28 REGENERATION 



lymph of animals) and the liquid contained in the spaces between 

 tissues and cells, the tissue sap. This latter sap can be caused 

 to collect in the lowest parts of the leaf under the influence of 

 gravity, while the sap of plants contained in vessels probably 

 cannot be caused by gravity to gather in the lowest parts. This 

 will become clear by experiments described in later chapters. 



It must strike every biologist as strange that gravity plays 

 such a great role in organ formation in plants, while it is rare to 

 find instances where the organ formation of animals is determined 

 by gravity. The reason may be that in plants the tissue sap 

 outside the vessels can move more freely than in animals, where 

 the influence of gravity on the tissue sap can only be dem- 

 onstrated in the case of edema. 



