THE HISTORY OF PLANTS 



407 



of coral reefs and forming marl deposits in fresh water. The group includes 

 both unicellular and multicellular types, and some of the latter are known 

 as far back as Cambrian time. The lime-secreting red algae form reddish 

 or whitish incrustations on shells and rocks and are among the important 

 rock builders in the tropical seas today, contributing as much to the 

 formation of coral reefs as do the corals themselves. First known from 

 the Ordovician period, the group has increased in abundance and impor- 

 tance to the present. The brown algae are the familiar 

 seaweeds called kelps. They are the most highly organized 

 of the algae, and some of them reach large size, but like 

 the others they lack roots and leaves and take their 

 sustenance directly from the water which surrounds 

 them. Lacking hard parts they are seldom fossilized, but 

 imprints ascribed to brown algae have been found in 

 Proterozoic rocks. 



Finally, the diatoms (Figs. 26.7 and A. 3) require men- 

 tion. These are microscopic unicellular plants with 

 delicate siliceous skeletons. Various species of this group 

 make up a large part of the phytoplankton of modern 

 seas. A single quart of sea water may contain thousands 

 of billions of diatoms during the reproductive season, and 

 the millions of tons of diatom protoplasm present in 

 the upper layers of the oceans constitutes the most 

 important basic food source for marine animals. Diatoms 

 and other phytoplankton are fed upon by protozoa, by 

 the larvae of free-swimming and bottom-dwelling 

 animals, and by other small fry at the base of a food 

 chain that extends to the largest carnivores. Many 

 diatoms occur in the plankton of lakes and ponds, and 

 other species live attached to submerged surfaces in both 

 fresh and salt water. The group is probably ancient, but 

 fossils are unknown from the older rocks, doubtless 

 because the fragile skeleton is so easily crumbled by pressure or dissolved by 

 ground water. In Cretaceous and Cenozoic strata, however, great beds of 

 diatomaceous earth have been preserved, and similar deposits are being 

 formed today. 



The Fungi (Figs. 26.8 and A. 5 to 9) are plants incapable of photosyn- 

 thesis for lack of chlorophyll. For the most part they are scavengers, 

 working with bacteria to destroy the dead bodies of organisms ; but many 

 have become parasitic on living plants or animals, and some have entered 

 into a cooperative relationship with higher plants, serving the latter as 

 functional replacements of root hairs. The fungi as a group are quite old; 

 mycelia and fungus spores have been found in fossilized decayed stems 



Fig. 26.7. A 

 modern species 

 of diatom, Na- 

 vicula crabro. 

 (Courtesy General 

 Biological Sup- 

 ply House, Inc.) 



