D. Diploid E. Most autotrophic, a few are heterotrophic 1. The two halves of the frustules fit together like a Petrie dish I. Reproduction mainly asexual. The two halves of the frustules open and each side generates a new half. Sexual reproduction takes place via oogamy J. Diatoms are abundant in the fossil record from at least 250 million years ago. Their silica shells have accumulated in large numbers to form large deposits of diatomaceous earth .
Kelp beds are some of the most productive ecosystems on earth.
They flourish to depths of 100 ft., and may extend out about 6 miles from the coast line. The free floating forms may form dense masses, e.g. Sargassum forms the Sargasso Sea in the Atlantic northeast of the Caribbean Islands. Sizes range from microscopic to giant kelps up to 200 ft. long. Some of the kelps have tissue differentiation into holdfast, stipe and blade. E. Some have conducting tissue in the stipe for food transport from photosynthetic blades way down into dark areas below sunlight penetration.