Fi)lk : CONTRIBUTION TO THE LIFE-HISTORY OF RUMEX. 149 



several layers of cells that they were scarcely visible in any 

 particular cell. After the mother cell has reached its full length, 

 there is no further downward growth, the further increase in 

 length of the sporogenous tissues and subsequently of the sac 

 being accompanied by a proportionate elongation of the nucel- 

 lus. By the time of the establishment of the macrospore, a 

 thickening of the walls of cells in the chalazal region for the 

 support of structures above has begun in a layer of cells ex- 

 tending transversely between the points of origin of the inner 

 seed coat. As the superimposed structures become heavier, the 

 thickenincr extends to several lavers of cells below the ones first 

 thickened and gives rise to quite a mass of thick-walled tissue 

 extending entirely across the chalazal region. 



As the macrospore matures and prepares to divide (Fig. 9), 

 absorption of tapetals above and pressure on surrounding cells 

 of the upper nucellus becomes evident and is apparent in all 

 subsequent stages of development of the gametophyte. How- 

 ever, the swelling of the cell walls of the upper nucellar tissue 

 just beneath the epidermis as observed by Strasburger* in Poly- 

 gomim divaricatnm as a result of absorption I have not seen 

 either in Riimcx or Polygomini. 



By the time development has proceeded to the condition rep- 

 resented in Fig. II, the subepidermal cells of the upper end of 

 the nucellus have all been absorbed, and from this time on till 

 the establishment of the conditions shown in Fig. 28, or possibly 

 not later than those shown in Figs. 25 or 26, the increase in 

 size is due, at least principally, to the absorption of cells of the 

 nucellus surrounding the middle portion of the sac, which still 

 continues to increase in size. During this time the sac is in- 

 creasing in length, and since there has been no further sinking 

 of its posterior end into subjacent tissues, as is shown by the 

 fact that its lower end is still removed from the lines connecting 

 the points of origin of the inner seed coat by two or three cells, 

 as was the lower end of the mother cell, this increase in length 

 is accompanied by an equal upward growth of the nucellus. 

 Dividing nuclei were seen in the basal region of the nucellus, both 

 in the epidermis and in the sub-epidermal cells, up to the latest 

 stages studied, indicating that this basal portion of the nucellus 

 is its chief region of growth at these stages. After the growing 

 gametophyte has absorbed all the sub-epidermal tissues of the 



*Strasburger, E. I. c. Figs. lo et seq. 



