248 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



not take place in the absence of light, the etiolated shoots grow then cylin- 

 drically. From occasional observation we know that the etiolated shoots in 

 some other species are flat but much smaller than normal, and if such 

 shoots be placed in the light the newly formed parts have the normal flat 

 construction and, according to Sachs, if the shoots be exposed to intense 

 illumination from one side the flat sides take a position so that the light 

 falls perpendicularly upon them. Those species of Opuntia are very 



FIG. 124. Genista sagittalis. The portion on the right shows the normal 

 winged shoot, that on the left is an unvvinged shoot produced in darkness. 



instructive in which the extension of surface is 

 brought about, not by flattening, but by the 

 formation of papillae upon the surface of the 

 stem. In Opuntia arborescens the formation of 

 these papillae is suppressed in the dark. In 

 Phyllocactus, which is only separated from Cereus 

 by its habit, the assimilation-shoots are leaf-like 

 and the clusters of spines are only found on the 

 margins. Phyllocactus latifrons, if grown in dark- 

 ness, forms relatively small shoots, but these are 

 bilateral although the wing is much diminished, 

 just as it is in the case of etiolated Marchantieae. 

 Phyllocactus phyllanthoides, which so readily 

 hybridizes with species of Cereus, has a much 

 greater tendency to produce many-ribbed wingless 



shoots ; in darkness it forms shoots with many rows of clusters of spines ; 

 if it is placed in illumination the formation of wings begins and then there 

 is reduction of the rows of spines to two. It is impossible here to go 

 further into details 1 . 



The influence of light which I have just depicted is by no means 

 restricted to the Cacteae. Fig. 124 shows a portion of a shoot of Genista 



\ 



1 See the literature cited by Vochting, Uber die Bedeutung des Ljchtes fur die Gestaltung 

 blattiormiger Kakteen, in Pringsh. Jahrb. xxvi. p. 438, 



