560 THE AMERICAN NATURALIST. [Vol. XXXIX. 



place of the gametophyte (12). The details of the nuclear 

 history in these embryo-sacs have not been followed but it is 

 plain that their eggs have the requisite number of chromosomes 

 to develop sporophyte embryos parthenogenetically. The vary- 

 ing proportions of parthenogenetically developed seeds which 

 may be found on plants of Thalictnmt piirpiirascens indicate that 

 the suppression of normally developed embryo-sacs is not very 

 firmly established in this form. 



We now come to a recent paper of Strasburger (: 04c) which 

 is the most important contribution to the subject of partheno- 

 genesis that has yet appeared. Strasburger studied a number 

 of species of Alchemilla from the section Eualchemilla, the 

 group which formed the subject of Murbeck's important discov- 

 eries. Most of the forms develop pollen in a normal manner 

 and Strasburger was able to follow reduction phenomena in this 

 process without difficulty. The nucleus of the pollen mother- 

 cell passes through a synapsis followed by a heterotypic mitosis 

 in which the structure of the chromosomes as bivalent elements 

 is apparent. The bivalent chromosomes are in the reduced 

 (gametophytic) number. Similarly Strasburger found that some 

 species (e. g., Alchemilla pentaphylla, gelida, and grossidens) 

 formed embryo-sacs in a normal manner with the presence of a 

 tetrad and a characteristic reduction division (heterotypic). But 

 the development of the embryo-sac in apogamous species {e. g., 

 Alchemilla speciosa, splendens, and fallax) cuts out the two 

 mitoses of sporogenesis and no tetrads are formed. The nucleus 

 of the megaspore mother-cell emerges from synapsis with the 

 sporophyte number of chromosomes and the first division which 

 follows is a typical mitosis and not heterotypic. The embryo-sac 

 then comes to contain a group of nuclei with the sporophytic 

 number of chromosomes in place of the gametophytic and a 

 parthenogenetic development of the egg takes place. Stras- 

 burger regards the parthenogenetic tendencies of Eualchemilla 

 as associated with excessive mutations among these forms through 

 which sexual processes are becoming displaced by apogamous 

 methods of reproduction. 



This clear evidence that the cause of parthenogenesis in 

 Antennaria, Thalictrum, and Alchemilla lies in the suppression 



