6 TRANSLOCATION IN PLANTS 



at that time also the sap contains the highest organic 

 solute content. Jones, Edson, and Morse (1903) have 

 published a valuable paper dealing with various aspects 

 of the flow of sugary sap from maple and other trees. 

 Sap flowing at that time of year from borings into the 

 xylem of sugar maples {Acer saccharum) and closely related 

 species {A. nigrum, saccharinum , rubrum) has commonly a 

 sugar concentration of from 2 to 4 per cent, and concen- 

 trations up to 8 per cent have been reported. A rather 

 extensive industry based on the collection of this sap 

 sugar has developed in northern United States and Canada. 

 According to the 1910 census, approximately 23 million 

 kilograms of sugar are collected annually from these trees 

 in the United States. A single fair-sized tree may produce 

 25 to 75 liters of sap and 0.5 to 3 kg. of sugar in a season. 

 There is no doubt that this comes entirely from the xylem, 

 and it is commonly assumed that the sugar solution is 

 carried in the transpiration stream to the developing 

 shoots. Many other kinds of plants also have been found 

 to exude a sap more or less rich in organic and inorganic 

 solutes. (Schroeder, 1871; Moreau and Vinet, 1923; 

 Priestley and Wormall, 1925). The total solute concen- 

 tration, however, or the concentration of any one solute, 

 except sucrose as in the maple, butternut, hickory, Ostrya, 

 and a few such plants, has been found to be rather low. 



3. Older Ideas as to Tissues Concerned in Downward 

 Transfer. — Even before anything was known about photo- 

 synthesis and before it was known that most of the organic 

 matter is manufactured in the leaves, it was recognized 

 that there was a movement backward from the leaves of 

 some substance or substances which are necessary for the 

 growth of the stem or roots, and that cutting the layers 

 external to the woody cylinder prevented this backward 

 movement. Malpighi (1679) considered that crude sap 

 ascended through the wood to the leaves where it was in 

 some way changed and then passed backward through these 

 outer tissues to regions of storage or growth. He was led 

 to these conclusions by ringing experiments. Hales (1727) 



