Cook: Potamogeton lucens 211 



side of the wall divides rapidly and produces an abundant endo- 

 sperm, which is usually non-cellular and parietal in character 

 {figure 12), but occasionally becomes cellular and fills the entire 

 sac {figure 13). The sac increases in size, especially in its long 

 axis, the nucellar tissue bounding it gradually undergoing disinte- 

 gration. At the same time the daughter-nucleus in the antipodal 

 end, which, in accordance with my second paper on Nymphaea- 

 ceae, I shall designate as the " nucleus of the nucellar tube," 



increases rapidly in size and moves farther and farther from the 



* 



partition-wall (figures p, 10, u, and text figures a-f). This in- 

 crease in size and activity continues until the embryo has reached 

 the age indicated in figure 22, after which it undergoes disinte- 

 gration {figure 11) and gradually disappears. Some time previous 

 to this the partition separating the two ends of the sac has also 

 disappeared. 



At about the time of the disintegration of the large nucleus of 

 the nucellar tube the endosperm is very conspicuous, consisting 

 of large spindle-shaped cells (figure 12) which stain deeply and 

 frequently contain several nucleoli and evidently are very active. 

 In most cases they are without cell-walls and form a parietal layer 

 one to three cells thick lining the sac, but occasionally they have 

 cell-walls and fill the entire sac (figure 13). With the breaking 

 down of the wall across the sac and disintegration of the large 

 cell of the nucellar tube the endosperm extends into the antipodal 

 end of the sac. Coordinate with the activity of the endosperm 

 there is a corresponding disintegration of the nucellar cells 

 bounding the sac. 



Soon after the division of the primary endosperm-nucleus a 

 mass of small nucellar cells may be distinguished at the antipodal 

 end of the sac (figure 14 and text-figures b-f) and from this mass 

 of cells there extends to the chalazal region a core of elongated 

 cells which eventually disintegrate, leaving a large third chamber 

 which is separated from the true embryo-sac. It is probable that 

 this sac finally becomes a part of the embryo-sac and is occupied 

 b y the cotyledonary end of the embryo. 



The gradual increase in size and the modifications in the shape 

 the ovule and sac are shown in text-figures a to /. Figure a 

 corresponds to figure p ; in figure b the sac is somewhat larger 



