What should be regarded as the single most significant structure in higher-plants?

Question

what should be regarded as the single most significant structure leading to rapid success of the so-called "higher-plants"?

Answer 1

cell wall

Answer 2

They differ from all "lower" plants by having flowers ... lots of innovations in flowers, but the most significant is probably the ovules being enclosed in an ovary.

Answer 3

By higher I believe you are limiting your question to the most recent adaptations of plants to terrestrial life. The more recently evolved plants include the largest variation plants have ever achieved. Angiosperms, flowering plants, comprise over 90% of the world’s plant species. The non-flowering Gymnosperms, Ferns, & Bryophytes (mosses) combined come to 27,100 - 30,100 species compared to 250,000 to 400,000 flowering species. Flowers with enclosed ovaries to shield the embryonic seed and their pollinator symbiosis has driven an explosion of diversity greater than the advent of plant life on land in the late Cambrian early Silurian period ~450 mya.

Among the flowering plants three families dominate with almost 25% of the species.

Asteraceae (asters & sunflower family) ~ 24,000 species

Orchidaceae (orchid family) ~ 20,000 species

Fabaceae (legume or pea family) ~ 18,000 species

Each has evolved in very effective but different symbiotic relationships between their flower and pollinators to ensure genetic recombination and enhance diversity. Seeds and their surrounding ovary that forms the fruit, whether dry or fleshy, both protect genetic investments and promote maximum distribution or dispersal of the next generation. The combination of seeds and attractive flowers with enclosed ovaries has driven much greater specialization and habitat partitioning than seeds and cones.

Species counts

http://waynesword.palomar.edu/trmar98.ht...

http://www.af-info.or.jp/eng/honor/essay...

http://www.theflowerexpert.com/flower-bl...

http://msuplants.com/SaylorPlants/Ref_In...

http://english.biodiv.gov.cn/images_biod...

http://msuplants.com/SaylorPlants/Ref_In...

http://amerfernsoc.org/lernfrnl.html

http://www.answers.com/topic/fern?cat=te...

http://books.google.com/books?id=g0aP4yq...

Fruit form & function

http://waynesword.palomar.edu/fruitid1.h...

http://waynesword.palomar.edu/termfr1.ht...

http://waynesword.palomar.edu/termfr2.ht...

http://waynesword.palomar.edu/termfr3.ht...

http://waynesword.palomar.edu/termfr4.ht...

http://www.northernontarioflora.ca/fruit...