the Determination of t$ex in Plants. 435 



plants, are alike in being either all microspores or all mega- 

 spores. The production of a particular kind of gamete by the 

 gametophyte of heterosporous plants is therefore not determined 

 by the qualitative reduction division which precedes the formation 

 of the spores. On the contrary it seems probable that the 

 conditions of nutrition determine the fate of the four cells formed 

 at the reduction division — whether they become microspores or 

 megaspores. 



In the Ferns and Equisetaceae the tendency of the gametophyte 

 toward dioecism is not fixed, and, as has been shown, may be 

 influenced by environment. (See also Strasburger, '00, b.) The 

 hermaphrodite prothallia invariably form a cushion, consisting of 

 a mass of cells which, compared with the cells of the lateral 

 wings, are rich in protoplasmic contents ; the result being that 

 only the most highly nourished prothallia assume the function of 

 rearing the next generation. Goebel ('02) has shown that the 

 antheridia and archegonia are exactly homologous structures, the 

 difference lying in (1) the cessation of cell-division in the female 

 organ as compared with the male; (2) the unequal division of 

 the primary cell into dissimilar daughter cells, of which the larger 

 alone is fertile, and, as a consequence, (3) the arrest of certain cells 

 in the female organ whose homologues in the male proceed to the 

 formation of antherozoids. This change in the course of the cell 

 divisions is then brought about by a state of high nutrition. 



In the heterosporous plants which still retain the primitive 

 independent condition of the gametophyte (e.g. Selaginella) the 

 megasporangia in the early stages are absolutely indistinguishable 

 from the microsporangia. The difference, which only appears at a 

 later stage, is that all the spore-mother-cells of the microsporangium 

 divide to form microspores, while in the megasporangium only 

 one of the mother-cells undergoes division, the others remaining 

 abortive, with the result that the four megaspores obtain a vast 

 quantity of nutriment as compared with the microspores, and the 

 protoplasm is abundant and rich in reserve food matter. 



In a gametophyte of this type it is to be expected therefore 

 that the development of the sexual organs will be characterized 

 by unequal divisions and by the arrest of one (the smaller) 

 daughter cell ; that is, that the gametophyte will bear only female 

 organs. 



To put it in another way, it seems as though the spores of 

 certain sporangia receive from the parent this tendency towards 

 the formation of antherozoids ; those of other sporangia receive 

 the corresponding tendency towards the formation of ova ; while 

 the sex-character carried by the gametes so formed, which is mani- 

 fested in the next sporophyte generation, is independent of this 

 tendency and perhaps may be determined at the reduction division. 



