ï~~1999
THE MICHIGAN BOTANIST
evolved basal and intercalary meristems, a hardened lemma and palea to protect
ingested seeds, and rhizomatous, trample-resistant sod. Grasses also shed the
ability to produce defensive secondary metabolites (e.g., tannins, alkaloids) during this period. These co-evolutionary adaptations permitted horse-like mammals to utilize grasses as food, and grasses to thrive under a grazing regime that
suppressed competing plant species (Clayton 1981; Stebbins 1981).
Coughenour (1985) noted that grasses existed as a distinct taxonomic group
for "quite some time" before abundant grazers appeared, and has suggested that
the adaptations attributed to grazing could have arisen in response to drought,
competition, or the need for physical support rather than in response to grazing
pressure. Nonetheless, the major grass adaptations attributed to grazing pressure
appeared in the fossil record at about the same time as did vertebrate adaptations
associated with a grazing habit (Stebbins 1981; Thomasson 1987), leading most
authorities to accept the co-evolution hypothesis (Clayton 1981; Stebbins 1987).
Fire adaptations
Fire was a factor in grass evolution even before the appearance of vertebrate
grazers (Clayton 1981). Fire benefits grasses by killing taller competitors, maximizing the light and nutrients that grasses can obtain (Weaver 1968). Annual
production of grass litter increases the frequency of grassland fires, which reduces the overall fuel load and the maximum temperature of a grassland burn.
Cooler-burning fires are less likely to damage basal grass meristems and subterranean grass roots and rhizomes, permitting them to rapidly sprout after a fire,
intercept light, occupy space, and recycle nutrients before competitors can become established (Weaver 1968).
Drought adaptations
The climate in African, South American, and North American grassland regions shifted from warm, humid sub-tropical conditions towards cooler, semiarid conditions between 25-60 MYA, during the Oligocene and Eocene epochs
(Stebbins 1981). Several grass adaptations suggest that this climatic shift influenced grass evolution. Grasses evolved an extensive network of highly ramified
roots, allowing them to efficiently scavenge moisture from the soil. When available soil moisture was insufficient to support metabolic processes, above-ground
grass stems and leaves died. Grasses survived these dry periods in underground
buds on roots and rhizomes.
Grasses also evolved the C4 carbon fixation pathway during this period. C4
grasses have a much lower CO2 compensation point, the point at which photosynthesis equals respiration, than C3 grasses (C.P. equals 5 parts per million for
C4 grasses versus 50 parts per million for C3 grasses). This means that C4 grasses
maintain higher CO2 diffusion gradients into their leaves than C3 grasses. This is
important because higher gas diffusion gradients permit C4 grasses to maintain
relatively high photosynthetic rates with partially closed stomata. Plants transpire less water with partially closed stomata than they do with fully-opened
stomata. Thus, the greater CO2 diffusion gradient in C4 grasses results in greater
