5: Seeds, Primary Growth, and Tissues

Two groups of plants, the Angiosperms (flowering plants) and Gymnosperms (including the cone-bearing plants) produce seeds. Each plant seed consists of an embryo and possibly an endosperm, surrrounded by a seed coat. The embryo has distinct parts such as the cotyledons, hypocotyl, plumule (epicotyl + primary leaves), and radicle.

Seeds and Seedlings

The seed germinates when it can absrob water, via imbibition, and begin growth. During germination the embryo uses the food materials stored in t

he cotyledons or the endosperm for this growth process. There are some developmental differences during germination and seedling establishment in different species, such as bean and corn.

Every seed results from the maturation of a single fertilized ovule. At maturity, some seeds will have a large endosperm as the source of stored foods. In others, the endosperm food will already be converted to grwoth of large cotyledons. Examine the parts of both types of mature seeds here. Label the seed coat, embryo, and endosperm on each.

A node is a place on a stem where one or more leaves develop, and the first node of any flowering plant is the one with the cotyledons, the cotyledonary node. The epicotyl is the part of the embryonic stem above the cotyledonary node. The hypocotyl is the part of the stem between the cotyledonary node and the beginning of the root (radicle).

The embryo will give rise to a new plant. You will see how the young embryo establishes distinctive developmental patterns from the two apical meristems. These are the shoot apical meristem at the tip of the epicotyl (the embryonic stem) and the root apical meristem at the tip of the radicle (the embryonic root). These two important apical meristems create all the new cells and tissues that will grow to form the primary organs of the flowering plant body (the leaf, the stem, the root, and the flower).

Meristematic Growth and Tissue Systems

The apical meristems found in the embryo give rise to all the subsequent primary tissues found in a young plant. Meristematic plant cells are uniform in size and shape. There are several kinds of meristems in plants, but for now you should know that two very important ones are those found at the growing tip of each stem and each root. These are the shoot apical meristem and the root apical meristem. The new cells produced at the meristems will quickly begin to undergo dramatic changes in size and composition. Each cell produced by the meristem will become differentiated or specialized in some way. Some cells develop primary or secondary walls in a distinctive fashion. Other cells specialize by changing their protoplasmic contents for a particular function. Some do both.

All cells in the meristems and those new cells produced by the meristem have a characteristic capacity known as totipotency. They are all initially undifferentiated but have the capacity to differentiate into very specific cell types, with distinctive specializations of wall structures, locations, and func-tions. As the cells differentiate, they become organized into three tissue systems, each containing characteristic tissues. Tissues are groups of cells with a common function. The tissues derived from the primary meristems are organized into three primary tissue systems: dermal, vascular, and ground.

The Dermal Tissue System produces epidermis, which covers leaves as well as young stem and root surfaces and provides for gas exchange with the external environment. It also prevents desiccation (drying) of the plant body. The epidermis is usually one cell layer thick. You will observe features of the epidermis of herbaceous dicot and monocot plants in the exercises that follow.

The Vascular Tissue System (usually in more interior parts of the plant body) enables transport of water, nutrients, and photosynthetic materials throughout the plant and provides structural support. The primary vascular tissue system produces primary xylem and phloem, which forms the veins of the leaf and the vascular bundles of stems and roots.

The Ground Tissue System (usually found between the dermal tissue system and the vascular tissue system) is composed of all tissue that are not dermal or vascular in origin. In leaves, ground tissue called mesophyll is the site of the photosynthetic reactions. In stems and roots, the primary ground tissues are cortex and pith. These tissues function in metabolism and storage.

Together, the tissue systems compose each of the organs of the plant body: leaves, stems, roots, and flowers.

Simple and Complex Tissues

Each tissue system (and therefore each organ) is composed of one or multiple simple or complex tissue types. Simple tissues are composed of only one cell type. Complex tissues contain two or more different cells types (xylem and phloem are examples). Some functions of these various tissues and specialized cells include gas exchange between the plant and the outside environment, movement of water, nutrients, foods, and growth regulators between different plant organs, structural support, zones of meristematic activity to produce specialized tissues in certain areas, and complex metabolic functions such as respiration, photosynthesis, and production of specialized plant compounds (pigments, toxins, attractants, and other chemicals). Simple tissues are composed of a single cell type. There are three basic kinds of simple tissues: parenchyma, collenchyma, and sclerenchyma.

In Today's Lab

Your instructor will guide you through the dissection and identification of the parts of a seed. The rest of the lab is divided into 6 stations that correspond to each of the tables in the room. Your lab instructor will provide you with instructions on circulating to all six stations. It is imperative that you take the necessary time to observe and draw all of the specimens at each station.

You will be using prepared slides, and you will make fresh mounts from living material. For prepared slides, there are a few conventions for labels and content that you should know. The label will be glued on the left side of the specimen. When you place the slide on the stage, be sure the label is to the left of the stage. This way, if there is an up -down directionality to the preparation, it will appear in the correct orientation when you observe it in the scope.

Learning Outcomes

 After completing this lab, you should be able to do the following:

1. Identify the parts of seeds and seedlings of bean and other examples.

2. Sketch the sequential development of seedlings of bean, corn, and others.

3. Identify and label the regions of the shoot and root apical meristems.

4. Distinguish the primary tissues and tissue systems that derive from the root and shoot apical meristems.

5. Diagram, label, and distinguish between parenchyma, collenchyma, and sclerenchyma simple tissues.

6. Diagram, label, and distinguish cell types found in xylem, phloem, and epidermis.

Station C: Simple Tissues: Parenchyma and Collenchyma

Parenchyma Tissue. This important simple tissue is composed only of parenchyma cells and is found throughout the plant. Parenchyma is the most abundant cell type in many fleshy plants. There are a number of specialized types of parenchyma used for short-distance movement, storage, food production, and general metabolism.

Collenchyma Tissue. This tissue is composed solely of collenchyma cells. These cells are typically found under the epidermis in leaves and herbaceous stems. Collenchyma is found only in the ground tissue system. They are initially parenchyma cells, but the primary walls gradually begin to thicken in an uneven way, most often in the corners of the cells. When they are sufficiently thickened, the uneven walls will appear distinct when stained.

Station D. Simple Tissue: Scelerenchyma

There are two types of sclerenschyma tissue, each composed of either fibers or sclereids. Both of these types of scelerenchyma cells have very thick walls in which the secondary wall in composed of many layers impregnated with lignin. Therefore, they are very hard and rigid. Fibers and scelerids are differentiated based on size, shape, and location.

Fibers are always a component of one of the complex tissues of the vascular tissue system, either phloem or xylem. They are usually very long and thin and found tightly grouped together as part of the vascular bundle or cylineder. Fibers that are freshly mounted and stained with phloroglucinol will appear red or orange.

Sclereids are frequently found in leaves, fruits, and seeds. Their main function is protection. They are variable in shape and size but are distinct when compared to fibers.

Xylem is the easier of the 2 major vascular tissues to locate. IT is almost always interior to phloem. It is composde of parenchyma cells, fibers, vessel elemtns, and tracheids. Xylem funcitions as a conductor of water and dissolved materials. Xylem vessels are usualy the largest cells visible in the vascular tissue system.

Phloem tissue is found interior to the cortex or periderm layers but outside the vascular cambium and the xylem. It is composed of parenchyma, fibers, and sieve-tube elements and their companion cells.

Some epidermal cells may be modified as glands that secrete substances or as hairs called trichomes. Trichomes can provide numerous functions such as reducing water loss, protecting the plant from herbivory by animals or insects, or secreting substances.