170 



COMPOUND ORGANS OF PLANTS. 



Fig. 72,* shows the different stages in the development of 

 Fig. 11. 



c d e f g 



Fig. 72. a. Vertical section of the pistil of Polygonum after fertilization, showing, 

 the pollen grains adherent to the stigma with their tubes passing down the style, the 

 erect orthotropous OTule in- the interior of the ovary, and the nascent embryo sac. 

 A pollen grain detached with its tube. &. The ovule more highly magnified, showing 

 the embryonal vesicle formed in the interior of the sac at a later period, c. The nas- 

 cent embryo and. its suspensor removed from the sac, and more magnified, d, «,/. 

 The embryo in succeeding stages of development, g. The embryo as it exists in the , 

 seed. 



the Dicotyledonous embryo of a species of Polygonum. Only 

 one ovule is contained in the ovary of the pistil, and this is 

 orthotropous. The plant has therefore been very properly 

 selected for illustration on account of the simplicity of its 

 pistil. The process is the same, although more complicated 

 when the ovules are more numerous. 



The change of the ovule into the seed. It will be perceived 

 that as the embryo is developed, the suspensory filament by 

 which it is attached to the summit of the embryo sac is 

 gradually absorbed ; also that great changes must necessarily 



* "Botanical Text-Book," by Asa Gray, M. D. 



