THE PTERIDOPHYTA : FILICALES, THE FERNS 



557 



Isolated though Azolla appears in its pecuHar structure, there are certain 

 points of resemblance to the Hymenophyllaceae, also a somewhat reduced 

 group. The basipetal order of development of the sporangia on the placenta 

 and the sheathing indusium have their parallels in this latter group, and it 

 seems probable that these are the nearest relatives to our present type among 

 the true homosporous Ferns. 



AzoUa Plant 



Microsporangium , 



Megasporangium 



Embryo 



Microsporet Megasporc 



Oosp/ierei 



/Antberozoids 



Archegonium Antheridium 



Female Protballus 



'Male Prothallus 

 Fig. 563. — Life-cycle oi Azolla filiculoides. 



Evolution of the Stelar Anatomy in the Filicales 



The examples of the Filicales which we have described exhibit a complex 

 stelar anatomy in the structure of the stems, and in order that this may be 

 clearly understood it is desirable to say something about the way in which it 

 probably evolved (Fig. 564). We can gain information about this problem 

 from three sources. One is the comparative study of different present-day 

 types of Ferns, another is the comparison of living with fossil genera, and 

 a third is the study of development in living forms. 



The simplest kind of stele is the protostele (Figs. 565 and 566). It 

 consists of a central solid mass of xylem surrounded by a ring of phloem and 

 bounded on the outside by a pericycle. Such a stele may be found in 

 Gleichenia and Lygodiiim among present-day forms, and in Botryopteris and 

 its allies among the earliest fossil Ferns. In such types the leaf trace is also 

 simple and consists of a single vascular strand which splits off from the 

 protostele and comprises a mass of xylem on the side towards the stem and 

 phloem towards the outside. This leaf trace is completely surrounded by 

 an endodermis and causes no break in the central xvlem core. 



