54 PHYSIOLOGY. 



gases in the plant become rarefied as a result of the excessive loss of water. 

 There are then successive rows of air or gas bubbles in the vessels separated 

 by films of water which- also line the walls of the vessels. The condition of 

 the vessel is much like that of a glass tube through which one might pass the 

 "froth " which is formed on the surface of soapy water. This forms a chain 

 of bubbles in the vessels. This chain has been called Jamin's chain because 

 of the discoverer. 



109 Why water or food solutions can be raised by the plant to the height 

 attained by some trees has never been satisfactorily explained. There are 

 several theories propounded which cannot be discussed here. It is probably 

 a very complex process. Root pressure and transpiration both play a part, 

 or at least can be shown, as we have seen, to be capable of lifting water to a 

 considerable height. In addition to this, the walls of the vessels absorb water 

 by diffusion, and in the other elements of the bundle capillarity comes also 

 into play, as well as osmosis. 



See Organization of Tissues, Chapter 38. 



110. Flow of sap in the spring. The cause of the bleeding of trees and 

 the flow of sap in the spring is little understood. One of the remarkable 

 cases is the flow of sap in maple trees. It begins in early spring and ceases 

 as the buds are opening, and seems to be initiated by alternation of high 

 and low temperatures of day and night. It has been found that the pres- 

 sures inside of the tree at this time are enormously increased during the 

 day, when the temperature rises after a cold night. This has led to the 

 belief that the pressure is caused by the expansion of the gases in the vas- 

 cular ducts. The warming up of the twigs and branches of the tree would 

 take place rapidly during the day, while the interior of the trunk would be 

 only slightly affected. The pressures then would cause the sap to flow 

 downward during the day, and at night the branches becoming cool, sap 

 would flow back again from the roots and trunk 



Recent experiments by Jones et al. show that while some of the pressure 

 is due to the expansion of gas in the tree by the rise of temperature, this 

 cannot account for the enormous pressures which are often present, for ex- 

 ample, when after a rise in the temperature of 2 C. there was an increase 

 of 20 Ibs. pressure. 



Then again, after the cessation of the flow in late spring there are often as 

 great differences between night and day temperatures. It therefore 

 seems reasonable to conclude that the expansion of gases by a rise in tem- 

 perature is not the direct cause. 



Activities of the cells. It has been suggested by some that the rise in 

 temperature exercises an influence on the protoplasts, or living cells, so 

 that they are stimulated to a special activity resulting in an exudation pres- 

 sure from the individual cells, which is known to take place. With the fall of 



