CONIFERALES 



293 



casionally it seems to be borne on the face of the sporophyll. What 

 is its homology? 



Fig. 297 Fig. 298 



Fig. 297. — Pinus banksiana: young ovulate cone still inclosed within the protecting 

 bud scales: b, the bract, or sporophyll; s, the structure which bears the ovule; X20. 



Fig. 298. — Pinus banksiana: longitudinal section of young vegetative shoot, show- 

 ing bracts {b) and spurs {s). The spur {s) on the right has not yet begun to develop 

 scale leaves; the one opposite, on the left, is developing scale leaves; all of the spurs be- 

 low show not only scale leaves, but the two needles. The drawing is somewhat dia- 

 grammatic, since so many spurs would not be shown in median view in a single thin 

 section; X20. 



As one traces the evolution of the sporophyte from forms like 

 Riccia, where nearly all of the sporophyte consists of spores, there is 

 strong support for the theory that tracing the evolution is tracing an 

 increasing amount of sterilization of sporogenous tissue. At the ly- 



