174 



BOTANY OF THE LIVING PLANT 



condition is only acquired in the course of the individual development, 

 and is not shown by the apical bud. This is seen in the rhizomes of 

 Acorus, where the apical bud shows a bilateral symmetry, with alter- 

 nate leaves. But in the mature rhizome the two rows are shifted to 

 the upper surface, giving a dorsiventral condition, with roots arising 



Fig. 130. 

 Successive sympodial shoots of Cait 



(.^fter Figuier.) 



from the lower surface. In other cases again, and conspicuously in 

 Ferns, the dorsiventrality is fixed already at the growing point, and 

 is not the mere result of any subsequent displacement of parts. This 

 is seen in the Common Polypody, which has from the first distichous 

 leaves placed obliquely on the upper surface of the rhizome, while 



