|Plant Taxonomy (BIOL308) - Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; firstname.lastname@example.org; http://www.employees.csbsju.edu/ssaupe/|
I. The gymnosperms are a diverse assemblage of plants.
Apparently, they are not closely related to one another (i.e., do not represent a single evolutionary line). They are considered primitive relative to the flowering plants. Modern angiosperms had a "gymnosperm-like" ancestor.
II. Characteristics of gymnosperms
There are three major groups including: cycads (class Cycadopsida), gnetophytes (Class Gnetopsida), and conifers (Class Pinopsida). The textbook classifies all in division Pinophyta. We will focus on the Pinopsida, and more specifically, the order Pinales.
IV. The Pinales (called Coniferales by some authors, hence the term "conifers").
This is the most common group of gymnosperms in Minnesota and is the predominant one in NA. Members of this order: (a) are trees and shrubs; no herbaceous members; (b) have berry-like or woody cones (also termed a strobilus, which is made of sporophylls (spore-bearing leaves); (c) have two types of branches - long shoots (for primary growth - growth in length) and short or spur shoots (extremely short internodes, leaf bearing); (d) leaves that are simple and acicular (needle-shaped); and (e) are monoecious.
In class, we will go over the pine life cycle. You may want to take a minute to review the life cycle of angiosperms, because the basic features of the two taxa are similar (i.e., alternation of generations, gametophytes, sporophytes, meiosis, fertilization). However, there are a few important differences that include: (1) the ovule typically secretes a sticky droplet that sits in the micropyle. The pollen grain lands in the droplet and is "pulled" into the ovule as the drop evaporates or reabsorbed; (2) eggs are produced in a specialized structure termed an archegonium; (3) there may be several archegonia per female gametophyte; (4) eggs in more than one archegonium may be fertilized and begin development; however, only one will finish the process; (5) there is only a single fertilization event. Thus, conifer seeds lack true endosperm; (6) completion of the process (pollination to seed) requires two growing seasons in the pines; (7) the seeds are not enclosed in a fruit; they are exposed (naked) on the cone scales; (8) although many conifer seeds superficially look like samaras, the conifer wing is an outgrowth of the seed coat not the ovary as in samaras.
VI. Families of MN conifers
There are four families represented in the flora of Minnesota. These are: (a) Gingkoaceae - the Ginkgo Family (Ginkgoales); (b) Taxaceae - the Yew Family (Taxales); (c) Pinaceae - the Pine Family (Pinales); and (d) Cupressaceae (including the Taxodiaceae of some authors) - the Cypress Family (Pinales).
VI. GINGKOACEAE - The Ginkgo Family
This family is represented by only a single species, Ginkgo biloba, the maidenhair tree. The common name is derived from the shape of the leaves which are reminiscent of those of the maidenhair fern (Adiantum pedatum). The tree is commonly cultivated in many temperate areas; in fact, this tree is not known outside of cultivation. It is very tolerant of pollution, drought and cold temperatures (to -30 C) and hence, makes an excellent street tree in cities. The ginkgo was originally known only from the fossil record and probably was at its peak in the Jurassic. It was long cultivated in Chinese temple gardens. It was introduced to Europe in 1730 and then to this country in 1784. The actual name was a typographical error in translating the Chinese characters - it should actually be called the ginkyo. The error was initially made in 1712 by Kaempfer and then formalized by Linnaeus in 1771. The seeds are edible, though the coat has a terrible odor - like rancid-butter (butyrate).
Distinguishing features: leaves fan-shaped, dichotomously veined, deciduous, borne on short spur shoots, seeds fetid odor.
VII. TAXACEAE - The Yew Family
Small trees and shrubs, much branched; leaves persistent, alternate, 2-ranked, linear to lanceolate, pale green bands beneath or with alternating bands of white and green; dioecious or rarely monoecious, staminate strobilus a cluster of 3-14 stamens, anthers 3-9 celled, borne on peltate scales; ovulate structure a solitary ovule, not in a cone, terminal, borne on a fleshy or reduced sporophyll, seed enveloped by a fleshy aril. The aril is tasteless, but edible; however - DO NOT EAT THE SEEDS - they are poisonous! Yew is currently being evaluated as a potential source of chemicals used to treat breast cancer. Taxol is isolated from the bark.
Distinguishing Features: Small evergreen trees or shrubs with alternate leaves; peltate microsporophylls, solitary, terminal ovules, seeds subtended by a fleshy aril.
One genus of Taxaceae occurs in Minnesota, Taxus. Other genera in the family include Torreya and Astrotaxus.
VIII. PINACEAE - The Pine Family
Shrubs or trees. Branching typically excurrent; leaves persistent, acicular (needle-like) to linear, spirally arranged in fascicles, on long shoots or spurs, two types of leaves; plants monoecious; male strobili small, herbaceous, sporophylls numerous, spirally arranged, microsporangia fused to lower side of sporophyll, pollen is shed in the spring, cones usually subterminal; ovulate strobilus with numerous scales (megasporophylls), spirally arranged, woody, 2 ovules borne basally on the upper side of each megasporophyll, each scale subtended by bracts, bracts shorter than the scales (except Pseudotsuga), cone scales persistent, cones remain closed until maturity; seeds winged, 2 per scale, embryo with 2-15 cotyledons.
Plants in this family exhibit a variety of adaptations for survival in cold, dry conditions. They are primarily wind pollinated. These include: waxy leaves, well developed hypodermis, sunken stomates, evergreen. These adaptations take minimize water loss which is a problem during cold dry winters (or dry summer season). Also evergreen leaves allow for ready photosynthesis whenever the weather becomes reasonable.
Distinguishing Features: Evergreen trees with excurrent branching, dimorphic shoots, flattened cone scales distinct from subtending bracts, winged seeds.
This is the largest and most important (commercially) family of conifers. It is composed of 9 genera, 6 of which are native to the United States, and approx. 220 species. Common genera of Pinaceae in Minnesota include:
Pines (Pinus). The leaves (needles) are produced in groups (fascicles), the cones have woody scales that don't disintegrate (are persistent). The cones are not typically produced at the end of branches, rather are subterminal. Common Minnesota species include Pinus banksiana (Jack pine), P. strobus (White pine), P. nigra (Austrian pine; this species is very similar to Red pine except that Austrian pine needles don't break with a snap, the buds are usually white and the bark is darker colored); P. sylvestris (Scot's pine); and P. resinosa (Red or Norway pine)
Spruces (Picea). The needles arise singly from the twig and are alternately arranged. The sessile (without a petiole) needles leave a woody peg when they fall off, are four-angled and will roll between your fingers. The cones are pendant, subterminal and the scales are persistent. Common species include: P. mariana (Black spruce), P. glauca (White spruce), P. abies (Norway spruce), P. pungens (Colorado blue spruce),
Firs (Abies). The needles are singly attached, alternate, sessile and leave a large, round depression when they are removed. The cone is upright, subterminal and disintegrates at maturity. They are slender, spire-like trees with a pointed top. Abies balsamifera (Balsam fir) is a native species. Other firs include A. concolor (White fir) and A. fraseri (Fraser fir).
Larch or tamarack (Larix). Needles occur in clusters at the end of short spur shoots. The needles are deciduous. The cone is upright, subterminal and has persistent scales. Larix laricina is common in boggy areas.
Douglas fir (Pseudotsuga menzesii). Evergreen with alternate needles that have a short petiole. The needles leave a small, round, raised scar when they are removed. The cone is pendant, subterminal and has two bracts that are exerted (looks like a snake tongue or mouse ears). Ornamental.
Hemlock (Tsuga canadensis). Evergreen, with alternate leaves that have a petiole that leaves a woody "cushion". The cones are terminal (produced at the end of the branches). Tip of the tree waves in the breeze. Not common in our area, though occasionally planted.
IX. CUPRESSACEAE - The Cypress (or Cedar) Family
Shrubs or trees; diffusely branched; leaves persistent, opposite or whorled, small, scale-like or awl-shaped, frequently dimorphic; monoecious or less often dioecious; male strobili small, clusters of 2-24 stamens borne on lower side of the margin of a broad peltate microsporophyll, anther 2-6 celled; ovulate strobilus scales few, flat and peltate or fleshy and connate, ovules erect, 1-12 per scale, subtending bract adnate to megasporophyll; cone small, rarely larger than 2.5 cm, dry and woody or fleshy and berry-like, seeds often winged.
The family (sensu stricto) is represented by approx. 140 species in 15 genera. Only 5 genera are native to the US; and of these, only Juniperus and Thuja occur in MN. Juniperus is a group of small trees and shrubs. The leaves are small and scale-like on young twigs, sharp pointed on older twigs. The produce a bluish berry-like cone instead of the typical woody cone. Thuja is similar to Juniperus except that the branches are flattened.
Our textbook considers the Taxodiaceae as a subgroup of the Cupressaceae; other taxonomists consider it a separate family. The description above is written primarily with the "old" Cupressaceae, sensu stricto, in mind.
Distinguishing Features: Monoecious or dioecious, evergreen; trees or shrubs; with small, whorled or opposite, scale-like or awl-shaped leaves, frequently dimorphic, cones small, less than 2.5 cm long.
X. TAXODIACEAE - The Bald Cypress Family
Trees; leaves persistent (our species deciduous), scale-like to needle-like, often dimorphic, alternate and spirally arranged; monoecious; male strobilus small, catkin-like, terminal or axillary, 2-9 microsporangia per sporophyll;; ovulate strobilus terminal, sporophylls flat or peltate, woody, bearing 2-9 erect ovules, subtending bracts adnate to scales; cones globose; seeds 2-9 per scale; wingless. Representative genera include Taxodium, Sequoia, Sequoiadendron, Metasequoia.
Distinguishing features: Trees, often with dimorphic leaves, peltate cone scales that lack distinct bracts, cones globose, 2-9 seeds per scale, seeds wingless.
XI. Key to Families of MN gymnosperms
1. Leaves alternate or borne in clusters or fascicles
2. Cone woody
3. Cone scale with distinct bracts . . . . . . . Pinaceae
3'. Cone scale lacking distinct bracts . . . . . Taxodiaceae
2'. Cone fleshy
4. Leaves fan-shaped, deciduous, dichotomously veined. . . . .Ginkgoaceae
4'. Leaves linear, persistent, parallel veined................................Taxaceae
1'. Leaves opposite or
11/20/2008 / � Copyright by SG