170 



BOTANY. 



of the cells. The growing point remains at the middle of the 

 advancing margin, and the cells which are cut off from either 

 side, as they become old, widen out. 

 In this way the ' ' wings, ' ' or margins 

 of the little, green, flattened body, are 



Fig. 142. 

 Germinating spores of 

 Pteris aquilina still in the 

 sporangium. 



Fig. 143- 



Young prothallium of a fern (nipho- 



bolus). 



in advance of the growing point, and the object is more or less 

 heart-shaped, as shown in fig. 139. Thus we see how the 

 prothallium of ferns is formed. 



280. Sexual organs of ferns. — If we take one of the prothallia 

 of ferns which have grown from the sowings of fern spores, or 

 one of those which may be often found growing on the soil of 

 pots in conservatories, mount it in water on a slip, with the 



