MOVEMENTS OF IRRITATION. 505 



e.g., by the presence of palisade parenchyma, while the side of 

 the reversed shoots directed towards the soil has assumed the 

 normal character of the under side. The dorsiventrality of Thuja 

 shoots is thus the consequence of local induction. It is brought 

 about by the action of light, not, as Frank sought to show, 

 through the action of gravity. 



If, at the time when the buds are bursting, i.e. in May, shoots 

 of Tax us baccata, in which the leaves are arranged more or less 

 accurately in two rows, are turned through an angle of 180, and, 

 without being removed from the parent plant, are tied fast in 

 that position, we observe that the young shoots (not of course the 

 older, already mature ones) after a few days return by torsion to 

 the position which they occupied at the beginning of the experi- 

 ment. If we rotate the Taxus shoots through 180 before the buds 

 begin to burst (my experiments began in the middle of March), 

 and fix them in this position, then, perhaps chiefly under the 

 influence of gravity, dorsiventrality is soon induced in the develop- 

 ing shoots, which corresponds with the new position, so that the 

 annual shoots after reversal return to the orientation assumed at 

 the time of sprouting. The difference between shoots of Taxus 

 which have developed in the usual manner, and shoots originating 

 from buds whose parent shoots early in the spring were rotated 

 through an angle of 180, is however essentially this, that in the 

 former the needles diminish in length from the lower to the upper 

 side, while in the latter the longest needles are on the upper side 

 (anisophylly). 3 



1 See Sachs, Arbeiten des botan. Inst. in Wurzburg, Bd. 2, p. 271. 



2 See Frank in Pringsheim's Jahrbiicher, Bd. 9, p. 147. 



3 See Frank, Die natiirliche wagerechte Richtung von Pflanzentheilen, 1870, 

 p. 24. 



193. Polarity, 



Numerous plant structures, especially many shoots, exhibit 

 well-marked polarity. The organisation, or even the physiological 

 behaviour of such structures, points to a distinction between base 

 and apex, and we will first conduct some experiments with a view 

 to obtaining a clear understanding of the noteworthy facts in 

 question (Vochting). 



Water is poured into a glass cylinder to a depth of about 1 cm. 



