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POPULAR SCIENCE MONTHLY 



THE EVOLUTION" OF SEX IN PLANTS. 



By Professor BRADLEY MOORE DAVIS, 



UNIVERSITY OF CHICAGO. 



TN" a former paper published in The Popular Science Monthly* 

 -J- we considered the origin of sex in plants, describing the progeni- 

 tors of the sexual elements or gametes and some of the conditions 

 under which these cells assume sexual characters. No attempt was 

 made to trace the later history of these primitive gametes as they 

 become differentiated into the two extremes of sexual cells, the egg 

 and sperm. This is a subject quite independent of the origin of sex 

 and deserves the separate treatment that we are now to present. 



Primitive gametes are sexual cells so similar in size, form and 

 internal structure that they cannot be distinguished as male or female. 



They are found in a number of the 

 lower groups of algae and have the 

 general appearance and same mode 

 of formation as the zoospores from 

 which they have been derived. Ex- 

 cellent illustrations are presented 

 by Ulothrix, Cladophora, Hydrodic- 

 tyon, JJlva and Ectocarpus. In these 

 types the gametes are small motile 

 cells, generally formed numerously 

 C) in the mother cell or gametangium 

 'fa^^f^^r an d discharged into the water where 

 ^fHWjL they conjugate in pairs. The prin- 

 cipal events of their formation and 

 behavior are illustrated for Clado- 

 phora and JJlva in Fig. 1. In my 

 former paper I described and figured 

 the conditions for Ulothrix and Hydrodictyon. (Popular Science 

 Monthly, November, 1901, pp. 70-73, figs. 2 and 3.) 



Any one familiar with the examples mentioned above will note im- 

 mediately that they are representatives of diverse groups which are not 

 closely related to one another. On the contrary, most of the forms 

 are associated with very clearly marked divergent lines of development. 



^r^70rm>- 



Fig. l. Gametes an t d Gametangia of 

 (a) Cladophora, (b) Ulva. 



* Davis: 'The Origin of Sex in Plants,' Popular Science Monthly, No- 

 vember, 1901, p. 66. 



