SIGNIFICANCE AND EARLY IDEAS 11 



cerned in transport is of major importance because explana- 

 tions relating to the other problems, such as the mechanism 

 involved, conditions determining the direction of move- 

 ment, and factors influencing the rate of movement, all 

 will depend upon an understanding of the tissues concerned. 

 It seems therefore desirable that the evidence for the differ- 

 ent hypotheses be more closely examined and compared. 



SUMMARY 



1. Since no cell of a plant that is differentiated into leaf, stem, and root 

 is totally independent but is dependent upon other cells for food, water, or 

 mineral salts, and since the behavior of individual cells, tissues, or organs, 

 as well as the plant itself, is largely determined by the ainounts and kinds 

 of these materials present, it becomes obvious that an understanding of the 

 tissues concerned in transport of substances from one part of a plant to 

 another, the mechanism of transport, and the factors influencing the rate 

 and direction of transport, is of major importance to one interested in plant 

 physiology or in interpreting behavior. 



2. Until rather recently it has been widely held by botanists that tran- 

 spiration largely controls both the absorption of inorganic salts from the 

 soil and their transport from the roots to the leaves, as well as the upward 

 transport of organic foods. This conclusion seemed substantiated by the 

 fact that large amounts of water are absorbed from the soil and transported 

 to the leaves and there evaporated; that colored solutions introduced into 

 cut stems or through incisions of one sort or another are quickly carried 

 through the wood to the transpiring leaves; and that cut stems often exude 

 a sap containing various salts and sugars. 



3. It has also been widely held that organic materials, moving basally 

 from the leaves are carried through the phloem tissue. This conclusion 

 has been based principally upon ringing experiments that have demonstrated 

 accumulation of foods and increased growth of certain tissues above rings, 

 and a diminished food content and diminished root growth below rings. 

 This conclusion has been further supported by observations showing high 

 carbohydrate contents in the phloem tissues, as well as by the fact that the 

 tissues seem to be so constructed as to favor transport. About 1920 the 

 older accepted theories as to tissues concerned in transport were called in 

 to question. Some investigators proposed that solute transport in both 

 directions takes place through the phloem, others that it takes place chiefly 

 through the xylem. A few years later several distinct theories concerning 

 the mechanism of transport and conditions determining the direction and 

 rate of transport were proposed. 



