8o HEREDITY AND EVOLUTION IN PLANTS 



cation has been two-fold: (i) simple individuals, whether 

 mountains, rivers, planets, animals, or plants, have become 

 more complex (e.g., compare the structure of the plant, 

 Pleurococcus, a simple spherical cell, with that of the fern) ; 

 (2) the relation between living things, and between them 

 and their surroundings has become more complex (e.g., 

 compare a unicellular bacterium, with its relatively simple 

 life relations, with the clover plant, highly organized, and 

 related to water, air, soil, light, temperature, gravity, 

 bacteria (in its roots), and insects (for cross-pollination)). 



Most of the steps of evolution have been progressive, 

 toward higher organization, greater perfection of parts, 

 increased efficiency of function, as, for example, from 

 algae having one or a few cells only, to flowering plants, 

 like roses and orchids; but not all the steps have been in 

 this direction. Some of the steps have been regressive, 

 toward simpler organization, less perfection of parts, 

 decreased efficiency of function, as, for example, from 

 green algae to the non-green, alga-like fungi (Phycomy- 

 cetes, such as bread mold) , from independence to parasitism 

 (mistletoe and dodder), or to saprophytism (Indian pipe 

 and toad-stools). 



The thirty odd species of the Duckweed family, related 

 to the Arum family (Jack-in-the-pulpit, calla, skunk cab- 

 bage, sweet flag, etc.), illustrate regression; they comprise 

 the simplest, and some of them the smallest of all flowering 

 plants. The plant body of Lemna is a tiny disc-shaped, 

 thallus, having a central vein (vascular strand) with or 

 without branches. Each plant has one root with no 

 vascular tissue. The flowers, borne on the margin or 

 upper surface of the thallus, have one simple pistil and 

 only one stamen (Fig. 44). The dozen or more species 



