DEVELOPMENT OF PLANTS 



369 



new departure. The microspores which have already begun to 

 germinate when discharged from their sporangia are carried by 

 the wind to the megasporophylls which are slightly spread apart 

 at this time, permitting the microspores to rattle down to the 

 megasporangia. The microspores fall into the micropyle either 

 by reason of the position and construction of the megasporangia, 

 which may be so placed that the spores naturally roll down into 

 the micropyle, or the microspores may be drawn through the 

 micropyle by mucilaginous excretions as in the cycads. The 



^M^io 



Fig. 254. Male gametophyte of the pine: 6, section of microspore, show- 

 ing the two air sacs, s, formed by the lifting up of the outer wall of the spore. 



8, stage of germination of the spore at time of discharge from its sporangium 

 — t, tube cell; a, antheridial cell, above which are seen two black lines, the 

 remains of cells formed by earlier division of the nucleus of the microspore. 



9, continuation of germination of the microspore after reaching the mega- 

 sporangium — /, tube cell forming a branching tube that disorganizes and ab- 

 sorbs the cells of the megasporangium. The antheridial cell (a) of 8 has 

 divided into a wall cell, w, and a body cell, b. 10, end of tube cell as it ap- 

 proaches the female gamete — m, m, male gametes. The nuclei of the tube 

 cell and wall cell are also seen (somewhat disorganized) in the end of the tube. 

 — ^After Coulter and Chamberlain. 



early stages in the germination of the microspore are essentially 

 as in Cycas, though generally more reduced. The microspore on 

 reaching the megasporangium has germinated, forming a tube 



