Seed Shooting

Published: December 15th, 2007 | Updated: 17/01/15

Summer starts at different dates when the indicators are different plants in various parts of the country. Many species of the Leguminosae ([Papilionaceae], [Mimosaceae], and [Caesalpiniaceae]) have fruit called legumen, in which the seeds look like projections on the fruit valves. The species that interest us here have three cell layers in their fruit wall. The most important layer consists of fibers that are arranged in a sharp angle to the length axis of the fruit valves. When the ripe fruit becomes dry, a twisting tension develops, owing to the differential contraction of the different layers.

The cells in the valves’ sutures hold the fruit closed against the twisting pressure. When temperatures rise and hot winds blow at the beginning of summer, the increasing tension leads to a sudden tearing of the fruits along the sutures. The seeds are then ejected in all directions. This tension release may be accompanied by a noise that may be related to the fruit’s size. Walking in an area rich with fruits ready for explosive seed dispersal, one may enjoy the rare pleasure of “hearing seed dispersal”. One has to train oneself to hear this sound and believe me – the reward for learning this element of nature is high. Collection of the right fruits when ripe but before exploding may enable a “public display” at home when hot air is blown on the fruits. Other plants “cooperative” in this manner are species of [Pisum] (pea), [Vicia] (vetches), or [Lathyrus].

Among the ornamental trees, Bauhinia purpurea (Figs. 3.4.1-3.4.4) is regarded as holding the world record in shooting distance – 15 m. When twisted valves are wetted they become straight again and the movement can often be clearly observed. The explosion and seed shooting is undoubtedly more dramatic than the straightening of the twisted valves.

Bauhinia purpurea

This tropical tree is named after two brothers from Switzerland, scientists who laid the foundations of botanical research in the 17th century, and whose studies are still quoted today. The tree became established in ornamental gardens in urban areas of Israel and of its neighboring countries. Its rich blooming (Fig. 3.4.1) attracts the eye in the late spring months. It is not competing for beauty with anemones or other wild plants of the country, it simply reveals its bloom after flowering in most wild areas has ceased. Bauhinia displays its dark petals, which carry dark lines indicating the position of the nectar for pollinators (“nectar guides” Fig. 3.4.2), at the narrow base of the dark spot.

It blooms when free of leaves, like many trees of tropical origin. When its fruits develop, rich in seeds, leaves develop as well and the observer is advised to wait for a hot, dry day. Figure 3.4.3 displays a pair of valves that became twisted upon the instant of explosion. The white spots/depressions are places where a large seed was positioned. Evidence of the opening mechanism can be seen in the two examples in Fig. 3.4.4. Many small, oblique crests are seen in its upper half, displaying the outside view of a valve. The fracture of the valve, in its center, is oblique as well, and runs parallel to the crests. The figure’s lower half displays the inner side of the valve with oblique lines in a similar angle and three fissures in a parallel direction. These oblique elements emphasize the fruit opening mechanism mentioned above.

Fig. 3.4.1: Bauhinia purpurea is a tropical tree that is planted in many parts of Israel and in many places blooms when the tree is leafless. This property is typical of many tropical trees and some interpret it as a way of allowing the pollinators to find the flowers more easily.

Fig. 3.4.2: Dark purple lines on the petals indicate the location of the nectar and are known as “nectar tracts” or “nectar guides”.

Fig. 3.4.3: The Bauhinia fruit (a legumen) opens immediately by releasing the tension accumulated in the valves, caused by twisted fiber formation.

Fig. 3.4.4: A thick layer of cellulose fibers organized obliquely to the longitudinal axis of the legumen influences the tearing direction of the valves.

[Vicia]

The vetch shown in Fig. 3.4.5 displays a closed fruit and an opened one, the valves of which became twisted in an explosion and shot the seeds to the distance typical of this species. A fruit that was artificially opened (Fig. 3.4.6) displays colored seeds that are connected by white pedicels to the fruit wall. If the opening were to take place spontaneously, no seeds would be seen, as the explosion would shoot them away from the fruit. This species ([Vicia lathyroides]) is found mainly in the Golan in the shade of Odem woodland and was photographed with fruits in pine woodland in central Turkey.

Fig. 3.4.5: Vicia lathyroides – an opened fruit with twisted valves (left) and a closed legumen (right).

Fig. 3.4.6: Vicia lathyroides, the fruit is slightly opened to be photographed, displaying the seeds. In nature, when opening starts, it happens immediately.

[Cardamine hirsuta]

I have known the [Cardamine] for many years, mainly as a weed in ornamental gardens and greenhouses. It also grows in the moist grasslands of Mediterranean territory. The most prominent feature of this plant is its leaf morphology; the leaves look like “compound leaves” (Figs. 3.4.7, 3.4.8). When Dalia Belkin sent me a photo of this plant and asked “what is the name of this seed shooter” – I had difficulty answering. On one hand the photo clearly showed a plant of the [Cruciferae (Brassicaceae)] family, but shooting seeds? I did not know of the phenomenon. I believed Dalia’s testimony, but in our telephone conversation I tried to direct the discussion towards a well known seed- shooting garden weed – [Oxalis corniculata] (will be discussed in the next chapters).

I asked my friend Ian Hedge in Edinburgh. He is one of the best experts on this family in Asia and the Middle East. He told me that Dalia is absolutely right and the English common name of the plant is “shot weed”. At an international conference in summer 2007, a world expert on the family increased my knowledge. This species is unique in the [Cruciferae (Brassicaceae)]. The cellulose fibers are arranged in the valves in such a way that they tend to fold the valve into a “spring.” As long as the fruit is not ripe, the soft tissue at the long margins of the valve holds the fruit closed. When dried, the developing tension leads to a vulnerable state.

When such a fruit is touched the fruit explodes, shooting the seeds about one meter from the mother plant – so said Dr. Ihsan Al Shabaz, world expert, from the Missouri Botanical Garden, St. Louis, USA. The valves separate and contract into a form unique in nature, and are thus easily determinable by the “spring-valve” alone. The first greenhouse I came to with this information was that of “Ron in the Valley” at the entrance to Kibbutz Yif’at. I found only a few minute seedlings in several pots, but below one of the display tables a plant with ripe fruits was waiting for me. The result of my touching it is displayed in Fig. 3.4.9.

Fig. 3.4.7: Cardamine hirsute – present in most greenhouses throughout the country. It has compounded leaves and special fruits.

Fig. 3.4.8: Half of the Cardamine fruits have dispersed their seeds and are recognizable by the translucent membrane separating the two halves of the fruit.

Fig. 3.4.9: Two coiled valves of Cardamine hirsute: they became detached and immediately coiled, shooting the seeds away from the mother plant. Three seeds are near the scale. No other plant in the world has similar valves.

[Oxalis corniculata]

[usefulplantsc4 When Dalia asked about exploding fruits and seed shooting] I tried to convince her that the plant she saw in her garden has five yellow petals, trifoliolate leaves with heart-shaped leaflets and long conical fruits (Figs. 3.5.2, 3.6.2). She replied that she also has [Oxalis] in her garden. When the fruits of [“Oxalis corniculata” O. corniculata] ripen, a white tissue develops on the brown seeds and the pressure in the fruit increases (Fig. 3.6.2). Touching the fruit lightly causes it to open along longitudinal sutures and the seeds to shoot to less than 1 m. Small ants collect the seeds with the white tissue and convey them to their crevices.

[“Oxalis corniculata” O. corniculata] is often seen in urban sidewalks or along the meeting zone of the sidewalk and the nearby wall (Fig. 3.6.3). Whoever wishes to demonstrate “magic” should pick-up stems without shaking them and touch the fruit over white paper (for the brown seeds) or a black background (for the white seeds). Touching the intact plants directly, causes “things” to jump above the plant.

Fig. 3.6.1: Oxalis corniculata, yellow flowered and having “heart-shaped” tri-foliolate leaves.

Fig. 3.6.2: A long capsule of Oxalis corniculata with seeds. Touching a ripe fruit causes it to shoot the seeds out through the longitudinal fissures.

Fig. 3.6.3: A carpet of Oxalis corniculata that developed in a side-walk crevice, near a wall. In many sites the seeds are carried into crevices by ants feeding on the oil body present on the seed surface.

[Ecballium elaterium]

Few readers need the warning against walking in a carpet of this plant (Fig. 3.6.4). When the fruits are ripe and become yellowish, and are touched (intentionally or not), the fruit becomes detached from the peduncle and shoots the black seeds with the accompanying fluids. A drop in the eye causes unpleasant burning; inside the mouth it may cause severe bitterness. Beware of contact between the fluids and your head or anybody else’s. Fluid pressure accumulates in the ripening fruit and as a result of human or animal touch, there is a sudden release of the fruit content.

According to a literary record, which should be tested, maximal seed ejection distance may reach 40 m. Teachers are invited to conduct “Olympic” experiments in their schools. We shall be glad to publicize the results communicated to us by e-mail. Children should be warned about the potential “practical joke” they will undoubtedly discover in this plant. Parents’ attention and warnings may save discomfort if the fluids come in contact with children’s heads.

Fig. 3.6.4: Ecballium elaterium – plant tempting children to play pranks.

Fig. 3.6.5: When the ripe fruit is touched, the high pressure stored inside is released and the seeds, with fluid, are instantly ejected to a great distance.