6i4 A TEXTBOOK OF THEORETICAL BOTANY 



tissue, the sporangiophore axis, into which the trace bundle from the stem 

 runs up and ends bhndly. 



When the sporangia are nearly full grown, small groups of cells in the 

 middle of the archesporium begin to divide repeatedly, while all the rest of 

 the cells disintegrate into a plasmodial mass, by means of which the spores, 

 developing from the central cells, are nourished. There is no true tapetum 

 formed. The spores are bean-shaped with finely reticulated walls. Mean- 

 while the sporangial wall cells have thickened considerably, except along one 

 vertical line running from the apex to base of each sporangium. These are 

 the lines of dehiscence, along which the entire structure opens into three 

 segments releasing the spores (Fig. 625). 



Development of the Gametophyte 



The spore wall opens along a dehiscence-cleft in one side, and reveals 

 a single, thin-walled cell, which by oblique divisions establishes an apical 

 cell, by whose activity a cellular body is formed. This elongates into a 

 cylindrical, slightly branched prothallus, covered with brown rhizoids and 

 growing by means of an apical meristem (Fig. 626). It closely resembles a 

 portion of the rhizome. The tissue is colourless, saprophytic and mycorrhizal, 

 as in the rhizome, but there is normally no vascular tissue, though cases are 

 on record in which tracheids were found (Fig. 627) in large prothalli, forming 

 a discontinuous strand. 



Antheridia appear first, each developing from a superficial cell, and they 

 are spherical structures with a single-layered wall. The antherozoids are 

 spiral and multiflagellate, like those of Isoetes and the Ferns. 



The archegonia develop as usual from a superficial cell which divides 

 periclinally. The outer cell forms a neck four to six cells in length. The 

 development of the inner cell is uncertain, but it probably forms an oosphere, 

 a ventral canal cell and two neck canal cells. When the archegonium is mature 

 the neck breaks ofl^, leaving only the lowermost one or two tiers of cells and 

 thus opening the neck canal to the antherozoids. 



Development of the Embryo 



Embryonic development is very simple (Fig. 628). The oospore divides 

 periclinally and the upper cell develops into the axis, the lower cell into a 

 foot, which sends out finger-like processes into the prothallial tissues. There 

 is a marked line of division between axis and foot, and when the developing 

 axis has broken through the surface of the prothallial tissue the foot and 

 the axis separate, the foot remaining attached to the prothallus. The 

 embryonic axis elongates vertically, before it becomes free, and develops 

 either one or two apical cells. In the latter case the axis is dichotomous 

 from the start, but in any case a first dichotomy soon occurs. The prothallus 

 tissue grows up like a calyptra round the young axis, but the growth activity 

 of the embryo soon breaks through this. When the axis is cut off from the 



