JACQUES LOEB 375 



gravity upon the distribution of sap. It is, therefore, not gravity 

 alone which determines the directive effect in these cases but gravity 

 in combination with the modification of the sap flow towards a 

 growing organ. ^ 



This reasoning is supported by the following facts. When we 

 prevent the growth of roots and shoots on the lower side of the leaf 

 by cutting off the dormant buds contained in the notches of the lower 

 side of a leaf of BryopJiyllum suspended vertically and sidewise, roots 

 and shoots will now develop as abundantly on the upper side as they 

 otherwise would have developed on the lower side ; the only difference 

 being that the roots begin to appear on the upper side slightly later 

 than they would have appeared on the lower side.^ 



The same proof can be furnished for the formation of roots on the 

 lower side of a horizontally placed stem. When we cut off the lower 

 half of such a stem the roots will now form abundantly on the upper 

 side of the stem.^ 



The fact that some sap collects on the lower side of a piece of stem 

 suspended horizontally or on the lower side of an isolated leaf 

 suspended sidewise in a vertical plane may be compared to the be- 

 havior of edematous liquid in animals which also follows gravity. 

 It is possible that only that part of the sap of an isolated organ is 

 thus affected by gravity which is not in active circulation in the 

 vessels. The circulation in the vessels will cause an abundant col- 

 lection of material at the ends of a piece of stem around the whole 

 circumference of the piece and this may account for the fact that at 

 the basal end of an excised piece of stem suspended horizontally in 

 air roots develop on both the lower and upper side in such abundance 

 that the slight quantity of sap collected through the influence of 

 gravity on the lower side of the base becomes a negligible factor. 



The real share of gravity in the directive influence on root and shoot 

 formation is, therefore, a modest one since it needs consist only in a 

 slightly greater collection of sap on the lower side of an organ, just 

 su£&cient to accelerate the growth of roots on the lower side. 



It occurred to the writer that this conception of the directive 

 influence of gravity on root formation might be tested in still another 

 way; namely, by suspending leaves or pieces of stem under water. 

 In this case the influence of gravity on root formation should dis- 



