CONIFERALES (PINACEAE) 271 



the archegonium initials are recognizable, the solitary pollen tube 

 has passed down the surface of the endosperm just within the mega- 

 spore membrane, until it lies in contact with the upper third or even 

 the upper half of the elongated gametophyte, bearing in its tip the 

 body cell. This position seems to determine the selection of arche- 

 gonium initials, which begin to appear in groups beneath the surface, 

 the groups being in the upper half of the gametophyte but not at 

 its micropylar extremity. There is hardly a trace of a jacket layer, 

 and no distinct complex. A very ephemeral ventral nucleus is cut 

 off. The whole situation is more suggestive of Sequoia and the 

 araucarians than of any other known forms. 



Among the Araucarineae sufficient work has not been done to 

 justify a general statement, but so far as known (117) the archegonia 

 are numerous and scattered irregularly, as in Sequoia, and some of 

 them are deep-seated, as in- the same genus. 



As the endosperm grows, the megaspore membrane grows also 

 and thickens, finally differentiating into two layers, the outer one 

 being completely suberized and the inner one containing cellulose. 

 Thomson (108) has made a comparative study of this membrane 

 and finds it thickest and most completely developed among the Conif- 

 erales in the Abietineae; less developed by the Taxodineae and 

 Cupressineae, among which it may be quite poorly developed, as 

 in Libocedrus (131); while among the Araucarineae the suberized 

 outer layer is wanting. He concludes that the condition of this mem- 

 brane may be used as a test of the relative age of the gymnosperm 

 groups, which when applied among Coniferales makes Abietineae 

 the most ancient and Taxineae the most recent, in the latter tribe 

 the megaspore membrane not appreciably developing. 



Ordinarily the endosperm invades the tissue of the nucellus uni- 

 formly, but among the Araucarineae (121) this invasion may be 

 irregular, resulting in an endosperm with a more or less dissected 

 contour. This is the so-called "ruminated" endosperm, which of 

 course is a misnomer, since it is the endosperm that is "ruminating" 

 the nucellus. This unusual character among gymnosperms is also 

 strongly developed by Torreya, in which connection it will be consid- 

 ered more in detail. 



