150 MINNESOTA BOTANICAL STUDIES. 



upper nucellus, the sac does not cease to expand laterally, but 

 presses the remaining epidermis of this portion of the nucellus 

 outward as it still further increases in size. These epidermal 

 cells contain cytoplasm and may divide even after the cells of 

 the inner nucellus, or their cytoplasmic contents, at least, have 

 been absorbed : but the increase in epidermal surface accom- 

 panyincr the continued increase in size of the sac is doubtless 

 due principally to increase in length of these upper epidermal 

 cells and the division of those near the base of the nucellus. 



Fig. 28 shows certain of these conditions of the nucellus 

 brought about by absorption of its tissues by the growing game- 

 tophyte and by its own growth. All of the epidermal cells ex- 

 cept those at the summit show elongation in the direction of 

 upward growth of the nucellus. All of the epidermal cells ex- 

 cept those at the summit are also well filled with cytoplasm, 

 indicating activity. The lowest sub-epidermal cells of the 

 nucellus shown in the figure are also well filled as were the cells 

 of five or six layers lying between the lowest shown in the fig- 

 ure and the area of thickened cells in the chalazal region. In 

 passing upward from the base to the summit of the nucellus, we 

 find greater and greater absorption of the cytoplasmic cell-con- 

 tents and finally a partial breaking down of the cell walls and a 

 beginning of the consequent collapsing of the cells. In the 

 lower portion of the nucellus the absorption by the gametophyte 

 has only affected the layers of cells near the sac, while in the 

 upper portions all of the sub-epidermal cells are affected. The 

 upper and older cells of the nucellus, where not too much disin- 

 tegrated, also show an increase in size over those of younger 

 nucelli, which accounts in part for the elongation of this organ 

 as it keeps pace with the growth of the sac. 



Methods. — After trying one per cent, solution of osmic acid, 

 one-half per cent, and one per cent, chromic acid, a saturated 

 solution of corrosive sublimate in seventy per cent, alcohol and 

 the last two plus a small addition of acetic acid to prevent 

 shrinkage, the corrosive sublimate with acetic acid was found 

 to give best results. The sections thus fixed also took best the 

 stains used. 



After this method of fixing, the tissues were washed in seventy 

 per cent, alcohol containing iodine, gradually transferred to ab- 

 solute alcohol, imbedded in pa affine through xylol, stained and 

 mounted in balsam in the ul aal way. Alcohol safranin fol- 



