24 PLANTS AND MAN 



needed in elaborating proteins from carbohydrates. Excretion 

 of wastes is also important in the economy of the plant organism, 

 though this takes place to a lesser extent in plants than in animals. 

 Added to these basic activities concerned with the intake and 

 outgo of materials, the plant grows and increases in size, protects 

 itself against unfavorable environmental factors, and adjusts 

 itself to a varying environment. 



In carrying out these maintenance functions, green plants 

 present a variety of structural modifications. There is a great 

 range in body size from the microscopic and unicellular algae, 

 to the giant Sequoias, even though both carry on all of the mainte- 

 nance activities essential to plant life. The range from simplicity 

 to complexity, in both structure and function, which exists 

 among living plants may represent the evolutionary sequence 

 through which plant life passed as it became more complex with 

 increasing specialization of functions. Considering a progressive 

 series of plant body types, chosen from living species, we can 

 visualize the probable stages through which plant life passed in 

 the evolution to its present-day complexity. 



This series, from the viewpoint of the maintenance organs 

 and activities of green plants, can be represented by six types of 

 plant body: (7) the unicellular body, as seen in a desmid plant; 

 (2) the multicellular aquatic plant body without any division of 

 labor as seen in the pond scum Spirogyra; (3) the multicellular 

 aquatic plant body with some degree of division of labor, as 

 seen in the kelps such as Laminaria; (4) the body of a primitive 

 land plant, the liverwort; (5) the body of a more advanced land 

 plant, the moss; (6) the most complex type of all plant bodies, a 

 woody land plant such as a maple tree. 



The Unicellular Plant Body 



Green plant metabolism is reduced to its simplest terms in the 

 bodies of the one-celled aquatic plants, of which a desmid will 

 be taken as representative. These minute algae are microscopic 

 free-living organisms dwelling among the leaves and stems of 

 submerged vegetation or floating about in the waters of ponds 

 and lakes. A drop of water from such a habitat reveals a world of 

 these minute plants and their relatives (fig. 8). The cell is char- 



