Montag, 25. April 2011

Article of the week - The Basic Structure of a higher land plant

During the last days I have used many terms to describe the plants of my blog like shoot, bract, foliage-leaf ore stalk. So in this article of the week, I want to give you a general overlook about the basic structure of a higher land plant. What is a flower exactly? And what is a bract? To clear these an other questions is the goal of this post. First we look at the basic structure of a higher plant.

A ordinary higher plant consists essentially of two parts: the root and the shoot. The term “shoot” in turn is the sum of all leaves (flowers inclusive) and the shoot axis; shoot axis and leaf are forming the shoot. On a shoot, you can have very different types of leafs. The most important leafs are

  • Foliage-leaves: The basic leaf type; responsible for photosynthesis
  • bracts: this are leafs, which carrying a side shoot and also often the blossom. It can be different shaped from the foliage-leaves (Germans be careful: In English “bract” is the same word for “Hochblatt” (leaves of a blossom) and “Tragblatt” (leaves, carrying a side shoot).
  • Cotyledons: a Cotyledon is the first leaf a plant creates after the germination. Some plants have one cotyledone (Monocotyledons) other have two (Dicotyledons) ore even more. The role of the cotyledons is the supply of the young seed.

     cotyledons of a young plant

A leaf always arises from a node. The area between to nodes is called internode. At some species, the internode can be very short, so you can't see it with your eyes any more. The area between the first node and the Cotyledons is called Epicotyl and the area between Cotyledons and root Hypocotyl

A side shoot in turn always arises from the axilla of a bract. A flower is also a side shoot with very short inter nodes ans special bracts, that are used for reproduction (That's also the definition of “flower”: “A compressed, inhibited growth in the shoot, which is used for reproduction”). The totality of all flowers is called “Inflorescence”. They are many types of inflorescences, but this should be the subject of another "Article.of the Week"

as you can see, a side shoot always
arises from the axilla of a (here big) bract


also flowers always arises from a bract

But now we will take a look at the basic building of a foliage-leaf.

Leaf

The basic foliage-leaf consists of two parts. The first one is the upper leaf, consisting the leaf blade and the stalk, and the second one is the lower leaf with the leaf base, where the leaf is sitting at the shoot axis. Sometimes the leaf base also have some small leaflets, that are called stipule. You can finde stipules for example at the Roses (Rosaceae).


stipules at the leaf ground of Rosa spec.
  
The leaves can be shaped very differently. In some cases the leaf blade is divided into smaller leaflets, in other cases, the leaf has no stalk or has been transformed into a thorn or a tendril. In this post, I only want to show you the basic structure of a leaf. The diffrent leaf shapes, metamorphoses and the Phyllotaxis (the kind, the leafs are sitting at the shoot axis) will be the topic of future "Article of the Week" in this Blog.

 the leafs of the walnut (Juglans regia L.) is a
good example for a normal Leaf. Here you can see
leaf blade and stalk

So for now we'll leave the leaves and come to the flowers.

Flower

A basic flower consists of the perianth, stamens (male part) and at least one carpel (female part). Flowers can be zygomorphical (like the flowers of the Lamiaceae) or radial shaped (e. g. the most (not all) Ranucluaceae).

Perianth

The perianth in turn consists of two different leaf types: the Sepals and the Petals. They are different shaped. The sepals are usually small, inconspicuous and green, while the petals are usually conspicuously coloured to attract pollinators. The leaves of the perianth can be free or grown together. In some cases sepals and petals do not differ morphologically. In this case we speak of a perigon, the individual blades are called tepals.

Stamen

A basic stamen consists of a filament and an anther. One anther in turn consists of two counters each with four pollen sacks (two small and two big ones). Sometimes, two of the sacs are reduced or have vanished completely.

Carpel

A Carpel consists of a basal part, the ovary, where you can find the ovules, and a distal part: the stigma, where the pollen is taking. Between the ovary and the stigma a sterile section can exists: the stylus.


In relation to the orientation of the ovary to the petals, there are three positions
  • inferior: the ovary is completely sunk into the receptacle. The sepals put on top of the ovary.
  • Half-inferior: the ovary is sunk into the receptacle to it's middle. The sepals are based middle of the ovary.
  • superior: the ovary is sitting one the receptacle and not sunk. The sepals set to below the ovary
The totality of the Carpels is called the Gymnoeceum. At many higher plants, the carpels are grown together. Such a Gymnoeceum is called coenocarp, the opposite is apokarp (Carpels are free).

A flower can have carpels and stamens. Such a plant is called monoicous. Other flowers have only one of them. That's called dioecious.

Root

The root is the underground organ of the plant. It has two main functions: anchoring the plant in the soil and the recording of nutrients and water. Roots consist of a main root and many small side roots. The side roots often has many hairs, the so called root hairs

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