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below is not localized in the stem perhaps we may say is not 

 transmitted in a direct line in the stem. A notch cut half-way 

 across the stem will not cause the cotyledonary but directly below 

 it to grow. In Bryophyllum, however, Loeb's 3 experiments have 

 indicated that the inhibitory influence of a leaf on the growth of 

 axillary buds passes downward in the same sector of stem as the 

 leaf itself. Moreover the inhibitory influence appears to be of 

 the nature of material flowing, because the pathway of the in- 

 hibitory influence is affected by gravity. This is illustrated in 

 Figs. 11, 12 and 13, pp. 349 and 351 of Loeb's paper. This in- 

 fluence of gravity is a fundamental fad whose importance for 

 the explanation of regulation phenomena must not be overlooked. 



It is obvious to one who seriously contemplates- the facts of 

 regulation that the influence of one part over another in the 

 organism, must be either similar to nerve influence and depend 

 on living protoplasmic continuity between the parts, or due to 

 the actual transport of material from one region to another. 

 Unless we are to assume the existence of a guiding all powerful 

 form-determining spirit or force, which is as difficult to prove as 

 to disprove, there is no other than these two explanations. These 

 we know to be two means of "action at a distance" in animals 

 and we might expect them to be operating in a plant also. 



Let us consider the second of these possibilities — transport of 

 material. Two views are prevalent regarding the nature of this 

 transport. (1) A growing stem may be supposed to form ma- 

 terial which inhibits shoot formation and a growing root to form 

 material which inhibits root formation. 4 These special inhibi- 

 tion substances pass downward and upward in the stem respect- 

 ively. Polarity is a direct consequence of the formation of 

 these substances and their direction of flow. Loeb has pointed 

 out several instances of growing roots inhibiting the formation 



3 Loeb, J., Journ. Gen. Physiol., I, p. 337 and 687, 1919. 



stances collect at the basal end of a cut stem and induce the formation of 

 roots there. That this assumption will not hold is indicated by McC'allum's 



stem of a whole bean plant is surrounded near its uppef end by water held 

 in a glass vessel. This gives favorable conditions for the development of 

 roots but none grow so long as the roots of the plants are intact. If they 



