THE PTERIDOPHYTA : FILICALES, THE FERNS 



549 



ANABAENA 



Fig. 552. — AzoUa fiUcuIoides. Longi- 

 tudinal section of the upper lobe of 

 the leaf showing the internal cavity 

 with filaments of Anabaena azolloe. 

 {After Campbell.) 



close overlapping of upper and lower lobes not only protects the growing 

 points but also serves to entrap air bubbles, which give the plant its buoyancy. 

 From the under side of the floating stems arise the small adventitious roots, 

 which hang down in the water. The primary root alone bears root hairs. 



STELE 



LEAF TRACE 



Fig. 553. — Azolla filiatloides. Transverse 

 section of the stem stele showing reduced 

 siphonostelic structure with leaf trace and 

 leaf gap. {After Eames.) 



Anatomy of the Axis 



The stem of Azolla grows by means of a wedge-shaped apical cell, 

 which cuts off two rows of segments. A leaf arises from each segment thus 

 formed. The vascular structure of the stem is very much reduced, as in 

 other aquatic plants (Fig. 553). It consists of a very small central stele 



