No. 464.] STUDIES ON PLANT CELL. VII. 559 



the embryo-sac in the parthenogenetic Antennaria alpina with 

 A. dioica whose ovules are normally fertilized. In A. dioica the 

 embryo-sac is one of a group of four cells (tetrad )_which are 

 formed through two successive mitoses (heterotypic and homo- 

 typic) showing the characteristic features of sporogenesis. A 

 clear stage of synapsis precedes the first mitosis. The type of 

 embryo-sac development in this form is then entirely normal. 

 Not only are tetrads suppressed in the parthenogenetic Anten- 

 naria alpina but there is no trace of the heterotypic and homo- 

 typic mitoses in the embryo-sac. The number of chromosomes 

 is very large (about fifty) and evidently the same as is found in 

 other periods of the life history. There is then no reduction of 

 the chromosomes during the formation of the embryo-sac in the 

 parthenogenetic species and the egg and other nuclei in this 

 structure have consequently the sporophytic number. There is 

 no need of fertilization to bring the egg to a condition when with 

 respect to chromosomes it is prepared to develop a sporophyte 

 embryo. Juel (:O4) notes certain peculiarities in the develop- 

 ment of the embryo-sac of Taraxacum officinale. Tetrad forma- 

 tion is reduced to a single mitosis and this is not heterotypic, 

 since there seems to be no reduction of the chromosomes. 

 Details are not given. 



Overton (: 04) finds normal reduction phenomena in the pol- 

 len mother-cell of Thalictrum purpurascens which establishes the 

 number of chromosomes to be 24 for the sporophyte and 12 for 

 the gametophyte generations. These mitoses are thoroughly 

 typical of sporogenesis being preceded by a synapsis stage. 

 The development of the embryo-sac is of two types. In some 

 cases a tetrad of four megaspores is formed from a megaspore 

 mother-cell. The nucleus of this cell passes through a synapsis 

 and the first mitosis is heterotypic showing the reduced number 

 of chromosomes. The lower cell of the tetrad becomes the 

 embryo-sac. But many embryo-sacs pass through a different 

 history. There is no heterotypic mitosis and no reduction of 

 the chromosomes which remain 24 in number. Thus in some 

 ovules the mitoses of sporogenesis are omitted and true tetrads 

 are not formed, with the result that the embryo-sac contains 

 nuclei with the sporophyte number of chromosomes (24) in 



