This section will deal with the sequence of events in the germination and emergence processes of seeds. We will look at the different types of germination exhibited by both the monocot and dicot type plants.

Figure 1

Figure 1 shows the typical steps involved in the emergence of a monocot and a dicot type plant. For illustration purposes the monocot type plant will be a corn seedling and the dicot plant will be a soybean seedling. However, we will see later that there are two different types of germination for the dicot type plants. Looking first at the monocot type germination, as exhibited in the left half of the picture, it may be noted that soon after planting the radicle and seminal roots emerge from the seed and begin to grow in a downward direction. At the same time the shoot or young leaves enclosed in the coleoptile emerge from the seed and begin growing in an upward direction. The second shot of the corn seedling shows the leaves after they have emerged from the coleoptile. Soon after the coleoptile with its enclosed leaves appear above the soil surface the coleoptile ceases growth. However, the enclosed leaves continue to grow and push their way out the end of the coleoptile where they continue to elongate. At the same time, the root system is continuing to grow and subsequently new roots will be forming at the lower nodes on the stem. In the dicot type germination, as depicted on the right half of this picture, the seed imbibes water and the young radicle or root begins to emerge from the seed. At the same time the hypocotyl begins to elongate and as it does so it pulls the cotyledons and enclosed apical meristem above the soil's surface. Once the cotyledons are above the soil surface, the hypocotyl straightens up, as indicated by the plant on the extreme right of this picture, and the cotyledons begin to unfold. After unfolding, the apical meristem continues growth and subsequently new leaves are displayed. At this time answer question number 1 concerning the emergence of a monocot and a dicot plant on page 1 of your study guide.

Figure 2

As you go through these figures on the "corn seedling" you will want to be answering questions 2 through 5 on pages 1 and 2 of your Study Guide.

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Figure 3

Figure 3 depicts a schematic drawing of a corn seed in the early stages of germination. Three structures may be noted at this time. They are number 1, the coleoptile, which as you recall, is a sheath that encloses the young leaves of the corn plant and provides protection for them until they emerge through the soil's surface. Structure number 2, although not visible, is an indication of the location of the apical meristem, or shoot apex of the developing seedling. Structure number 3 is the radicle or primary root system of the plant which emerges from the basal end of the seed and grows in a downward direction. Shortly, additional roots, referred to as seminal roots, will also emerge from the seed and the combination of the seminal roots and the radicle are commonly referred to as the primary root system of the plant. These roots, the primary root system, will be functional for about 3 to 6 weeks, at which time the secondary or permanent root system of the plant will take over the functions performed by these roots.

Figure 4

Figure 4 is an actual photograph of a young seedling in the early stages of development. Again, the coleoptile at the top may be observed to be emerging from the seed coat, and at the same time, the radicle or young primary root system is emerging from the lower end of the seed.

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Figure 5

Figure 5 is a schematic drawing of a young corn seedling about 7 days after initiation of germination. Structures to be noted here are Number 1, the young initial leaves of the plant emerging from the coleoptile. These may be noted to be green in color. Although not labeled, the light area slightly above and extending below the soil surface is the coleoptile. As indicated in a previous section, once the coleoptile emerges through the soil surface, it ceases any further growth and the young leaves push their way through the end of the coleoptile. Structure #2 is the mesocotyl and is that part of the stem between the scutellar node and the coleoptile node. Elongation here pushes the remainder of the stem and leaf tissue to near the soil surface. At this point in time, however, the growing point, or apical meristem, is still located at or very near the soil itself. At the bottom of the slide is Structure #3, which is the radicle or primary root system of the developing seedling. Structure #4, are the seminal roots that are also emerging from the seed, and as indicated earlier, in combination with the radicle, comprise the primary root system of the plant. Again, the primary root system is only functional for about 3 to 6 weeks.

Figure 6

Figure 6 is an actual photograph of a seedling approximately 7 days after germination. The green area at the top represents the leaves that are emerging through the coleoptile, which at this point in time has ceased growth. The light area extending from this point down to the seed is the coleoptile, and again functions to protect the young leaves as they elongate or protrude through the soil. At the base of the coleoptile is the seed and extending to the left of the seed is the radicle which has emerged. To the right of the seed are three small seminal roots that, in conjunction with the radicle, make up the primary root system.

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Figure 7

Figure 7 represents a schematic drawing of a corn seedling approximately 14 days after the initiation of germination. Structures to be observed are as follows: No. 1 is the young leaves of the corn plant. No. 2 down at the bottom of the picture would be the seminal roots. It may be observed that the seminal roots are about 3 in number and already profusely branching. In the lower left portion of the root mass, the radicle may be observed growing in a downward direction. No. 3 represents the nodal roots of the plant. The nodal roots, in contrast to the primary roots of the plant, are called the secondary, or permanent roots. As indicated earlier, the primary roots of the plant sustain it for approximately 3 to 6 weeks, at which time the permanent root system has developed to the point that it can supply the needed nutrients, water, etc. for the entire plant. At this time, the stem area (or mesocotyl) between the primary root system and the secondary root system will decay and separate. Structure No. 4 is the remnants of the coleoptile as the leaves have already punctured through it. Structure No. 5 is the mesocotyl area and as referred to earlier is the area that will decay and separate the primary root system of the plant from the remainder of the plant.

Figure 8

Figure 8 is an actual photograph of a plant approximately 14 days after the initiation of germination. Here the leaves may be observed at the top of the plant as well as the beginning of the permanent root system at the basal portion of the stem. Coming from the seed are the remnants of the seminal roots and the radicle. At this point in time the shoot apex, or growing point, is still deeply imbedded in the stem and only very slightly above the seed itself.

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Figure 9

One common misconception in the planting of corn is that planting depth plays a very important part in the depth at which the corn plant develops its roots. This is true only for the primary root system and not for the secondary root system. The picture on the left of Figure 9 represents a seed planted very shallow as opposed to the picture on the right showing a seed planted deeper. It may be observed that the deeper planted corn seed has the primary root system located deeper in the soil. However, the secondary or adventitious root system of the two plants or seedlings are at about the same depth in the soil. The primary reason for this is that in the more deeply planted seeds the mesocotyl portion of the stem elongates more, such that the adventitious roots generally develop at about the same depth regardless of the planting depth. Therefore, in summary planting depth does affect the depth of rooting only for the primary root system and has no effect on the depth at which the secondary roots develop. At this time answer questions number 5 and 6 on page 2 of your Study Guide. After you have answered questions number 5 and 6, label the diagram under number 7 on page 3 of your Study Guide.

Seed to Seedling:  Soybeans

At this time we will look at the germination and early seedling growth of two dicot plants, those being the soybean and the pea.

Figure 10

By way of review, this drawing depicts the schematic of a soybean seed. The schematic represents a side view of each half of a soybean seed. Structures to be observed are the cotyledons or the reserve storage materials in the dicot type plants. The seed coat or protective structure of the seed is also noted. Also to be observed is the young seedling root, or the radicle, and the first true leaves or unifoliate leaves of the plant. The unifoliate leaves are attached to the embryonic axis at the node immediately above the point of attachment of the cotyledons. The hypocotyl, or that portion of the stem between the radicle and the cotyledonary node is also labeled. Although not observable on this photograph, the portion of the stem above the node where the cotyledons are attached is the epicotyl. At the very tip of the epicotyl is the apical meristem of the shoot, and this is the region where cell division and later cell elongation occur which gives rise to developing shoot tissue.

Figure 11

Figure 11 is an actual photograph of a soybean seed in the very early stages of germination. The seed located at the top of this picture represents the seed shortly after the imbibition of water begins. The swelling noted at the right of the seed is the radicle that is beginning to elongate and will be the first structure to rupture the seed coat. The seed at the bottom of this photograph represents one that is in the same stage of development as the seed at the top. However, this seed has been cut in half. Most of the tissue observed is the cotyledons; however, the upper half of the seed shows the radicle that is beginning to elongate. At this time answer question number 8 on page 3 of your Study Guide.

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Figure 12

Figure 12 depicts the soybean seedling approximately 7 days after the initiation of germination. You will want to answer questions number 9 and 10 on page 4 of your Study Guide as we go through this series. Figure 12, as indicated earlier, depicts a soybean seedling approximately 7 days after the initiation of germination. At this point in time the young root system is developing as well as the stem and leaf tissue. The stem has elongated to the point that the cotyledons are now erect and are beginning to unfold. The soybean seedling exhibits a type of germination referred to as epigeal. Epigeal germination is a type of germination in which the cotyledons rise above the soil surface in the developing seedling. This figure depicts soybeans, which exhibit epigeal germination, and here the cotyledons may be observed to have risen above the soil surface.

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Figure 13

Figure 13 shows a developing soybean seedling approximately 14 days after the initiation of germination. The young root system, which is a taproot system, is developing very profusely at this point in time. Proceeding up the stem the lowermost leaf structures represent the cotyledons. The cotyledons at this point in time have just about served their function and shortly will turn yellow and drop from the plant. The next two leaves above the cotyledons are referred to as the unifoliate, or first true leaves of the plant. Subsequent leaves that develop in the soybean plant will be referred to as trifoliates in that each petiole will bear three individual leaflets. Enclosed at the very tip of the shoot is the shoot apex.

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Figure 14

Figure 14 is a summary drawing showing a set of sequential schematics of soybean germination. The left most drawing represents the soybean seed shortly after imbibition of water has started. Note that the young radicle or root is beginning to push out and downward from the seed. The second drawing shows the root system beginning to develop and at the same time the hypocotyl or that part of the stem between the radicle and the cotyledons is now elongating and is pulling the cotyledons above the soil surface. The third drawing represents a seedling in which the hypocotyl has now straightened with the cotyledons now assuming a horizontal display. At the same time, the shoot has developed to the point that the first unifoliate leaves are now being displayed. The schematic on the extreme right in this drawing shows the continued development of the root system; it also shows the hypocotyl. The cotyledons are now withering away since they have served their function in getting the seedling established.

Figure 15

Figure 15 shows an actual photograph of soybean seedlings in the various stages of growth discussed in the previous slide. The left most seedling now has the radicle emerged and as we look further to the right the shoot tissue is now developing to the point that the shot at the extreme right shows the cotyledons, the unifoliate leaves, and the first trifoliate now exposed. At this time  answer question number 11 on page 5 of your Study Guide. After you have answered question number 11, label the diagram under number 12 on page 5 of your Study Guide.

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PEA

This next series will be a depiction of a dicot, the pea seedling that exhibits a different type of germination. This type of germination is referred to as hypogeal. This is a type of germination in which the cotyledons remain below the soil surface during the process of germination. In this case, only the developing shoot area above the cotyledons emerges through the soil surface. As you go through this section answer question numbers 13 through 17 on pages 6 and 7 of your Study Guide.

Figure 16

Figure 16 depicts a schematic of a cross section of a pea seed. Structures to be observed are number 1, the cotyledons, number 2, the seed coat; number 3, the radicle or embryonic root system; number 4, the first true leaves of the plant; and number 5, the hypocotyl or that part of the stem below the cotyledonary attachment and above the radicle.

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Figure 17

Figure 17 is an actual photograph of pea seeds in the very early stages of germination. Here they are imbibing water and the radicle is the swollen area on the seeds and will soon emerge through the seed coat.

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Figure 18

Figure 18 is an actual photograph of a pea seedling in the initial stages of germination. This picture shows two structures coming from the seed. The structure coming out and turning downward is the radicle or the primary root of the plant. This will later develop into the taproot as was the case with the soybeans and for other dicot-type plants. The structure coming out the top of the seed and turning upward is the epicotyl or that part of the stem above the cotyledonary area of attachment. In this type of seedling emergence, that is hypogeal germination, the cotyledons stay enclosed in the seed coat and remain below the soil surface. Therefore, that part of the stem below the cotyledon area attachment known as the hypocotyl also remains in and near the seed. Born at the top of the epicotyl, or emerging shoot, is the leaf tissue and enclosed in between the leaves is the shoot apex.

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Figure 19

Figure 19 depicts the developing pea seedling approximately 7 days after initiation of germination. It may be observed here that the radicle, or beginning of the taproot system, has elongated downward to a great extent in the soil and that on the upper portion of the radicle, branches are beginning to develop. The seed shown here has the cotyledons enclosed since this type of plant exhibits hypogeal type of germination. Protruding upward from the seed is the developing epicotyl or shoot and the young leaves.

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Figure 20

Figure 20 depicts the developing pea seedling approximately 9–11 days after initiation of germination. Here the taproot is developing very nicely with the branch roots just beginning to develop. The epicotyl has elongated to the point that the leaves are now above the soil surface and the leaves are beginning to unfold.

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Figure 21

Figure 21 shows the developing plants approximately 14 days after initiation of germination. Note that the root system, both the taproot and the branch roots, are profusely developing as well as many additional leaves are now being exposed.

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Figure 22

Figure 22 is another shot of a plant approximately 21 to 25 days after initiation of germination showing the developing root and shoot systems.

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Figure 23

Figure 23 shows the pea seedling in several stages of development. The left seedling has begun to elongate the radicle and the shoot system. The central seedling has developed to the point that the leaves are now above the soil surface and the seedling on the right has developed to the point that a very extensive root system has been initiated as well as numerous leaves are now exposed. At this time answer questions 18 and 19 on pages 7 and 8 of your Study Guide

Figure 24 Also refer to number 20 on page 8 of your Study Guide. At this time you may want to again review the anatomy of a monocot and a dicot-type plant. The monocot or grass plant anatomy and morphology is shown in Figure 7 while this drawing depicts the typical dicot type of anatomy. Refer to and make notes on these as you feel the need.