CONIFERALES (PINACEAE) 



257 



mother cell, which persists into the early stages of endosperm-for- 

 mation (fig. 280). This conspicuous zone has been taken to repre- 

 sent sporogenous tissue; but the most usual mistake is to interpret 

 it as endosperm tissue, overlooking the comparatively inconspicuous 

 embryo sac, a mistake first made by Hofmeister (10) and later cor- 

 rected by Strasburger (25). In the more recent literature of the 

 subject it has been referred to as the "spongy tissue." 



This differentiated region has been studied 

 in considerable detail in Pinus (87) and 

 Taxodium (76), but it appears to be a general 

 feature of Pinaceae, as distinct from Tax- 

 ineae. In its later development it becomes 

 differentiated into three zones : (i) a tapetal 

 zone, consisting of large, glandular cells, 

 that are evidently digestive and are very 

 active during early endosperm-formation; 

 (2) a tabular zone, consisting of flattened 

 and finally disorganized cells; (3) a starch 

 zone, consisting of cells rich in starch. This 

 elaborate nutritive mechanism persists until 

 the endosperm tissue invades the surrounding 

 nucellar tissue. This region may not always 

 be so complex as in Pinus, but the tapetal 

 cells are always evident. 



The megaspore mother cell, deeply placed 

 in the nucellar tissue, initiates the two re- 

 duction divisions, and a linear tetrad is 

 formed (fig. 281) . Sometimes the micropylar 



daughter cell of the first division does not divide, so that there 

 appears a row of three cells, as in Larix (25), Pseudotsuga (156), 

 Taxodium (76, 86), and Juniperus (132), but the row of four cells 

 has been observed in a sufficient number of cases to indicate that 

 the full number of divisions is usual. In Sciadopitys, according to 

 Lawson (17s), the division of the mother cell nucleus is not accom- 

 panied by wall-formation, but walls appear in connection with the 

 subsequent divisions of the two daughter nuclei; so that the tetrad 

 comprises three cells, the middle one of which is binulecate. The 



Fig. 281. — Pinus Lar- 

 icio: a linear tetrad of four 

 megaspores derived from 

 the megaspore mother cell; 

 the innermost megaspore 

 enlarging and the others 

 beginning to disintegrate; 

 X666. 



