332
T.G.A. Green and O.L. Lange
environment around the photobiont cells, which are not surrounded by a
cuticle, in a manner similar to the mesophyll environment of higher plant
leaves (Green et al. 1985).
The actual photobiont gas exchange surface area in a lichen does not
appear to be high. A ratio of internal surface area to projected surface area
of 4.6m 2 m- 2 has been calculated for Xanthoria parietina (Collins and Farrar
1978) but, since only about half the photobiont surface is actually in contact
with air space, this reduces to about 2m 2 m- 2 . Measurements of leaf area
index in lichens are rare but must be around 1 for foliose and crustose
lichens, and for fruticose species known values are 6, Stereocaulon virgatum,
and 19, S. tomentosum (Coxson and Lancaster 1988).
16.4 Water Location and Transport
16.4.1 Bryophytes
The mechanisms by which bryophytes can store and move water have been
elegantly described by Proctor (1979, 1981, 1982, 1984, 1990). Bryophytes
can form sheets of photosynthetic cells and these can be arranged so that
photosynthetic exchange surfaces and stored water can be kept separate. An
excellent example would be Pleurozium schreberi where the shoot is effectively a column of water with a shell of photosynthetic tissue around it
(Proctor 1990). Each photosynthetic cell has a water store on one side and
an open CO2 exchange surface on the other. Overlapping of leaves and the
presence of papillae or folds on surfaces are also systems for the movement
and storage of water (Proctor 1980). Areas can be kept free of water by
slight cuticle development (water repellancy), convex surfaces, and papillae.
Perhaps the most impressive example of the separation of water storage and
photosynthetic tissue is in the genus Sphagnum, where the water is held in
large, dead, hyaline cells between the living, photosynthetic cells. Proctor
(1980, 1984) has pointed out that the capillary systems developed by "leaf"
arrangements and structure can transport water from a high potential source
as long as evaporative demand is not great. Effective use of photosynthetic
tissue structures allows bryophytes to have high thallus water content with
only slight depression of A. The above comments apply to the vast majority
of bryophytes that are ectohydric. Endohydric bryophytes, with their capacity
for some internal water supply, behave more like homoiohydric higher
plants and lichens, and show depression in A when excess water is present
(Table 16.4).
16.4.2 Lichens
Water storage is still quite an enigma in lichens. Various tissues, such as the
cortex, photobiont layer, and medulla, have been suggested as storage sites
T.G.A. Green and O.L. Lange
environment around the photobiont cells, which are not surrounded by a
cuticle, in a manner similar to the mesophyll environment of higher plant
leaves (Green et al. 1985).
The actual photobiont gas exchange surface area in a lichen does not
appear to be high. A ratio of internal surface area to projected surface area
of 4.6m 2 m- 2 has been calculated for Xanthoria parietina (Collins and Farrar
1978) but, since only about half the photobiont surface is actually in contact
with air space, this reduces to about 2m 2 m- 2 . Measurements of leaf area
index in lichens are rare but must be around 1 for foliose and crustose
lichens, and for fruticose species known values are 6, Stereocaulon virgatum,
and 19, S. tomentosum (Coxson and Lancaster 1988).
16.4 Water Location and Transport
16.4.1 Bryophytes
The mechanisms by which bryophytes can store and move water have been
elegantly described by Proctor (1979, 1981, 1982, 1984, 1990). Bryophytes
can form sheets of photosynthetic cells and these can be arranged so that
photosynthetic exchange surfaces and stored water can be kept separate. An
excellent example would be Pleurozium schreberi where the shoot is effectively a column of water with a shell of photosynthetic tissue around it
(Proctor 1990). Each photosynthetic cell has a water store on one side and
an open CO2 exchange surface on the other. Overlapping of leaves and the
presence of papillae or folds on surfaces are also systems for the movement
and storage of water (Proctor 1980). Areas can be kept free of water by
slight cuticle development (water repellancy), convex surfaces, and papillae.
Perhaps the most impressive example of the separation of water storage and
photosynthetic tissue is in the genus Sphagnum, where the water is held in
large, dead, hyaline cells between the living, photosynthetic cells. Proctor
(1980, 1984) has pointed out that the capillary systems developed by "leaf"
arrangements and structure can transport water from a high potential source
as long as evaporative demand is not great. Effective use of photosynthetic
tissue structures allows bryophytes to have high thallus water content with
only slight depression of A. The above comments apply to the vast majority
of bryophytes that are ectohydric. Endohydric bryophytes, with their capacity
for some internal water supply, behave more like homoiohydric higher
plants and lichens, and show depression in A when excess water is present
(Table 16.4).
16.4.2 Lichens
Water storage is still quite an enigma in lichens. Various tissues, such as the
cortex, photobiont layer, and medulla, have been suggested as storage sites