CHAPTER X 



THE INFLUENCE OF GRAVITY ON THE POLAR CHAR- 

 ACTER OF REGENERATION IN A STEM OF 

 BRYOPHYLLUM 



1. Introductory Remarks. — The decision between the two 

 possibihties for the explanation of the polar character of regener- 

 ation in the stem can be given with the aid of gravity. It had 

 been suggested in a preceding chapter that we must distinguish 

 in a plant between the sap which flows in regular vessels and the 

 tissue sap which exists in the tissue spaces. This latter sap can 

 follow the influence of gravity when the tissue spaces are suffi- 

 ciently large; while the sap in the vessels cannot follow gravity at 

 all or only to a neghgible extent. We shall see a corroboration 

 of this suggestion later in this chapter. The fact that the tissue 

 sap follows gravity gives us a chance to decide whether the polar 

 character of regeneration in the stem is due to any chemical differ- 

 ences between ascending and descending sap or whether it is due 

 to any difference in the nature of the tissues which these two kinds 

 of sap reach primarily in the plant; the ascending sap reaching 

 primarily the anlagen for shoots while the descending sap reaches 

 primarily the tissues which give rise to roots. We shall see that 

 in a stem suspended horizontally the descending sap from a leaf 

 collects in the lower side of such a stem, giving rise to the forma- 

 tion of roots; and that this formation of roots on the under side 

 of a stem increases with the mass of an apical leaf. If now the 

 polar character of regeneration in a stem were due to a chemical 

 difference between the ascending and descending sap from a leaf, 

 such a root formation should not occur on the under side of the 

 apical part of a stem which has a leaf at its base, since, in this 

 case, the ascending sap should contain shoot-forming but no root- 

 forming substances. We shall see, however, that a leaf at the 

 base of a stem causes also root formation on the under side 

 of the apical part of a stem placed horizontally, and that this 

 root formation is exclusively due to the ascending sap sent out 

 by the basal leaf. This leaves little doubt that the influence of a 



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