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 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 organs are diverse. The root anchors the body of the plant to the soil and absorbs water and mineral salts. The stem gives support for aerial organs like leaves, flowers, seeds and fruits, and it transports both water and mineral salts from the root toward aerial organs, and complex organic molecules from leaves to the rest of the plant body. Leaves are specialized organs for capturing Sun light and make photosynthesis, which is a source of organic molecules. They also release water by transpiration.
During reproductive periods, some plants have flowers or inflorescence, which are regarded as an organ by some authors, or as several organs divided into somatic and reproductive parts by others. Macrospores or female gametes and microspores or male gametes are formed in the flowers. Fertilization, also happening in flowers, results in the formation of embryo, which remains latent until germination takes place. Embryos are a component of seeds, which also have nutritional and protecting tissues. Seeds may be surrounded by other tissues, altogether forming the fruits. Germination of an embryo gives a new plant.
Plant organs are divided into three systems:
Protection system includes epidermis and peridermis. They are the superficial tissues of the plant body.
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.
Vascular system are 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. Monocotyledons, herbaceous dicotyledons, and young woody dicotyledons and gymnosperms have primary grow. Secondary grow is found in woody dicotyledons and gymnosperms, and a few monocotyledons. The difference 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 left descendants. Seed plants may have discovered 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.