Plants, flowering plants in particular, exhibit a tremendous diversity in size and
shape, ranging from just a few millimeters in the tiny duckweeds to almost 100 meters in giant eucalyptus trees. Some may complete their life cycle in a few
weeks, while others live thousands of years. The amazing diversity results from
the adaptation to different, oftentimes hostile environments, as exemplified by the
early evolution of land plants. The colonization of land by plants, dating back some
480 million years according to fossil records (Kenrick and Crane 1997), marks the
beginning of an evolutionary success story, with flowering plants now occupying
every habitat on Earth except the regions surrounding the poles, the highest mountaintops,
and the deepest oceans (Soltis and Soltis 2004). The colonization of land
was a major event in the history of plant life, and at the same time, paved the way
for the explosive evolution of terrestrial ecosystems. Despite the vulnerability of
plants as sessile organisms to adverse biotic and abiotic conditions, they actually
dominate over much of the land surface. This apparent success of flowering plants
relies on the evolved ability to persist in unfavorable and variable environments by
virtue of effective resistance systems that are based on a combination of physical,
chemical, and developmental features (Schoonhoven et al. 2005). It was recognized
by Stahl in 1888 that the great diversity of mechanical and chemical ‘means of
protection of plants were acquired in their struggle for existence within the animal
world’ leading to the conclusion that ‘the animal world [...] deeply influenced not
only their morphology but also their chemistry’ (Stahl 1888; Fraenkel 1959). Hence,
not only thorns and spines as morphological resistance traits, but also the bewildering
variety of plant secondary chemicals attest to the selective pressure exerted by
phytophagous animals (Fraenkel 1959; Ehrlich and Raven 1964).