CONIFERALES 309 



is shed ; and others differ in the number of gametes or in the organiza- 

 tion of gametes. 



Prothallial cells are a constant feature of the Abietaceae; but in 

 most of the Taxodiaceae (Sciadopitys, Cunninghamia, Sequoia, 

 Cryptomeria, Taxodium), they are generally lacking. In many of 

 the Cupressaceae {Callitria, Widdringtonia, Lihocedrus, Thuja, 

 Cupressus, Chamaeceparus, Juniperus), they are also lacking. There 

 are no prothallial cells in the Taxaceae. Wherever there are no pro- 

 thallial cells, the microspore is the antheridial initial, as in angio- 

 sperms. In all of the Podocarpaceae, except Phcrosphaera, there is a 

 vigorous development of prothallial cells. The greatest display of 

 these cells in the whole order is in the Araucariaceae, where the de- 

 velopment of prothallial tissue is far greater than in any living 

 heterosporous pteridophyte (figs. 312, 313). 



In Araucaria, and others with two or more prothallial cells, the 

 first two cells, and sometimes a third cell, are formed as in Pinus. 

 The large number of prothallial cells is due to the subsequent divi- 

 sion of these two or three cells. More than two cells, formed as in 

 Pinus, are rare. When the first prothallial cell divides periclinally, it 

 looks as if three cells had been formed by the Pinus method. The 

 first division in the primary prothallial cells is nearly always anti- 

 clinal ; and even when there are a dozen prothallial cells, there may 

 be no perichnes. Where the number is large, sometimes as many 

 as forty, there will be both anticUnes and periclines. 



There can be no doubt that prothalUal cells are vestigial. Original- 

 ly they were green, independent plants, bearing the antheridia. In 

 some heterosporous pteridophyte ancestor, the prothallium (gameto- 

 phyte) became included within the spore, became parasitic, and then 

 became reduced, and finally disappeared entirely. In angiosperms 

 there is no normal occurrence of a prothallial cell. 



Throughout the order the generative cell divides, forming two 

 cells which are called the "stalk cell" and "body cell," the stalk cell 

 getting its name from its position in forms hke Pinus, where it looks 

 hke a stalk, bearing the body cell (fig. 311 L). In the Araucariaceae, 

 in Dacrydium, Phyllocladus, and Podocarpus, there is no stalk posi- 

 tion, but the generative cell divides, forming two cells, one of which 

 aborts, while the other gives rise to gametes. 



