Reproduction and fruiting

Most woodland herbs produce chasmogamous flowers (CH) that open and expose the reproductive parts to wind, insects or other pollinating agents, encouraging cross-pollination (allogamy), so enhancing biological diversity in the next generation. But the flowers of some species remain entirely closed: cleistogamous flowers (CL) are structurally modified for self-pollination (autogamy); bud-like, never open, and are much reduced compared with the more familiar chasmogamous flowers. Petals are absent or greatly reduced, nectar and scent are not present, and stamens are frequently reduced in number and size. The advantages and disadvantages of the two forms of flower are reviewed by Berg and Redbo-Torstensson (1998), who investigated the sexual reproduction of wood-sorrel Oxalis acetosella, a perennial of European woodlands that produces both types. Figure 4.8 shows the seasonal distribution of the two types of flower. Chasmogamous flowers were produced in spring; ramets (individual members of a clone) with one or more CH flowers left unfertilized generally produced more CL buds than those in which all CH flowers were fertilized.

Chasmogamous And Cleistogamous Flowers

Figure 4.8 Left: Cleistogamous flower of wood-sorrel Oxalis acetosella borne on a flower stalk (pedicel) with two small modified leaves (bracteoles). The flower remains closed during development of the seeds until, as in normal opening chasmogamous flowers, they are discharged explosively through slits in the carpel walls to a distance of about a metre. (Drawn by John R. Packham.) Right: Pooled data for seasonal percentage distribution of chasmogamous and cleistogamous flowers of wood-sorrel in three woods near Uppsala, Sweden dominated respectively by birch/oak, Scots pine and Norway spruce. (From Berg and Redo-Torstensson, 1998. Journal of Ecology 86, Blackwell Publishing.)

Figure 4.8 Left: Cleistogamous flower of wood-sorrel Oxalis acetosella borne on a flower stalk (pedicel) with two small modified leaves (bracteoles). The flower remains closed during development of the seeds until, as in normal opening chasmogamous flowers, they are discharged explosively through slits in the carpel walls to a distance of about a metre. (Drawn by John R. Packham.) Right: Pooled data for seasonal percentage distribution of chasmogamous and cleistogamous flowers of wood-sorrel in three woods near Uppsala, Sweden dominated respectively by birch/oak, Scots pine and Norway spruce. (From Berg and Redo-Torstensson, 1998. Journal of Ecology 86, Blackwell Publishing.)

Detailed analysis of the numbers of CH and CL flowers produced, the season in which they grew and the numbers of seeds maturing in each capsule at the three sites over a period of 3 years, demonstrated the impact of climatic variation and the resources available to the plants concerned. The general view of cleistogamy is that it is a fail-safe mechanism for back-up seed production unaffected by changes in resource supply and environmental conditions. In contrast, these studies showed wood-sorrel Oxalis acetosella to be a perennial with a bet-hedging strategy optimizing reproductive output in fluctuating environments.

Trees near the end of their lives sometimes fruit very heavily, as was the case in a common beech at the Patcham Place site (see Fig. 4.9), which became so dangerous that it had to be cut down in the autumn of 2002. The stump had a hollow centre, evidence of spalting (fungal rot) and a thin zone of active tissue adjoining the bark. The crown, however, bore an immense number of nuts, scoring the top grade 5, while the other trees in the Patcham Place site averaged

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Worm Farming

Worm Farming

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