REPRODUCTION: ALTERNATION OF GENERATIONS 269 



ALTERNATION OF SEXUAL WITH ASEXUAL METHODS OF 

 REPRODUCTION 



In many plants, and in some animals, there is a regular 

 alternation in the methods of reproduction, i. e., that with 

 sex union and that without sex union. This is commonly 

 spoken of as the al- 

 ternation of genera- 

 tions. One of the 

 simplest and most 

 easily understood ex- 

 amples is in that of 

 the common fern. 



Life History of the 

 Fern. At certain 

 seasons of the year, 

 usually in the fall, 

 there appear upon the 

 under surface of the 

 fern leaves, or fronds, 

 which grow every- 

 where by the road- 

 side, little rounded 

 disks known as son; 

 Fig. 124 B-. They 

 are sometimes cov- 

 ered by a little scale 

 called an indusium. 

 A study of these 

 disks with a micro- 

 scope shows that they 

 are made up of a number of little sacs, containing minute 

 reproductive bodies; Fig. D. When mature the sacs burst 

 and the reproductive cells are thrown out into the air. If 

 they fall upon some surface where they have the proper tem- 

 perature and moisture, they begin to grow at once; and since 



FIG. 124. COMMON FERN 



A, the fern attached to its root-stock; B, the back 

 of two leaflets, showing the sori ; C, a leaflet more highly 

 magnified showing the sporangia within the sori; D, one 

 of the sporangia still more highly magnified discharging 

 spores. 



a, sori; 



sp, spores; 



spg, sporangia. 



