PLANT STRUCTURE AND COMPOSITION 195 



plant. The germ or embryo is particularly rich in vitamin E. Al- 

 though no definite metabolic role in the plant can be ascribed to these 

 vitamins, there are suggestions that they play a role in electron trans- 

 port, particularly in the photosynthetic process. The fat-soluble 

 vitamins A and D are not found in plants, although the precursors 

 /^-carotene (page 208) and ergosterol (page 94) are present in many 

 plant tissues. 



The distribution ol the nucleotides in plants follows the same 

 pattern as that of the vitamins, the greatest concentrations occurring 

 in the rapidly metabolizing cells. The same is true of many of the 

 essential minerals. Manganese, zinc, magnesium, potassium, molyb- 

 denimi, copper, and iron are either components of, or form com- 

 plexes with, enzyme systems in performing their cofactor roles. Mag- 

 nesium is a component part of chlorophyll (page 207). Iron and cop- 

 per are constituents of oxidases and electron receptors (pages 150 and 

 172). Calcium is found in both the cytoplasm and in many cell walls 

 where it occurs as calcium pectate, a hydrophylic colloid which presum- 

 ably plays a part in water imbibition by the cell. Calcium pectate acts 

 also as mortar in cementing the cells together. Boron is distributed 

 throughout the plant, although the greatest concentrations are found 

 in the growing tips. The remaining requisite elements have already 

 been described in preceding chapters as constituents of proteins. 



INCIDENTAL AND SPECIAL SUBSTANCES 



Non-essential Elements 



In addition to the above-mentioned essential elements, the plant 

 absorbs many of the diverse inorganic ions found in the soil. Sodium 

 ion is found in all plant tissues grown under usual conditions. Al- 

 though it cannot be considered essential for growth and reproduction, 

 many plants, such as beets, celery, and cabbage, respond to sodium 

 additions when grown in deficient soils. Silicon and aluminum, two 

 common elements in most soils, are accumulated in small quantities, 

 particularly in the woody or stem tissues. Silicon may serve as a stiffen- 

 ing agent in the cell walls. Cesium and rubidium compete with potas- 

 sium in the absorption process. Plants grown in soils containing 

 appreciable quantities of these and similar elements contain measur- 

 able amounts. In most cases such absorption is inconsequential, both 

 to the plant and to the animal which consumes it; however, selenium 

 provides a well-known exception (pages 390 to 392). 



