Land Plants: Still working on the reproduction issue
The development of flight in insects was to have a major consequence on the evolution of plants. Early plants including tree forms existed in two alternating forms, a sexual and an asexual generation. Becoming tall would have no effect on the transport of spores and may even enhance their wind-dispersal, however, the distribution of sex cells which, hitherto, was achieved by the male cells swimming through a droplet of water and reaching a female cell. This demanded that the sexual generation was small and grew close to the ground, a situation that is found today in ferns, club mosses and horsetails. The spores of such plants develop into a filmy plant called a thallus which produces sex cells on the undersurface where there is permanent moisture. After fertilization of the female egg cells the thallus develops into the tall spore-bearing plants.