Part II Reproductive Anatomy
As indicated, this section of Crop Plant Anatomy will focus on the reproductive anatomy of monocot and dicot crop plants.
Dicot Flower Anatomy
In this section you will be observing the reproductive structures of both the dicot and monocot type flowers. As you are aware, dicot-type plants are those that contain two cotyledons within the seed and are characterized as broadleaf-type plants. Examples of dicot-type plants would be cotton, sunflower, soybeans, and various other plants of this nature. The monocot type plants are those that have only one cotyledon contained in the seed and are characterized as the grasses. Examples of monocot type plants would be corn, grain sorghum, sugar cane, and various other species that have narrow leaves.
Figure
1
This figure depicts a typical dicot flower. Also, at this time refer to page 2 of the study guide. The dicot flower is composed of several structures as may be noted from this figure. In addition, these structures are defined on page 1 of the study guide for your convenience in following along. The floral unit itself in a dicot plant is mounted on a structure called the pedicel. The pedicel may be defined as a branch or stalk that bears a single flower or floret in the flower cluster or inflorescence. It may be noted that the tip of the pedicel is somewhat enlarged and, at this point, the structure is called the receptacle. This enlarged tip of the pedicel, the receptacle, bears the rest of the floral unit. Progressing up the floral unit, next in order would be the sepals. The sepals are green leaf-like structures mounted directly on the pedicel (or receptacle) and are often times referred to as the outer set of floral leaves. These are generally green in color. Each one of these floral leaves is termed a sepal and collectively, they are referred to as the calyx. Therefore, the calyx may be defined as the outer part of the floral envelope that is composed of the sepals. The next layer of structures is the petals. The petals may be defined as the inner leaf-like parts of a flower and are generally very colorful in nature. Each one of these is referred to as a petal and collectively, they are referred to as the corolla. The corolla may then be defined as the inner set of floral leaves composed of the petals. The next set of three structures comprise the female reproductive unit. These structures are referred to as the stigma, style and ovary. The apical portion of the female reproductive structure is referred to as the stigma. The stigma may be defined as that part of the female reproductive structure upon which pollen adheres and germinates. As indicated earlier the stigma is generally terminal in location as far as the female reproductive structure is concerned. The next portion of the female reproductive structure is the style. The style, as may be noted in the figure, is the stalk-like part of the female reproductive structure and connects the stigma and the ovary. The third portion of the female reproductive system is the ovary. The ovary may be defined as the enlarged, basal portion of the female reproductive structure and contains one or more ovules. Collectively, the stigma, style and ovary comprise the pistil and is, thus defined as the entire female reproductive system. During pollination and fertilization, pollen from the male reproductive structure lands on top of the stigma and, if conditions are conducive, it will germinate. Upon germination, the pollen forms a pollen tube and grows downward through the style, and upon reaching the ovary releases the male nuclei into the ovum and fertilization subsequently occurs. The next set of structures is the male reproductive system. The male reproductive system is composed of two parts, that being the anther and the filament. The anther is the pollen containing part of the male reproductive structure and usually consists of two sacs. It is within the anther that the pollen is formed and matures. The second part of the male reproductive structure is a filament. The filament is the stalk-like portion of the male reproductive system and generally bears the anther at its tip. Collectively, the anther and the filament form the stamen with the stamen being defined as the entire male reproductive system.
Label the diagram under No. 2 on page 2 of the study guide as per figure 1. As indicated previously, the structures that you have just observed are typical of many of the dicot species. However, not all dicot species will have this type of flower in that several modifications may be observed. This modification of dicot floral anatomy will be described in more detail later on in this section.
Monocot Flower Anatomy
The next area to be described is that of the monocot flower anatomy. Also refer to number 3 on page 2 of your study guide.
Figure
2
This figure depicts three types of floral anatomy generally found in the monocot-type plants. The one on the left is the panicle type of inflorescence, the center one is the raceme type of inflorescence, and the one on the extreme right is the spike type of inflorescence. Again, all three are fairly common types found in monocot-type plants.
Figure 3
Figure 3 depicts a close-up of the panicle-type inflorescence. The central portion of the inflorescence is called the rachis. This is the vertical branch or structure comprising the inflorescence. Coming off of the rachis are a number of individual branches. It may be noted in this drawing that there are five branches coming from the central rachis. Coming off of each main branch are other small branches referred to as pedicels. Borne at the tip of each pedicel is the individual spikelet, which contains the floral units of the monocot flower. Examples of plants that have the panicle type of inflorescence would be oats, sudangrass, Kentucky bluegrass, tall fescue, and timothy, and grain sorghum.
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Figure
4
Figure 4 depicts the raceme type of inflorescence. Again, it may be noted that the central stalk of the inflorescence is referred to as the rachis. Coming off of the central rachis are a number of structures referred to as the pedicels. In this particular type of inflorescence there are no branches as was described for the panicle-type of inflorescence. Borne at the tip of each individual pedicel again are the individual flowering units referred to as the spikelets. This type of inflorescence is relatively uncommon among the monocot-type plants. An example of a plant that has a raceme-type of inflorescence would be foxtail millet.
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Figure 5
Figure 5 depicts the third type of monocot inflorescence referred to as the spike. It may be noted in this type of inflorescence that there are no branches and no pedicels. The flowering units, the spikelets, are attached directly to the central axis, or rachis of the stem. Examples of crops that have this type of inflorescence would be wheat, barley, ryegrass, and wheatgrass.
In summary then, the spike type of inflorescence and the panicle type of inflorescence are the most common types found among our monocot plants. The raceme type of inflorescence is relatively uncommon.
At this time you will want to fill in section number 3 on page 2 of your study guide.
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Figure
6
This figure shows the close-up detail of the individual flowering units, referred to as the spikelet, of the monocot type of inflorescence. At this time refer to number 4 on page 3 of your study guide. This section will identify and describe the individual structures of the grass spikelet. The grass inflorescence flower cluster is referred to as a spikelet and consists of two glumes and one or more individual florets. As was noted earlier, the central axis of the inflorescence in the monocots is referred to as the rachis. Attached directly to the rachis is the individual spikelet, thus this type of inflorescence is characteristic of the spike-type of inflorescence that was described earlier. The first two structures constituting the spikelet are referred to as the glumes. The glumes are a pair of empty scale-like bracts that are located at the base of the grass spikelet. Contained within the glumes is the individual floret or florets constituting the flower. The floret, itself, may be defined as a small flower, and in grasses would include the lemma, the palea, and the enclosed reproductive components. Dependent upon species, the spikelets may contain one or more individual florets. In this particular example, the spikelet contains three florets.
Figure 7
Figure 7 shows an individual spikelet. This actual photograph
depicts or shows the two glumes, which in this particular photograph
are laid out horizontally, and then the three enclosed florets.
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Figure
8
Figure 8 shows a blown-up sketch of the dissected floret. Note in the left-hand portion of the figure the structure labeled the lemma. The lemma is a bract that usually encloses or partially encloses a flower in the spikelet of the grasses. On the opposite side of the floret is another leaf-like structure that is referred to as the palea. The palea is the inner and usually smaller of two scaly bracts immediately subtending the grass flower in a spikelet. In other words, the entire floret itself is enclosed in two bracts, referred to as the lemma and the palea. The outer bract is the lemma and the inner bract is the palea. Another structure not defined in your study guide is the awn. The awn is an extension of the vein or mid- rib of the lemma. The presence or absence of this structure is variety dependent in that some varieties may have a very prominent awn while other varieties may have no awn at all. Those in which no awn is present are referred to as awnless. Those in which a very prominent awn is evident are referred to as awned, or bearded varieties. Still, in other instances the varieties may only have a very small awn, hence, they are referred to as awnletted varieties. Located to the interior of the lemma and the palea are the male and female reproductive structures. The first is the pistil. Again the pistil is defined as the entire female reproductive system, consisting of the stigma, style and ovary. The terminal portion of the pistil is the stigma and is the organ in which the pollen adheres and germinates. If conditions are favorable, the pollen grain will germinate and the male nuclei will advance through the next portion of the female reproductive structure called the style. The style is the stalk-like portion of the pistil that connects the stigma and the ovary. The basal portion of the pistil is referred to as the ovary. It is that part of the pistil, or reproductive structure that contains one or more ovules that upon fertilization will develop into the seed or seeds.
The male reproductive system is referred to as the stamen. The stamen consists of the anther and the filament. The anther is the enlarged terminal portion of the stamen and is the structure that contains the pollen. The filament is the stalk-like portion of the stamen, which bears the anther at its tip. This then is the complete description of the individual grass flower referred to as a floret. The floret or florets in combination with the associated glumes would be referred to as the grass spikelet.
At this time turn to page 4 of your study guide and label the structures under numbers 5 and 6.
Figure 9
By way of review, label on a separate sheet of paper the structures numbered in this particular slide.
Now that you have labeled these structures according to numbers, let's review them.
- palea
- style
- stigma
- filament
- anther
- lemma
- ovary
- awn
- lodicles
We did not define the lodicles in the study guide, however, these are two small structures located at the base of the pistil that function to open the floret during the time of pollination and fertilization.
Figure
10
Figure 10 is an actual photograph of a developing floret. The structure on the left is the lemma that has been removed and it may be noted that it does have an awn at the top. The remainder of the floret is pictured at the right. Not clearly shown and to the rear of this structure on the right is the palea or the other small retaining bract. The two fuzzy structures protruding from both sides of the structure would be the stigma and part of the style. At this time the ovary is not very prominent. The orange colored structure in the lower right quadrant is the anther. Upon further maturation the anther will be pushed further up into the floret and eventually out of the floret by the elongating filament. Thus far we have looked at the typical anatomy of both the monocot and dicot flowers. However, as indicated earlier, not all flowers will be typical in nature in that one or more of the structures studied may be absent from the individual flower. At this time there are certain terms or definitions that are used to describe the flowers that lack one or more of these structures.
Complete Flower: A flower having all floral structures, including the calyx (sepals, corolla(petals), stamen(anthers and filaments) and pistil (stigma, style, and ovary).
Refer to No. 7 on page 4 of the study guide.
As we go through these definitions you will want to copy them into your study guide. The first definition is that of a complete flower. A complete flower may be defined as a flower that has all of the floral structures that we have studied so far. These would include the calyx, or the sepals, the corolla, or the petals, the stamen composed of the anther and filament, and the pistil which includes the stigma, style, and ovary. If all of these structures are included in a particular flower, then the flower is referred to as a complete flower.
Incomplete Flower: A flower which lacks one or more floral structures.
The next definition is that of an incomplete flower. This may be defined as a flower, which lacks one or more of the floral structures. In other words, a flower lacking the calyx, the corolla, the stamen, or the pistil or any combination of these structures would be referred to as an incomplete flower.
Perfect Flower: A flower which has male and female floral structures in the same flower.
The next definition is that of a perfect flower. A perfect flower would be defined as a flower, which has both male and female floral structures located in the same flower. Wheat is an example of a crop with a perfect flower.
Imperfect Flower: A flower which has male and female floral structures in separate flowers.
An imperfect flower on the other hand would be defined as a flower, which has male and female floral parts located in separate flowers. Corn is an example here in that the male flower is the tassle and the female flower is the ear. These previous four definitions all refer to the flower itself.
Monoecious Plant: A plant species having separate male and female flowers on the same plant.
The next definition is that of a monoecious plant. A monoecious plant may be defined as a plant species having separate male and female flowers but yet located on the same plant. A very good example of a monoecious plant would again be corn in that both the male and female structures are located in separate flowers but yet both are located on the same plant. The stamen, or the male floral structures, are the tassles located at the top of the plant. The pistil, or female reproductive structure, is the ear, which is located about midway up the stalk of each plant.
Dioecious Plant: A plant species having male and female flowers that are on separate plants.
A dioecious plant is a plant in which the male and female flowers are on separate plants. In other words, the female reproductive structures would be located on one plant and hence referred to as a female plant while the male structures would be located on a separate plant, and therefore referred to as the male plant. Buffalograss would be an example here. These latter two definitions (monoecious and dioecious) refer to the plant as a whole, whereas the former four definitions (incomplete, complete, perfect, and imperfect) referred only to the flower itself.
The next area is that of seed anatomy. We will be looking at both the anatomy of a monocot seed and a dicot seed. Again, an example of the monocot seed that will be used is that of corn, and the soybean seed will serve as an example of the dicot seed.
Figure
11
Refer to #8 on Page 5 of the study guide.
Figure 11 is a depiction of a typical monocot seed and for our purposes we will use corn as an example. This particular example is dent corn, or what is usually referred to as ordinary field corn. The term dent corn comes from the fact that there is an indentation at the top of the seed in the mature corn kernels. The first structure or outer covering of the seed is referred to as the pericarp. The pericarp may be defined as the fruit wall or seed coat, which is developed from the ovary wall. Just to the interior of the seed coat or pericarp is the endosperm of which there are two types. The soft endosperm is defined as the soft nutritive tissue formed within the embryo sack of seed plants. By the same token the hard or horny endosperm is the same nutritive tissue formed in the embryo sack, but is of a hard consistency. The endosperm functions in the seed as a source of reserve materials and energy for the germinating process of these seeds. During the process of germination the endosperm will be broken down and used by the developing embryo for growth. The next structure is the embryo. The embryo is the rudimentary plant located within a seed that will develop into the seedling and later the plant. The next structure is the scutellum. The scutellum is the cotyledon in the grass family. The function of the scutellum is to absorb the materials from the endosperm as they are broken down, and hence supply these materials to the developing embryo. The next structure is the coleoptile. The coleoptile is a sheath that encloses the apical meristem of the embryo. The primary function of the coleoptile is to provide protection to the developing shoot as it is passing through the soil. Once emergence has occurred, the coleoptile stops growing and the remainder of the developing shoot emerges through the coleoptile. The next structure is the plumule. The plumule is defined as the apical portion of the shoot. This includes the rapidly dividing apical meristem as well as any leaves that might be associated with the developing shoot. The apical meristem may be defined as a group of meristematic cells located at the tip of either the root or the shoot and that by dividing produce the precursors of the primary tissues of the root or shoot. In other words, these cells are very rapidly dividing, and upon division constitute the cells that will later mature and differentiate into the stem and root tissue. The next structure is the scutellular node. This is the node or position on the main stem at which the scutellum or cotyledon is attached. The next structure is the radicle. The radicle is nothing more than the embryonic root of the plant. It constitutes the basal continuation of the hypocotyl in the embryo structure. The next structure is the root cap. The root cap is a thimble like mass of cells that cover the apical meristem of the root. The function of the root cap is to protect the sensitive cells of the root apical meristem as they proliferate and elongate through the soil. The next structure, the coleorhiza, is a sheath that encloses the radicle or developing root in the embryo. Again, the coleorhiza affords protection to the developing radicle in much the same way that the coleoptile affords protection to the apical meristem of the developing shoot. The last structure is the root tip, and may be defined as a terminal layer of cells that constitute the root. At this time label the diagram as indicated under number 9 on page 6 of the study guide.
Figure 12
Figure 12 is an actual longitudinal section through a corn
seed. This particular seed has been stained with dye in
which the red coloration develops only in the embryo
or living tissue. The red area, being the embryo, may be
noted to lie along one side or face of the seed. Note that
approximately 2/3 of the seed is comprised of the white
to off-white colored material, referred to earlier as endosperm.
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Figure
13
Figure 13 depicts three views of a corn seed. The view on the left is a top view in which the prominent dent of the field type corn is shown. The middle view is a broad faced shot of the corn seed and the view on the right is a longitudinal section of an unstained corn seed. The tissue that is off-white in color in the right-hand portion of this longitudinal view is that of the embryo itself. The remainder of the tissue grading in color from white to brown would be the endosperm with the white or light colored area being the soft endosperm and the dark brown area being the hard endosperm.
Figure 14
Refer to #10 on Page 6 of the study guide.
Figure 14 depicts a typical dicot seed in terms of its external morphology. There are three structures that are somewhat prominent when looking at a dicot seed such as soybeans or the various other beans. The first is the micropile. On the mature seed the micropile is nothing more than a small scar. However, during the development of the ovum into the mature seed the micropile was a small pore that allowed for the entrance of the pollen tube and the sperm nuclei during the process of pollination and fertilization. The next structure, the hilum, in a mature seed is a scar left over from its attachment to the mother plant. During the development of the ovum the ovum was attached to the mother plant by a small stalk called a funiculus. After the seed matured, the funiculus became separated from the seed, thus leaving the scar that is referred to as the hilum. The third structure, in terms of external morphology, is the raphe. During the development of the seed and at maturity when the funiculus or stalk supporting the seed become disjoined, part of it remains with the seed. The part of the funiculus that remains with the seed and becomes a part of the seed is referred to as the raphe.
Figure
15
Figure15 is an actual photograph of a soybean seed during the early stages of germination. The structure labeled 1 on this photograph is the hilum. The structure labeled 2 represents the seed coat or outer covering of the seed, and the structure labeled 3 represents the enlarging radicle just beneath the seed coat. Very shortly the radicle will be emerging from the seed coat and will develop into the seedling root.
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Figure 16
Figure 16 shows a schematic of a cross section of a dicot seed. At this time, also refer to #11 on Page 7 of your study guide. The outer layer of the seed is the seed coat and its primary function is that of protection. Just to the interior of the seed coat are the two cotyledons, one in each half of the seed, which serves as a reserve energy and materials storage for the developing embryo. During the germination process, the material in the cotyledons will become available to the developing embryo such that a new seedling and plant may be established. The lower portion of the embryonic axis is referred to as the radicle. The radicle again is the embryonic root, which will later develop into the main taproot system of the seedling and the plant. Moving up the embryonic axis, above the radicle the remainder of the tissue is the stem. The stem is divided into two sections - one being the hypocotyl and the other being the epicotyl. That part of the stem below the point of attachment of the cotyledons is referred to as the hypocotyl. That part of the stem above the attachment of the cotyledons is referred to as the epicotyl. Contained in the epicotyl portion of the stem will be the beginning of the first true leaves of the plant as well as the apical meristem. At this time label the diagram of the dicot seed under #12 on Page 7 of the study guide.
This concludes the section on reproductive anatomy. Please disregard page 8 in the study guide.