AND THE EmHRYO-SAC IN BiGNONIA VEXUSTA 



93 



placental tissue is a small mass of parenchymatous cells, and in this 

 mass of tissue, growth is much more rapid on one side than on the 

 ■other, consequently giving the anatropous ovule. In the nucellar 

 mass a large hypodermal cell is soon differentiated as the primitive 

 archesporium. It is readily distinguished by its greater size and 

 by the richness of its protoplasm, see Fig. 12. As this archc- 

 sporial cell elongates apically, it is enveloped in the epidermal layer 

 alone, and there is no further nucellar development. The funicu- 

 lus is considerably enlarged, and the cells beneath the archesporial 

 cells are considerably elongated, as in Fig. 13. The single thick 

 integument develops from below these elongated basal cells. Dur- 

 ing the subsequent expansion of the archesporium into four cells, 

 the nucellar cells are greatly compressed and stretched, as in Fig. 

 16. Finally, the growth of the embryo-sac causes the complete 

 disorganization of these apical nucellar cells, which disintegrate as 

 the embryo-sac pushes itself out to such an extent as to encroach 



upon the cells of the integument. • 



of the embryo-sac become thick-walled and further elongated. 



M 



Macrosporic Archcsporiuiii 

 In the microsporangia, both of the divisions of the pollen 

 mother-cells are complete before there is any differentiation in the 

 macrosporangium of the primitive archesporium, or initial cell. 

 When first recognizable as the primitive archesporium, this hypo- 

 dermal cell is about i 5 n in length. No tapetum is cut off, and 

 growth is rapid. At the time of synapsis In the nucleus of this 

 archesporial cell, the latter is 45 /i in length, and its size, w^hen the 

 spindle begins to form, is about 60//. The cell is rich in proto- 

 plasm, sometimes with a single vacuole in the vicinity of the nu- 

 cleus. Owing to the narrow transverse diameter of the cell, the 

 nucleus is often oblong in form. During the formation of the first 

 spindle, the transverse diameter of the cell increases appreciably, 

 Fig- 39- The two equivalent cells resulting from the first division 

 rapidly divide again, usually synchronously in all details. Fig. 15, 

 and there result the four highly differentiated and equivalent cells 

 regarded as potential macrospores, as in Fig. 16. As a rule, the 

 fourth cell of this axial row immediately begins to enlarge at the 

 expense rf3f the others, as shown in Fig. 17. I have noticed sev- 



