INFLUENCE OF EXTERNAL STIMULI. LIGHT 



247 



The roots of Podostemaceae l which are spread out upon stones 

 show an analogous behaviour. 

 In Fig. 1 22 a tranverse section 

 is shown of their dorsiventral 

 configuration, and the flatten- 

 ing on both sides is evident. 

 This flattening reaches an un- 

 common degree in Dicraea 

 algaeformis and in Hydro- 

 bryum 2 . We have no ex- 

 perimental investigations up 

 till now upon these cases, yet 

 we may assume that at least 

 in many of them the increase 

 in surface is the result of the 

 direct influence of light, and 

 we shall probably not be 

 wrong if we assume that in 

 other cases the condition has 

 ' become inherited.' 



In most Cacteae the shoot- 

 axes have become transformed 

 into organs of assimilation 

 and storage concurrently with 

 an arrest of their leaves. These 

 shoots often exhibit increase 

 of surface which may be 

 brought about in different 

 ways 3 . In many species of 

 Opuntia the shoot-axes are 

 strongly flattened ; they de- 

 velop out of a radial vegetative 

 point and they show their 

 radial origin in their possession 

 of tufts of spines on all sides. 

 It has now been established 4 

 that in some forms, for 

 example Opuntia leucotricha 

 (Fig. 123), the flattening does 



FIG. 123. Opuntia leucotriclia. Plant which has developed 

 shoots in darkness. These shoots are cylindric, not flattened 

 as are the normal shoots produced in the light. The internodes 

 of the shoots formed in darkness are not elongated as is 

 commonly the case in etiolated shoots. 



1 For the biological relationships of this remarkable group see my ' Pflanzenbiologische Schil- 

 derungen,' ii. p. 2. 2 See the chapter upon the Root in Part II of this book. 



3 Goebel, Pflanzenbiologische Schilderungen, i. 



4 Goebel, Uber die Einwirkung des Lichtes auf Kakteen nnd andere Pflanzen, in Flora, 1895, 

 p. 96. Sachs' work is referred to in this paper. 



