REPRODUCTION AND DISPERSAL 



815 



(i.e. Marsilea, Salvinia, and Azolla) and in Selaginella and Isoetes there 

 are two kinds of spores, namely, small spores or microspores, and large 

 spores or megaspores; such a condition is known as heterospory (fig. 303). 

 Upon germination the microspores give rise to male plants and the 



FIGS. 1128, 1129. Reproduction by asexual spores in a fern (Aspidium): 1128, a 

 leaf segment (pinnule) with fruit dots (sori), each with a shield-shaped cover (indusium) ; 

 1129, a cross section through a sorus, showing the indusium (z')and long-stalked sporangia 

 (5) ; 1129 considerably magnified. After WOSSIDLO. 



megaspores to female plants, whereas the spores of most homosporous 

 ferns give rise to plants that bear both male and female organs. 



The spores of most pteridophytes are scattered by the wind, and they 

 are well fitted for such dispersal by their small size, by their resistance 

 to wetting (particularly in Lycopodium), and by their elevation upon 

 foliage leaves or special stalks (figs. 266, 332, 353). Fern sporangia 

 dehisce in a somewhat complicated manner (p. 351), a ring of dead 



FIGS. 1130-1132. Dehiscence of a sporangium in a fern (Polystichumacrostichoides): 

 1130, the sporangium cracked; a, the annulus; 1131, position of reversal, exposing the 

 spores; 1132, position after recoil, the sporangium having been emptied; highly magnified. 

 After ATKINSON. 



tissue, the annulus, springing back and releasing the spores when a 

 certain stage of desiccation is reached (figs. 1130-1132). Probably no 



