HOW PLANTS ARE INFLUENCED 



329 



All these features are very characteristic of desert 

 plants ; in fact, the whole appearance and behavior of 

 this plant reminds one of a Cactus. 



On the other hand, we may, by growing a Cactus 

 in the dark, with abundant moisture, cause the spines 

 to grow out into succulent, leaf -like organs. Fig. 181 

 shows a sprout which 

 has grown from a 

 Prickly Pear under 

 these conditions. It 

 will be noticed that the 

 sprout appears cylin- 

 drical, and that in its 

 whole appearance it is 

 more like an ordinary 

 plant than is the Potato 

 plant shown in Fig. 

 179. This sprout was 

 obtained by taking a 

 joint of Prickly Pear 

 which was preparing to 

 sprout, placing it in a 

 pot of sand, and set- 

 ting this inside a closed 

 box to exclude the light. 

 It was watered frequently, and in the course of about 

 two months produced the sprout shown in the figure. 



We believe that the ancestors of the Cactus had 



181. Sprout of Prickly Pear grown in the dark, 

 showing leaf-like organs. 



