Sonntag, 15. Mai 2011

Article of the week - Inflorescences

The sum of all flowers of a plant is called inflorescence. Inflorescences exist in many different forms and shapes, so in this week's “Article of the Weel” I want to show the basic and common variants of inflorescences. I'll not show all of them, but only the main types.

So first, let us check out some basic terms. When you look at an inflorescence, you have maybe noticed, that some of them are ending into a terminal flower and others not. In Botany we speak of a monotelic (closed) inflorescence in first case and of a polytelic (opened) inflorescence in the second

It's also makes a different, if an inflorescence only consists of single flowers or in turn are compounded of small inflorescences by itself. First we look at inflorescences, which consist only of single flowers.

I. simple Inflorescences

a) ear: the ear is possible the most simple type of an inflorescence. Here, the flowers are sitting directly into the axil of their bracts. Ears can be found at some species of the Poaceae (e. g. the Genus of Hordeum), but also at the Genus Plantago. A cone is also a variant of an ear.

b) raceme: the raceme is not very different from the ear. The only difference is, that the flowers of a raceme are sitting at the end of a short stalk, growing from the axil of a bract. This type of inflorescence can be found e. g. at the Genus of Viccia.

c) umbel: At the umbel, the flowers also are sitting on a stalk. All stalks starts at the same point of the shoot axis. The bracts of the flowers often forms a such called involucrum. So the whole inflorescence looks like a umbrella. Umbels are the typical character for the Apiaceae family, which is also called Umbeliferaceae. Be careful, because sometimes, a raceme is formed like an umbrella (e. g. at Capsella bursa-pastoris L.), but this are no real umbels, because here the stalks are not starting at the same point.

Sometimes, an umbel consists of smaller umbels. This is a double-umbel (d)

e) & f): catkin: a catkin in basically no more than an inverted and hanging ear or raceme. This type of inflorescence is typical for many genera of our native trees like Alnus, Populus or Salix.

note: in this picture the small bracts of 
the spadix and the head 1 has not be draw

g) spadix: A spadix is a special type of an ear, but here, the floral axis is thickened and cylindrical. Spadices can be found e.g. at Zea mays L. (corn) and the most species from the Araceae. They are also typical for the Genus Typha (bullrush or corndog grass) of the Typhaceae.

e) head 1: The first type of the head-formed inflorescence is similar to the spadix. The transition is fluently indeed. The flora axis is also thickened but more jolted. This inflorescence is typical for the Trifolium Genus (e. g. Trifolium medium ssp. medium L. in this Blog)

f) head 2: The second head inflorescence is the characteristic inflorescence of the Asteraceae family (with species like common daisy, common dandelion or the sun flower). Here the floral axis is compressed even more than the first head type. The flowers are sitting directly at this floral axis and their bracts are forming a involucrum.

II. compounded inflorescences

O.k. Folks, now it becomes complicated. In the previous inflorescences we always have a single flower per bract. But sometimes, a inflorescence is multi-branched, that means instead of a flower it has smaller floral axis, growing out from the axial of the bracts.

In this case it depends on how the inflorescence is branched. First, we have two basic types again: the panicle and the cyme.

a) panicle: At a panicle, a new floral axis grows from a single bract. This axis can be branched by itself. The branching doesn't follow a strict scheme and flowers can grow everywhere. A panicle is always monotelic.

b) cyme: If the successive branches follows a scheme, we speak of a cyme. If there is only one new axis per branching, we call it a monochasium. With two new axis per branching, we called dichasium.

Now we subdivide the monochasial and dichasial cymes into three types again.

c) thyrse: A thyrse is simply a inflorescence with dichasial cymes as branches. The inflorescence of Aesculus hippocastanum L. (conker tree) is a good example for a thyrse

d) depranium: A depranium is a monochasial cyme. Here, the floral axis are helical applied towards the main axis.

e) rhipidium: The rhipidium is basically the opposite of the depranium. The successive floral axis grow in a zigzag pattern. The inflorescence of the tomato is e. g. a rhipidium.

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