Thoughts on an enigmatic plant
Thoughts on an enigmatic plant |
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Pachytheca, a small fossil sphere from Silurian/Devonian, has been known for long and has been described from several sites [1] but nevertheless is considered enigmatic. It is regarded as one of several failed attempts of algae to colonize the land. It has been found well preserved in the Rhynie chert and presented at the 3rd Chert Meeting 2004 in Chemnitz. The apparently simple structure is an incentive to try an explanation just now, without waiting for more details eventually to be discovered. Starting from the filamentous habit of aquatic algae, the tightly packed radial array of the filaments to form a sphere is optimal with respect to the reduction of water loss in ambient air. The presence of a cuticle, which is the usual means of land plants to avoid dehydration, is suggested by the observation that the surface of the sphere is very smooth and apparently mechanically weak so that eventually present cracks in the chert readily use it as an easy way of propagation, as seen in Fig.1. Since the cuticle is a barrier against diffusion in either direction, its presence poses the problem of how the necessary water and CO2 could get in. A radial "borehole", which is not seen in the present specimen but shown in [1], could possibly be involved in the problem in an unexpected way: Provided that there were a few tiny pores in the cuticle, liquid water could quickly get soaked in by capillary action and flow along the narrow spaces between the radial filaments (Fig.2) towards the spongy centre, while the air would be driven out through the mentioned hole. Some elementary physics: Without air escaping the water would not enter. In the presence of a water cover, as by a raindrop, the air would come out in bubbles, which form more easily at large openings than at small ones. This is because the pressure in the bubble due to surface tension is inversely proportional to its diameter. Therefore, bubble formation at small openings requires a correspondingly high pressure. Hence, bubbles come out at the largest available opening (if not at the uppermost opening in case of prevailing hydrostatic pressure due to gravity). Water uptake without pores could be possible along the mentioned hole if its inside were well wettable. Then a water film would creep along the internal surface towards the centre while the air could flow out simultaneously.
Whether one of these thinkable means of water uptake was realized in Pachytheca remains an open question. Living plants inevitably lose water since there must be openings to let CO2 in. The well known controllable openings called stomata, which are present in "proper" land plants, serve for this purpose. Since the H2O molecule is lighter than CO2, it gets quicker out by diffusion than the latter gets in, but the uptake of water by liquid flow can vastly outweigh the loss. The soaked up and stored water can be used by the alga filaments during dry periods. It cannot be excluded, of course, that Pachytheca had already been able to control its water regime in a more subtle way instead of passively benefiting from the flow and diffusion phenomena, but the simple scheme outlined here appears to be a natural option. The difficulties encountered by alga-like ancestors in the transition to land have been formulated in [2] as follows: "A primary consideration is the problem of obtaining and conserving water. ... A plant growing on land is not surrounded by water; the nearest source is the soil. Thus early land plants had to evolve a system whereby water could be absorbed from the soil." Now it seems that Pachytheca may have chosen a simpler solution: Quickly soaking in liquid water if available and slowly giving it off by diffusion through an opening and/or small pores in a waterproof cover. text: H.-J. WEISS/Rabenau, photographs: H. SAHM/Dresden [1] H. STEUR: Pachytheca website
(English) http://www.xs4all.nl/%7Esteurh/engpach/epachy.html |
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