CHAPTER XI 



THE INFLUENCE OF GRAVITY ON THE POLAR CHAR- 

 ACTER OF REGENERATION IN A STEM OF 

 BRYOPHYLLUM (CONTINUED) 



When a completely defoliated stem is suspended horizontally 

 in moist air the polar character of regeneration is not changed as 

 far as shoot formation is concerned. Shoots continue to form in 

 the apex. The polar character of root formation undergoes, 

 however, a change which becomes striking when the leaves are 

 not all removed. It is true that roots continue to be formed at 

 the extreme base of the stem and this root formation undergoes 

 no change when the stem is put into a horizontal position. The 

 change manifests itself in the fact, however, that in addition to 

 this polar form of root formation there appears now a second form, 

 namely, along the whole lower side of the stem. This latter 

 effect is due to the collection of tissue sap on the lower side of the 

 stem under the influence of gravity. This peculiar influence of 

 gravity on the regeneration of horizontally placed stems finds its 

 explanation in the assumption already referred to that there are 

 two channels for the distribution of sap in the stem; first, by the 

 vascular system, and second, by the gaps between cells and tissues. 

 The sap flow in the vascular bundles is little, if at all, influenced 

 by gravity, and thoseformsof regeneration which depend upon the 

 vascular sap are little if at all affected by gravity. We notice 

 therefore that shoots continue to form in abundance at the apex 

 and roots at the extreme base of a stem placed horizontally. 



The tissue sap, however, follows gravity, collecting on the lower 

 side of the stem where roots but no shoots are formed. In this 

 respect a stem suspended horizontally differs from a leaf which 

 when suspended horizontally forms both roots and shoots on the 

 lower edges. This difference in the influence of gravity on regen- 

 eration in the leaf and the stem is a strong argument in favor of 

 the tissue theory of polarity and against the hormone theory. For 

 if it is true that the descending sap which is responsible for the 

 root formation at the base of a stem has this effect because it 



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