344 Mineral Nutrition of Plants 



lower leaves to comparative xeromorphy of upper leaves on the same 

 stem (y6, 84). It is also obvious that data on root physiology are essen- 

 tial to a comprehensive grasp of ontogeny. This fact explains the prefer- 

 ence of investigators for liquid and gravel cultures from which roots 

 are readily retrievable. Such cultures, in addition, permit better control 

 and study of the substrate than in the case of soil-grown plants. Finally, 

 a running inventory on the composition and behavior of the nutrient 

 solution or substrate is a great aid to an understanding of the plant's 

 relationship to and effect upon its edaphic environment (/, 14,26, jy, $y, 

 59,60,106). Thus, Hartel (52) reports translocation of carbon dioxide 

 from roots to shoots where it can be used in photosynthesis, and Ulrich 

 (7^5) has noted the buffer action of organic acids in roots when cation 

 absorption exceeds anion intake (79). 



In scanning the numerous studies of developmental physiology, it is 

 surprising to find many similarities in fundamental processes despite 

 the extreme diversity of plant types and environmental conditions in- 

 volved. As might be expected, a preponderance of data exists concern- 

 ing herbaceous annuals, and the most consistent portion of this infor- 

 mation relates to plants grown under controlled environmental con- 

 ditions. Comparable data for biennials and perennials, often of neces- 

 sity grown under field conditions, are more difficult to obtain, hence, 

 often relatively incomplete and more difficult to correlate and interpret. 

 Because of this fact the present discussion is limited to those aspects of 

 developmental processes which seem common to the wide range of ordi- 

 nary herbaceous annuals most frequently investigated. 



THE VEGETATIVE STAGE 



Under favorable growing conditions, the phase of rapid vegetative 

 enlargement in typical annuals is characterized by progressive incre- 

 ments in absolute amounts of inorganic elements, carbohydrates, and 

 proteins. Due to the accelerated synthesis of organic compounds, the 

 proportion of ash on a percentage basis begins to fall even though abso- 

 lute amounts of the latter may continue to rise until well toward ma- 

 turity (29, 7/, log). Many annuals tend to absorb the major portion of 

 their total mineral supply in very early life (iy.yj, 142) and early ab- 

 sorption is, in general, in excess of current needs when the external 



