THE MEGASPORANGIUM AND SEED 



229 



important structures of the seed in one of the Abietineae or pine 

 family. It is provided with an integument in which a small pore 

 known as the micropyle is present at the apex of the seed. Within 

 the integument is accommodated the nucellus or megasporangial 

 portion of the seed. This is not characterized, as is that of the 

 older and fernlike gymnosperms, 

 by the presence of a pollen cham- 

 ber. The pollen grains or micro- 

 spores are received, in fact, on the 

 smooth apex of the nucellus and 

 very soon send out pollen tubes 

 which bore their way through the 

 tissues of the nucellus in order to 

 reach the archegonia, situated on 

 the apex of the gametophyte. 

 The absence of the pollen cham- 

 ber and the presence of functional 

 pollen tubes are features which 

 most clearly distinguish the seeds 

 of the modern gymnosperms from 

 those of the lower largely extinct 

 types included under the heading 

 of the Archigymnospermae. The 

 nucellus is not provided with a 

 tracheary mantle, such as is found 

 in the ovular structures of certain 

 extinct gymnosperms. The soft 

 sarcotesta often presented by 

 more ancient types of seeds is 



likewise conspicuous by its absence in the case of the more typical 

 representatives of the higher gymnosperms. 



In the angiosperms the situation in regard to the seed is still 

 further modified by the fact that the pollen is no longer received on 

 the apex of the ovule or young seed, but comes to rest on a special 

 region of the closed sporophyll known as the stigma (Fig. 170). 

 The microspores or pollen in germinating send out pollen tubes, 

 of greater or less length, which penetrate first the tissues of the 



FIG. 169. Diagram of seed of a conifer 



