226 



THE ANATOMY OF WOODY PLANTS 



such as appears in the seedlike structures described above for the 

 lycopods. Such a layer was doubtless originally present, but has 

 ceased to be necessary as a protection on account of the shelter 

 afforded by the integument; moreover, it could no longer be func- 

 tionally useful in 

 opening the spo- 

 rangium since the 

 megaspores in the 

 case of true seeds 

 are permanently 

 inclosed. The 

 elimination of the 

 ectokinetic me- 

 chanical layer of 

 the older seeds 

 must lie far in the 

 geological past, be- 

 cause no evidence 

 of its presence has 

 been observed in 

 the oldest seeds 

 with the structure 

 of which we are 

 acquainted. The 

 pointed apex of the 

 megasporangium is 

 occupied by a 

 cavity, the pollen 

 chamber, in which 

 the pollen grains or microspores come to rest before germination. 

 This cavity has its capacity much reduced by the presence of a 

 central column known as the columella. Below the pollen cham- 

 ber lies the germinated megaspore with its somewhat thickened 

 megaspore membrane. The membrane incloses the gametophyte, 

 bearing the archegonia in its upper region. Surrounding the part 

 of the seed corresponding to the megasporangium and fused with 

 it, except in the upper region, is the integument, and this consists 



FIG. 167. Diagram of seed of a cycad 



