544 



A TEXTBOOK OF THEORETICAL BOTANY 



The Sporangium 



The sporangia are spherical and separate, with massive walls (Fig. 545). 

 There is no annulus and dehiscence takes place between two transverse rows 



Sporangium wall 



Spore mother cells 

 Tapetum 



Vascular bunch of . 

 rachis 



<^. 







,J^.- 



.y 



Fig. 545. — Botrychiiwi hinoria. Transverse section of 

 fertile leaf pinna passing through two of the 

 thick-walled sporangia. 



of specially small cells. Each sporangium produces from 1,500 to 2,000 

 spores (Fig. 546). The sporangia are sessile on the two margins of the rachis 

 which bears them. 



The Gametophyte 



The prothallus is very difficult to find, as it is a very small tuberous body, 

 only a millimetre or two long, which lives saprophytically underground. It 

 grows slowly by a group of apical cells and is wholly parenchymatous (Fig. 

 547). It contains a mycorrhizal Fungus which enters through the rhizoids 

 of the prothallus and infects principally the lower parts of the tissue. Along 

 the upper surface of the prothallus is a ridge, with antheridia (Fig. 548) 

 on top and archegonia along its flanks. The antheridia and archegonia are 

 very similar to those of Marattia. The archegonia (Fig. 549) usually mature 

 before the antheridia, in contrast to those of the Leptosporangiate prothallus. 



Development of the Embryo 



The embryo is at first an oval mass of undifl^erentiated cells. When it 

 begins to grow out from the prothallus it forms a central strand of elongated 

 cells which connect the primary root (outwards) to the stem apex (inwards). 

 The innermost part of the tissue swells up, becoming the foot, and the stem 

 apex is thus pushed to one side. Around the stem apex there arises a ring wall 

 of tissue which is called the cotyledon. It is later broken through by the 

 first leaf, whose base then surrounds the apical cell. We may perhaps regard 

 this cotyledon as a leaf base of which the upper portion does not develop. 



