THE PTERIDOPHYTA : FILICALES, THE FERNS 



555 



The archegonia are of a simple type, immersed in the prothallus, with a 

 four-tiered neck which projects sHghtly. The venter has no wall (Fig. 561). 



Inside the megaspore a considerable amount of nucleated protoplasm 

 remains, filling the cavity below the prothallus (compare Selaginella, p. 587). 

 It is finely granular and apparently contains food reserves. 



As the megaspore opens by a three-rayed slit the prothallus has a corre- 

 spondingly triangular outline in transverse section. 



The microspores, after being liberated in massulae by the decay of the 

 sporangial walls, form extremely reduced male prothalli (Fig. 560). The 



ANTHEROZOID 

 MOTHER CELLS 



Fig. 560. — Azolla filindoides. Development of the male 

 prothallus, which consists of a single antheridium. 

 {After Smith.) 



spore wall splits by a three-rayed slit and part of the contents bulge out and 

 are cut olT from the rest of the spore contents by a wall. The cell so formed 

 becomes the antheridium. The rest of the spore contents represent the 

 vegetative part of the male prothallus, which consists of two cells, as a second 

 small cell is cut off inside the spore. The antheridial cell undergoes further 

 divisions, cutting off a cover cell and wall cells, enclosing a central cell, which 

 divides to form eight antherozoid mother cells. 



The whole male prothallus is formed embedded in the substance of the 

 massula, and the antherozoids, which are of the Fern type, make their w^ay 

 out through it. 



Neither male nor female prothallus ever becomes free from the spore and 

 has no independent life. They are nourished entirely by the reserves in the 

 spores themselves, and consequently are only short-lived. 



The Embryo 



The early development of the embryo resembles that in the Lepto- 

 sporangiatae. The basal part of the embryo remains embedded in the 

 prothallus as a foot, the primary root being more or less lateral (Fig. 562). 

 The first leaf envelops the apex like a funnel and is called the cotyledon. 

 Large intercellular spaces develop in it, and the whole embryo plant, often 

 carrying with it the megaspore still attached to the foot, floats to the surface, 

 where growth commences. 



