Montag, 20. Juni 2011

Article of the Week - Morphology of the grass flowers

In this “Article of the Week” (the first since two weeks) I want to give you an overview about the flower-morphology of the three great families of grasses: the Poaceae (true grasses), the Cyperaceae (sedges) and the Juncaceae (rushes). This has became necessary, after I introduced two species from the Poacae and with them, I also used some new terms, which I want to explain now.

I. Basic informations

All members of these three families belong to the Monocotyledons. The Monocotyledons are one of the two big subgroups of the Angiosperms (the other ones are the Dicotyledons). Many well known species and genera belongs to this sub-group, e. g. the Orchids (Orchidaceae), the lilies (Lilaceae) and also the grasses.

All Monocotyledons share some similarities between each other:

  • The embryos of all Monocotyledons have only one cotyledon (name)
  • (Almost) all Monocotyledons have parallel leaf-veins
  • The perianth of the Monocotyledons isn' t morphological divided into petals and sepals; there are only two circles of leaves. This type of a perianth is called Perigon and its single leaves Tepals
  • All Monocotyledons have threefold flowers (3+3 tepals, 3+3 stamen, 3 Carpels)

While the most Monocotyledons make beautiful flowers, the grasses have adapted themselves towards wind pollination and wind dispersal (Aerochorie). This leads to some changes within the morphology of their flowers and their inflorescences

II. The flowers of the Poacae

Basically, the structure of the flowers of the Poaceae corresponds to the structure of all Monocotyledons with three outer tepals, three inner tepals, three outer stamens, three inner stamens and three carpels.


However, by the time they've undergone a drastic change in their morphology. One of the outer tepals has vanished completely, the other ones has fused and became a complete new structure, which is called palea (“Vorspelze” in German). The inner tepals are reduced to small leaflets, which are called Lodiculae (Lodicula in Singular). The bract of each flower is very narrow and called lemma ("Deckspelze"). Sometimes a lemma has a long extension on their ventral side, which is known as awn ("Granne"). The bract of the whole inflorescence is called glume ("Hüllspelze").

The inner stamens have been reduced completely, and two of the three carpels are stunted.

The inflorescences is a simple raceme, but with very, very short internodes, so you think, that each of them is just a single flower. Many people often call the inflorescence of the grasses “ear” but botanically this ear is in fact a double-raceme. There are three different types of double-racemes at the Poaceae.

  • Panicle grasses: The inflorescence are sitting at the end of a long stalk. Typical Genera of this type are oat-grass (Arrhenatherum P. Beauv.), bluegrass (Poa L) or the reed canary grass (Phalaris arundinaceae L.)
  • ear grasses: At the ear grasses, the racemes are not stalked. Morphologically, this type is a double-raceme, not a real ear. Ear grasses are Barley (Hordum vulgare L.), rye (Secale cerale L.) and wheat (Triticium aestivum L)
  • Panicle-ear grasses: The inflorescences are very short stalked, so on the first sight, they may look like an ear. Grasses from this type are e. g. the foxtail (Alopecurus pratensis L.) or Timothy-grass (Phleum pratense L.)
III. The flowers of the Cyperaceae

The flowers can be the divided into two types: the flowers of the genus Carex L. and the flowers of the rest (Eriophorum L., Scirpus L., Schoenoplectus (Rchb.) Falla. and so on). While the flowers of the most genera correspond to the basic scheme of the Monocotyledons, the genus of Carex have a very unique type of flower.

At the genus of Carex, we have always pure male and female flowers, no hermaphrodites. The perigon has vanished completely, so we have no tepals. The male flowers only consists of three stamens (one circle), which is sitting in the armpit of a bract. Thereby, the male inflorescences are real ears.

The female flowers are double-ears. The fused carpels are located in the armpit of a branch, which grows from the armpit of a bract (a rudiment of this branch still exists). The bract of the flower has transformed into a tube like structure: the utriculus. Each utriculus has a small opening, where the styli of the carpels are looking out.

IV. The flowers of the Juncaceae

The flowers of the Juncaceae follows the basic scheme of the Monocotyledons overall, but their flowers are often very small and inconspicuous compared to the beautiful flowers of the lilies and orchids. In the most cases, the inflorescences are panicles or a drepanium.

V. Final Comparsion

The morphology of the flowers is not the only difference between these three families. The following chart gives you an overview about the main differences.


Poaceae
Cyperaceae
Juncaceae
shoot-axis
Round, with nodes, always hollow (except the nodes)
Triangular, without nodes, not hollow
Round, without nodes and not hollow
flower formula
P(2)+2 A3 G(3)
Male: A3; Female: G3
P3+3 A3+3 G(3) or
P3+3 A3 G(3)
fruit
One seeded nut (Karyopse)
One seeded nut
3 to many seeded capsule
inflorescences
double-racemes
double-ears
Panicle or drepanium
leaf-pattern
Alternate, in two
Alternate, in three
Alternate, in two or three
leaf-shape
Flat or rolled
Flat or rolled
Flat and ciliated

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