Page:Popular Science Monthly Volume 62.djvu/306

From Wikisource
Jump to: navigation, search
This page has been proofread, but needs to be validated.
300
THE POPULAR SCIENCE MONTHLY.

THE EVOLUTION OF SEX IN PLANTS.
By Professor BRADLEY MOORE DAVIS,
UNIVERSITY OF CHICAGO.

IN a former paper published in The Popular Science Monthly[1] we considered the origin of sex in plants, describing the progenitors 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.

PSM V62 D306 Gametes and ganetangia.png
Fig. 1. Gametes and Gametangia of (a) Cladophora, (b) Ulva.

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. Excellent illustrations are presented by Ulothrix, Cladophora, Hydrodictyon, Ulva and Ectocarpus. In these types the gametes are small motile cells, generally formed numerously C) in the mother cell or gametangium discharged into the water where they conjugate in pairs. The principal events of their formation and behavior are illustrated for Cladophora and Ulva 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 immediately 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.

  1. Davis: 'The Origin of Sex in Plants,' Popular Science Monthly, November, 1901, p. 66.