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TEXTBOOK OF BOTANY 



others thrive with their roots and rhizomes in soil continuously below 

 the water surface. The erect stems of such plants extend above the 

 water surface, or because of long petioles their leaf blades are exposed 

 to the air above the water. Among the plants of the world there appear 

 to be all gradations between those limited to land and those limited 

 to water habitats (Fig. 268). Those on land can endure varying degrees 



Fig. 268. Plants in various degrees ol submergence. Many plants not evident 

 in the pictvire are wholly submerged. Water milfoils, hornwort, elodeas, pond- 

 weeds, and many others may be wholly submerged or with their tips slightly 

 above the surface (foreground). Leaf blades of water lily and pond lily may be 

 floating or raised above the surface. The leaves of cat-tail and bulrush are mostly 

 aerial, and all but the root systems or parts of root systems of dogwood, alder, and 

 willow are above the water. Photo by E. S. Thomas, Ohio State Museum. 



of exposure to the air and to water loss bv exaporation. Those below 

 the water surface can endure low concentrations of oxygen. 



Some submerged plants live wholly suspended in water. Others 

 have roots in the underlying substrate. The general processes of nutri- 

 tion and growth in submerged plants are the same as those in land 

 plants, and they are dependent upon the same external factors. These 

 factors, however, are not similarly distributed or equally available in 

 air and in water media. A few of the conditions in land habitats and 

 in water habitats are compared in the accompan}dng summary. 



