GROUP III. PTERIDOPHYTA : FILICIJUE. 869 



The development of the prothallium commences with the rupture 

 of the outer coat (exospore) of the germinating spore, which takes 

 place either along three lines meeting at an angle, when the spore 

 is tetrahedral, or by a longitudinal slit when the spore is bilateral, 

 the contents covered by the inner coat (endospore) being exposed. 

 Most commonly this cell grows out into a filament, cell-divisions 

 taking place in the transverse plane only, so that the prothallium 

 consists of a longitudinal row of cells. At length a longitudinal 

 wall is formed in the terminal cell of the filament ; cell-division 

 then proceeds in two planes, giving rise to a flattened plate of cells, 

 further growth being effected by means of a two-sided apical cell. 

 After a time the activity of the apical cell ceases, a periclinal wall 

 being formed in it ; whatever further growth takes place is effected 

 by the marginal cells. At this stage the prothallium becomes 

 somewhat heart-shaped, the anterior depression indicating the posi- 



L" 



FJG. 223. Polypodium. vulgare. A Youug arcbegonium, not yet open: K' neck-canal- 

 11 ; K" ventral canal-cell : o young oosphere. B Mature archegonium open. ( x 210 



after Strasburger.) 



tion of the organic apex (Fig. 221). The cells lying anteriorly in the 

 middle line now begin to divide in a plane parallel to the surface, 

 with the result that the prothallium becomes thickened in this 

 region, and eventually a " cushion :> of tissue, several layers of 

 cells in thickness, is produced, which projects on the lower 

 (ventral) surface, and bears the archegonia. 



The sexual organs. The antheridium is developed from a 

 single superficial cell. The free surface of this cell grows out into 

 a blunt protuberance, which is cut off by a transverse wall. The 

 projecting cell thus formed generally undergoes division by the 

 formation "of a transverse wall near its base, so that it comes to 



M.B. B B 



