1897 | THE LIFE HISTORY OF SAGITTARIA VARIABILIS 269 
from one or two tiers above the stem apex tier, while the root 
apex develops from an undetermined tier above the hypocotyl 
region. 
20. Beyond the root apex a short suspensor of a single chain 
of cells, variable in number, connects the embryo with the large 
vesicular suspensor cell. 
21. Inthe mature embryo the dermatogen, periblem, plerome, 
and calyptrogen are well differentiated, the plerome strand and 
periblem cylinder terminating in a single initial cell just within 
the calyptrogen layer. 
THE UNIVERSITY OF CHICAGO. 
EXPLANATION OF PLATES XX-XXVI 
PLATE XX, 
Fig. 1. Section of’a nectary showing the position of the secreting cells. 
Fic. 2. Pollen mother ¢ells in the mother star stage, with centrospheres 
at the poles. x 800. 
Fig. 3. Pollen mother cell with two centrospheres at each pole. x 800. 
Fie. 4. Tetrad stage. x 800. 
Fic. 5. Microspore. x 800. 
Fic. 6. Pollen grain with two nuclei; the generative nucleus has two 
centrospheres. x 800. 
Fig. 7. Pollen grain with the generative nucleus divided into the two 
Sperm nuclei. x 800. 
Figs. 8 and 9. Mature pollen grains. x 800. 
Fie. to. Embryo sac with four nuclei. 800. 
Fic. 11. Mature eight-celled sae sac; ee antipodals are — sur- 
rounded by —- cell walls. x 
PLA TE XXT, 
Fr IG. 12. Outline sketch of mature nine aps = wie showing ra 
_ tive size of the nuclei. x 600. 
: Fis 16. = ese peations aga nuclei, showing one pair of centonpheres x 
vena a X 600. 
Fae: 14. Reabeyé sic with the sea nuclei ae the = amp o 
