal? Why isn't there a "Society for the prevention of cruelty to plants?" We all know we can pull a leaf off a tree without killing it, but we would harm an insect by removing its wings or legs. What is the difference? Chopping up a plant (and some animals) results in clones (Halle, Fig. 39).
D. In fact, plants ARE quite different than animals. The entire DNA sequence of Arabidopsis is known, but a significant number of its genes still cannot be matched with those archived on GenBank. Why is this so?
IV. Goals of this Course
V. Hierarchy of Organization
A. The overall hierarchy
B. Molecules / Macromolecules
C. Organelles or subcellular components (example plant cell)
D. Cell types
E. Tissues (example: leaf)
F. Organs (example an angiosperm)
G. Organisms