86 MARGARET C. FERGUSON 



circumference to the center of the prothallial cavity. The cells 

 are long, it is true, the walls delicate and wavy in outline, but 

 a ring of tissue composed of longer or shorter cells is formed 

 rather early in the inward growth of the prothallium. The cells 

 of the innermost row always remain open on their outer free 

 sides, their cytoplasm is more abundant than in the other cells 

 of the prothallium and their nuclei invariably retain a position 

 near the open side of the cells (fig. 162). As observed by 

 Jaccard ('94), and Jager ('99), the nuclei of the prothallium cease 

 to divide synchronously after individual cells have been organ- 

 ized. When the center is reached the cells close and thus, one 

 year after pollination, the endosperm becomes a solid mass of 

 tissue. 



The prothallium grows rapidly after it has become a con- 

 tinuous cellular body and in a few days it fills all the central 

 and lower portion of the ovule. Above it is the prominent 

 nucellar cap, while only a few cells of the nucellus remain along 

 the sides separating the gametophyte from the integument (fig. 

 73, plate VII). Cell-divisions continue to take place, and the 

 cytoplasm becomes more abundant, though the prothallial cells 

 are never richly supplied with cytoplasm. Strasburger ('80), 

 Jager ('99), and several more recent students have noted many 

 nuclei in the endosperm cells. I have not observed multi- 

 nucleated cells in the prothallium of Piniis up to the time when 

 the suspensor has elongated and carried a several celled embr3^o 

 to a considerable depth into the endosperm. Later stages than 

 this have not been studied. There is often an appearance of 

 more than one nucleus in a cell, but careful study never fails to 

 demonstrate a delicate cell-wall between the nuclei. At an 

 early stage in prothallial development the cell-walls are very 

 delicate, scarcely more than condensations of the ectoplasm, so 

 that they might easily be mistaken, in Pinus^ for strands of 

 cytoplasm. Doubtless the cells become plurinucleated during 

 a more advaned stage in embryo formation. 



THE SO-CALLED SPONGY TISSUE. 



The Fh'st Period of Growth. — When tiie macrospore- 

 mother-cell first becomes apparent it is surrounded by a group 



