51« 



LECTURE XXXI. 



in this case experiment teaches that it is an effect of gravitation. In the first place, we 

 have sometimes an opportunity of observing in gardens where Opuntias are cultivated 

 the case represented in Fig. 337 a. This figure shows a very large shoot-segment of 

 the plant, /, which has given rise in the usual manner on the right and left angles to 

 the lateral segments marked //. After the production of the latter, the segment / 

 had accidentally bent over at its apex, so that one of its flat sides was turned down- 

 wards and the other almost horizontal and facing upwards. When new segments 

 began to shoot forth in the next spring, they appeared not exactly at the angles of /, 

 but rather more on the uppermost surface of the segment, but nevertheless not far 



Fig. 337 A.—Opicntia Fktts indie, 



from its two margins, as shown at /// in the figure. Only in the following year did 

 a further shoot /Farise, quite independent of the two lateral angles, on the up-turned 

 surface of the mother-segment. It thus required two years to overcome the tendency 

 to form shoots at the angles, through the influence of gravity, and perhaps with 

 the co-operation of light, so that the shoot IV could arise at the middle of the 

 upper side of the shoot /. To see how far the relations of configuration of this plant 

 are subject to external influences, however, it is first necessary to know that the two- 

 edged flat form of the shoot-segments is in its turn due to illumination, since the 

 segments of Opiintia assume this flat two-edged form only under the influence of light, 



