328 



MORPHOLOGY OF GYMNOSPERMS 



tinct enough to follow, and the enlarging mother cell is the first evi- 

 dence of its existence. The number of mother cells varies as among 

 the Pinaceae, but the only recorded cases of more than one mother 

 cell are Taxus, in which there are one to five, and Podocarpus, in 

 which there are occasionally two. In Taxus the occasional appear- 

 ance of more than one developing embryo sac has been known ever 

 since Hofmeister's investigations. 



An important fact in con- 

 nection with the megasporangium 

 of taxads is the general absence 

 of any tapetal zone about the 

 megaspore and embryo sac, so 

 characteristic a feature of the 

 Pinaceae. Among podocarps, 

 however, "spongy tissue" occurs 

 in Phyllocladus (174), Podocarpus 

 (69), Dacrydium (174), and 

 Saxegothaea, which indicates that 

 it is of general occurrence in the 

 tribe. The general lack of differ- 

 entiation of any glandular cells in 

 taxads may be regarded either as 

 a primitive character or a degra- 

 dation. 



The deep-seated megaspore 

 mother cell usually forms a 

 linear tetrad, the innermost 

 megaspore functioning. As in 

 Pinaceae, the second division may not occur in the upper daughter cell 

 of the mother cell, resulting in the row of three cells originally reported 

 for Taxus, and also in Phyllocladus (174); but complete tetrad- 

 formation has now been observed in a sufficient number of cases to 

 estabhsh it as the usual condition. Even the linear arrangement of 

 the tetrad is occasionally departed from, as has been observed in 

 Taxus (100). There are exceptions as to the functioning megaspore 

 of a tetrad; while the innermost one usually functions, any one may 

 function, and two contiguous ones have been observed to begin 



Fig. 380. — Taxus baccata: three 

 megaspore mother cells, each at the end 

 of a row of cells derived from a primary 

 tapetal cell; X37S. — ^After Strasburger 

 (100). 



