550 A TEXTBOOK OF THEORETICAL BOTANY 



containing a few spiral tracheids and phloem cells arranged around a very 

 minute pith. In spite of its reduction it is therefore essentially a siphonostele 

 and there are distinct, though minute, leaf gaps where the leaf traces are 

 inserted. This simple stele is surrounded by a pericycle and an endodermis 

 the brown-coloured walls of which appear to be suberized. There is a rather 

 loose cortex with no distinct epidermis and no cuticle. 



The root structure is similar, but still smaller (Fig. 554). The root cortex 

 consists principally of a single row of very large cells. The root hairs are 



Fig. 554. — Azolla filiculoides. Transverse 

 section of root showing outer and inner 

 cortical layers and well-marked endo- 

 dermis. {After Queva.) 



arranged in short, transverse rows, alternating at intervals on two sides of the 

 root. A final crop of root hairs is produced underneath the root cap, which 

 then drops off, leaving the apical mass of root hairs exposed. 



The Sporangia 



On the side branches of the stem are produced the fertile leaves or 

 sporophylls, which are always the lowest leaves on the branch. Their 

 structure is complex, but it has been explained by von Goebel as follows : 

 The sporophyll, like the vegetative leaf, consists of two lobes. The lower 

 lobe divides very early into two portions, at the apices of which arise the sori. 

 The indusium grows out as a ring-wall below the sorus and encloses it 

 completely except for a micropyle-like opening at the top, resembling very 

 much an integument round the ovule of a Seed Plant. The upper lobe of the 

 sporophyll has the form and structure of the corresponding lobe in an ordinary 

 leaf, including the Anabaena cavity, but its margins are produced into wings 

 which partly enclose the sori. 



The sori are of two kinds : microsori and megasori, so called according 

 to whether they contain microsporangia or megasporangia. Although 

 the sori are " unisexual " they have been reduced from a hermaphrodite 

 condition, which they still retain in their early development. In both cases 



