The evolution of sex in plants / by John Merle Coulter.

  • Coulter, John Merle, 1851-1928.
Date:
[1914]
Catalogue details

Licence: In copyright

Credit: The evolution of sex in plants / by John Merle Coulter. Source: Wellcome Collection.

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    conditions. For example, the beginnings of sexual cells are seen among both the green algae and the brown algae, in regions of the two groups that could by no chance be related; and the same fact is true in refer- ence to several distinctly differentiated subdivisions of these two groups. Gametes, therefore have appeared as a natural response to certain conditions, and sexuality is a product of plant evolution as natural and as distributed as is the production of spores. One of the fresh-water algae (Ulotlirix) is commonly used to illustrate the origin of gametes, which means the origin of sex. It is no better for this purpose than many other algae, but it is a common form and is an excellent example (Fig. 5). Its body con- sists of a single row of cells, form- ing a simple filament. All of the cells are vegetatively active so long as vegetative conditions are favor- able, and all of them are meriste- matic, the cell divisions resulting in the elongation of the filament. When the conditions become less favorable for vegetative activity, some of the protoplasts become detached from their walls and usually begin to divide. Such divisions naturally do not result in the growth of the body, but in spore-formation. The size of the spores depends upon the number of successive divisions, and in the Fig. 5.—Ulothrix, dis- charging spores (below) and gametes (above): above the filament are pairing gametes, and above them are two zygotes.