Movement in Thistles' Flowers

פורסם: January 15th, 2008 | עודכן: 17/01/15

Being armed with thorns or spines is a phenomenon found in several plant families. The most common spiny plants in Israel belong to the [Compositae (Asteraceae)] and in that family, to the tribe Carduineae. Species of [Centaurea], [Carduus], [Carthamus], [Onopordum], [Silybum] and [Notobasis] belong here. Many species of the listed genera have spiny involucral bracts that protect the small florets, and repel nature lovers to a certain extent. The small florets of the family members are rather similar and can be seen in a schematic drawing (Fig. 3.6.7/3). The floret’s petals are united and their tips look like triangular teeth (d) at the upper end of the corolla tube (e). There is a wide ring-like zone at the lower part of the corolla tube, in the place where there are hairy filaments (‘a’ in Fig. 3.5.10/1). The lower ovary is marked ‘g’ in Fig. 3.6.7/3 armed at its top with a group of hairs.

The anther tube, which typically developed in the whole family from fusion of the anthers, passes inside the corolla tube. The anthers open inwards as seen in Fig. 3.6.7/1 and 3.6.7/2. The style, terminating in a stigma, passes inside the anther tube. Often there are hairs and small, sharp teeth on the parts of the style facing the anthers’ tube. Sometimes there is a ring of hairs below the stigma; it functions like a piston, pushing the pollen upwards. The two stigma’s lobes are closed in Figs. 3.6.7/1 and 3.6.7/2 and open in Fig. 3.6.7/3. The anthers tube is enlarged in Fig. 128/1 and the style passes inside it with a closed stigma. Movement takes place when the floret or inflorescence is touched. Water escapes from the filaments’ cells into the inter-cellular space and the filaments become arch-like (Fig. 3.6.7/2).

A wide part of the corolla tube (below ‘e’ in Fig. 3.6.7/3) enables the arch-shaped-filaments to pull the anther tube downwards, thus pushing pollen grains above the anthers (b in Fig. 3.6.7/2). Fig. 3.6.6a displays a capitulum ready for activity, but no pollen grains are seen yet at the top of the anther tube. The white pollen grain are easily seen above the purple anthers tube in Fig. 3.6.6b; in the same place in Fig. 3.6.6c the styles project above the anthers tube. A similar situation is seen in one form of [Centaurea iberica] (Fig. 3.6.8a); the florets are waiting to be touched and in the second form, displayed in Fig. 3.6.8b, the pollen is already pushed upwards.

Fig. 3.6.6: Crupina crupinastrum – (a) no pollen grains are seen yet in the youngest inflorescence; (b) white pollen grains are seen at the top of the anthers’ tubes; (c) the tops of the styles project from the anthers’ tubes.

Fig. 3.6.7: Schematic representation of a single floret of Centaurea. 1. Anthers tube with 5 free hairy filaments at its base (a), 2. When the flower or the flowering head is touched, the filaments instantly become arch-like, the anthers tube is pulled downwards and pollen grains are pushed upwards (b), 3. A tubular floret: (c). Anthers tube, (d). Corolla lobes, (e). Corolla tube, (f). Hairy pappus, (g). Achene.

Fig. 3.6.8: Centaurea iberica – most florets in capitulum “a” display no pollen grains, whereas most florets in capitulum “b” carry white pollen grains.

Fig. 3.6.9: An inflorescence of Carthamus tenuis with extending pollen-covered styles.

Fig. 3.6.10: Three florets of Carthamus tenuis: (a) A floret from the periphery of the capitulum, having a scaly pappus; (b) A floret from the center of the capitulum; it has no pappus and is designated to remain in the same place as the mother plant. (c) The swollen corolla tube, (d) Opening the corolla tube reveals the hairy filaments.

Fig. 3.6.11: Notobasis syriaca with pollen grains at the top of the anthers tube of most of the flowers.

Fig. 3.6.12: A bee enjoying the beauty of nature from the Notobasis capitulum.