160 MORPHOLOGY OF SPBRMATOPHYTES 



served in Pinus and in Gnetum. An exception must be made 

 of Tumboa, in which the persistent beak is formed, which is 

 riddled with passageways but contains no definite pollen cham- 

 ber. Sequoia also presents an interesting modification, the 

 micropylar sterile region of the nucellus being but feebly devel- 

 oped, the sterile tissue beneath the sporogenous mass growing 

 in a remarkable way and being penetrated by the elongated and 

 tubelike megaspore. 



The origin of the deep-seated sporogenous cell or group is 

 an open question for most forms. The general statement is that 

 it is derived from an archesporium consisting of hypodermal 

 cells, and that periclinal divisions cut off outer sterile cells 

 which build up the remarkable mass of sterile tissue. This has 

 been definitely observed in a very few forms and has been in- 

 ferred for the rest. It coincides with the well-known sequence 

 in Angiosperms, and may prove to be true of Gymnosperms in 

 general, but we have not been able to verify it as yet in Pinus. 

 The first differentiation of sporogenous tissue usually visible is 

 the appearance of one or several sporogenous cells in the chalazal 

 region of the ovule. 



III. THE GAMETOPHYTES 



THE FEMALE GAMETOPHYTE 



With the exception of Tumhoa and Gnetum, the structure 

 of the female gametophyte is very uniform in Gymnosperms, 

 and recalls in a general way the female gametophyte of such 

 heterosporous Pteridophytes as Selaginella and Isoetes. This 

 resemblance, however, is very general, and should not be pushed 

 too far, as is shown by recent investigations. 



The general statement is that the mother cell divides and 

 organizes a row of potential megaspores, the lowest of Avliicli 

 becomes the functional megaspore. We have recently succeeded 

 in verifying this statement for Pinus (Fig. 106). In this genus 

 we have repeatedly observed a single cell, usually centrally placed 

 in the chalazal region, which by its increase in size and its con- 

 tents is evidently the functional mother cell ; but we have only 

 a single preparation to show that it develops a row of potential 

 megaspores. We suspect that in Pinus, and in many other 

 Gymnosperms, there is but a single primary sporogenous cell. 



