322 



GENEBAL BOTANY 



the similar structures in the pine, and the numerous sporangia are 

 usually grouped in sori, like the ferns (Fig. 187, C, a). The micro- 

 spores resemble those of Selaginella and are called pollen grains. 

 The megasporophylls are shield-shaped and bear two megaspo- 

 , : . rangia on each sporo- 



FIG. 186. The habit of a cycad, Dioon edule, 

 growing in Mexico 



The trunk of this specimen was 15 meters (nearly 



50 feet) high, and the age was estimated to he over 



nine hundred years. After Chamberlain 



which exhibit many 

 characteristics of the 

 early types of ovules. 

 and seeds found in 

 the fossil plants of 

 the coal period. Each 

 megasporangium con- 

 sists of a mass of cellu- 

 lar tissue enveloped, 

 as in the higher seed 

 plants, by an integu- 

 ment (Fig. 187, #), 

 which forms a long 

 micropyle leading to 

 the sporangial tissue. 

 A single megaspore is 

 ultimately developed 

 from a mother cell 

 which IS deeply bur- 

 i e( j w ithin each young 



megaSpOrailQ'ium, as 



. L ^ 5 



m * lg- Io7, />, a. 



GAMETOPHYTES, EMBRYO, AND SEED 



The microspores and megaspores both form gametophytes 

 within the spores by germination, as in Selaginella, and the 

 female gametophyte and archegonia are permanently retained 

 within the sporangium. The megasporangium is also furnished 

 with an integument, which marks a new departure in the plants 

 studied thus far in our consideration of the great plant groups. 



