Atlas of plant and animal histology

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Plant organs


This section of the Atlas is dealing with vascular plant organs, and how their tissues are organized. There are more than 250 thousand vascular plant species. Their ancestors are probably shared with green algae, because green algae and vascular plants have a and b chlorophyll, store true starch in chloroplasts, may have motile cells with flagella, and form phragmoplast and cell plate during cell division. The closest relative is a group of green algae known as Charophyceae. However, vascular plants have independently developed a very complex body (Figure 1) with unique organs adapted for terrestrial living.

Plant body
Figure 1. Main organs of a dicotyledon vascular plant.

Groadly speaking, there are two main vegetative (non-reproductive) organs in plants: root and shoot. The root anchors the plant to the ground and absorbs water and mineral salts. It is a homogeneous structure that may develop lateral roots. Some roots may also emerge from the aerial part of the stem. The shoot includes the stem and the leaves. The stem support the aerial structures of vegetative organs, such as leaves and branches, and reproductive organs, such as flowers and fruits. It is the main avenue for conducting water and minerals coming from the root and organic molecules from the leaves. The stem grows in length by adding units consisting of nodes and internodes. The leaves and branches emerge at the nodes. Leaves are very specialized organs for transforming the light into organic molecules by photosynthesis, and for releasing water by transpiration. They are designed to fetch sunlight and to resit the wind. In the base of the leaves, at the attachment point to the stem, the axillary buds are the starting points to grow new branches.

During the reproductive period, some plants develop flowers or inflorescence, which are regarded as an organ by some authors, or as several organs divided into vegetative and reproductive parts by others. All the flower structures are actually modified leaves. Male and female gametophytes are formed in the flowers, which give rise to male and female gametes, respectively. Fertilization, also happening in flowers, results in the formation of the embryo, which remains latent until germination takes place. The embryo is a component of the seed, which also have nutritional and protecting tissues. Seeds may be surrounded by other tissues, that can be fleshy or dry, and altogether form the fruits. The germination is the development of the embryo to give a new plant (Figure 2).

Plant life cycle
Figure 2. Alternation between sporophyte and gametophyte stages during the life cycle of an angiosperm plant.

Plant organs are divided into three systems:

The protection system includes epidermis and peridermis. They are the superficial tissues of the plant body.

The ground system is composed of parenchyma and support tissues. They are found below the protection tissues in stems and roots, but they can also be found in the inner parts or medulla.

The vascular system is made up of xylem and phloem. They are found in all plant organs, with different location and organization depending on the organ type and plant species.

The three systems have particular features in different organs, type of growing and plant species.

Plant stems and roots may have primary and secondary grow. The type of grow determines the organization of the three systems of tissues. Monocotyledons, herbaceous dicotyledons, and young woody dicotyledons and gymnosperms have primary grow. Secondary grow is found in woody dicotyledons and gymnosperms, and in a few monocotyledons. The differences between primary and secondary grow can be observed in the organization of the vascular bundles and meristems. During primary grow, plants grow mostly in length, whereas secondary grow allows plant growing in thickness. Although secondary growth is only found in current seed plants, fossils indicate that ferns and lycopodium had secondary grow, but they did not leave descendants. Seed plants may discover the secondary grow 400 million years ago.

In the next pages, we are studying the organs of gymnoperms and angiosperms, including both monocotyledons and dicotyledons.

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