328 BOTANICAL GAZETTE [October 



Fig. 54. — The tetrad with a few spindle fibers still present. 



Figs. 55-59. — Stages in the division of the pollen grain into vegetative 

 and generative cells; in fig. 57 the 10 chromosomes are shown. 



Fig. 60. — Fertilization: the pollen tube curved around the upper part of 

 the synergid, which is quite dark; apparently the sperm has already fused with 

 the egg; the endosperm of this sac is two-nucleate. 



Fig. 61. — The male nuclei have already fused with the egg and the primary 

 endosperm nucleus; the dark mass underneath the synergid and outlined 

 through it is the other synergid and some material from the pollen tube. 



Fig. 62. — Pollen tube entering one synergid; the other synergid is just 

 back of this one ; fertilization has already taken place ; the endosperm nucleus 

 has divided, the other endosperm nucleus being near the antipodals. 



Fig. 63. — The upper end of an embryo sac; in each synergid is a synergid 

 nucleus and another smaller dense nucleus (a male nucleus with fine-grained 

 cytoplasm around it) ; one synergid has a third nuclear mass, the tube nucleus. 



Fig. 64. — Mitosis in both nuclei of a two-celled embryo. 



Fig. 65. — Five-celled embryo with endosperm nucleus still undivided. 



Fig. 66. — An older embryo with traces of a synergid and showing one 

 endosperm nucleus. 



Fig. 67. — Older embryo with dermatogen layer differentiated; the endo- 

 sperm forms a layer about two cells in thickness all around the sac with a few 

 free nuclei in the interior, especially around the lower end of the embryo. 



Fig. 68. — The basal part of older embryo, showing dermatogen, plerome, 

 and periblem. 



Fig. 69. — The same embryo outlined; a typical straight dicotyledonous 

 embryo which fills about two-thirds of the sac except for the layer of endosperm 

 about two cells in thickness; the other third of the sac is filled with endosperm. 



Fig. 70. — A diagram of an abnormal flower; one anther has developed 

 on a staminodium, two are normal, the others, more or less imperfect, are on 

 the carpels, which are not so completely united as usual. 



■ 



Saxifraga 



Fig. 71. — 5. sponhemica: outer half of young ovule showing archesponal 

 cell. 



Fig. 72. — Same: the archesporiai cell has divided. 



Fig. 73. — S. crassifolia: one mother cell. 



Fig. 74.— Same: one mother cell, but two other cells are quite large and 

 stain like sporogenous cells. 



Fig. 75. — S. cordifolia: one sporogenous cell. 



Fig. 76. — S. crassifolia: mother cell with three parietal cells. 



Fig. 77. — Same: entire ovule with less magnification. 



megaspores 



sac. 



Fig. 79. — Same: some megaspores showing nucellar tissue above. 



