INFLUENCE OF EXTERNAL STIMULI. LIGHT 



235 



position is observed, and by a peculiar displacement a bilateral forma- 

 tion is developed out of a radial one in the course of individual develop- 

 ment l . The plant grows in places of slight illumination and this position 

 of the leaves enables it to use them to the best advantage. 



Is light then, as Hofmeister conjectured, directly concerned in the 

 change of the leaf-position? I can answer this question in the affirmative 

 on the basis of my prolonged cultures. At first it was assumed that the 

 direction of the light was of importance, that unilateral illumination had 

 to do with the displacement. Were this so, then a plant grown upon 

 a klinostat with a vertical axis of rotation ought to have a radial leaf- 

 position ; but this is 

 not the case. Plants 

 in such circumstances 

 are still bilateral' 2 ; 

 although occasional 

 torsion of the stem 

 and other deviations 

 from the normal occur, 

 the chief result re- 

 mains the same. On 

 the other hand, plants 

 of Schistostega grown 

 in very low light- 

 intensity retain their 

 radial character ; they 

 become in these cir- 

 cumstances positively 

 heliotropic, and a 

 change of the leaf- 

 position does notseem 

 necessarily bound up 

 with this. Under 

 luxuriant cultivation, 



shoots which in their lower part are bilateral may become radial in their 

 upper part (Fig. 116, right and left), because the newly laid down primordia 

 are radial (Fig. 116, in the middle); the leaf-insertions are then often displaced 

 towards the longitudinal axis, not however in two rows but equally all round. 

 Between radial and bilateral constructions there are then all gradations. 

 There is however, apart from torsion of the shoot-axis, a source of error to 

 consider in this instance : the sexual shoots have leaves in a radial position. 



FlG. 116. Schistostega osmundacea, after cultivation in feeble illumina- 

 tion. The shoots on the right and the left of the figure were grown at first 

 in normal illuminatk n and in their lower parts have normal construction, 

 the upper portions which developed in feeble illumination are radial. The 

 middle shoot was grown from the first in feeble illumination and its leaves 

 are radially placed throughout, usually with an oblique insertion. This 

 shoot had not concluded its growth and might have produced sexual 

 organs had the experiment continued. 



1 See Fig. 26 and Part II of this book. 



2 Plants cultivated for long on the klinostat show disturbances which are especially expressed in 



reduction of the leaves. 



