316 BOTANICAL GAZETTE [OCTOBER 
Fic. 21.—Cross-section of contents of microsporangium soon after chro- 
matic substance of microspore mother cells has formed a spirem following 
synapsis; X 500 
Fic. 22.—Section of microspore mother cell slightly older than those in 
preceding aun nucleus contains spirem showing its double nature; 1620. 
—Section of microspore mother cell soon after spirem has seg- 
caanteds 2 clea present, small one directly over large one was omitted from 
sketch; 20. 
IGS. 25.—Sections of 2 microspore shothice cells after bivalent chromo- 
somes ise icine! and become smooth in outline and intranuclear fibers 
have appeared; in fig. 25 a thick wall is beginning to form about the mother 
cell; 1620 
IG. 26. Section of microspore mother cell with multipolar polyarch 
spindle; heterotypic division; 1750. 
1G. 27.—Section of microspore mother cell showing bipolar spindle of 
heterotypic division; bivalent chromosomes arranged at equatorial plate; 
1620. 
Fic. 28.—Cross-section of tetrad; 1620. 
Fic. 29.—Vertical section of tetrad with but 2 of the microspores visible; 
X 1620. 
Fic. 30.—Vertical section of young microspore; nucleus has passed through 
a very short resting stage and has entered upon a prolonged prophase; X 1620. 
Fic. 31.—Cross-section of young pollen grain; large central vacuole 
present and tube and generative nuclei at one side of pollen grain; 1620. 
Fic. 32.—Vertical section of slightly older pollen grain than fig. 31; genera- 
tive cell lies near center of pollen grain; 1620 
PLATE XV 
Fic. 33.—Section of one of large multinucleate tapetal cells which occur 
within microsporangium after microspores have separated from tetrads; nuclei 
in various stages of disintegration; 810. 
IGS. 34, 35-—Vertical sections of nearly mature pollen grains; note 
change in size and cytoplasmic contents from fig. 32; cytoplasm packed full 
with food which is doubtless starch grains; in fig. 34 generative cell lies against 
wall of pollen grain, while in fig. 35 it lies free within cytoplasm of tube cell; 
1620. 
Fic. 36.—Cross-section of mature pollen grain; cytoplasm of generative 
cell cannot be distinguished from that of tube cell; generative nucleus forms 4 
crescent and contains 7 distinct chromatic masses; 1375. 
Fic. 37.—Vertical section of ovule after pollen tube has entered embryo 
sac; sketch is largely in outline and was derived from 3 serial sections;  509- 
Fic. 38.—Vertical section of micropylar region of embryo sac after pollen 
tube has entered sac; X1000. _ 
Fic. 39.—Two sperms near egg nucleus; 1620. 
