This is important, for it shows that sex did not arise at one period only and at a certain level of the plant kingdom, but, on the contrary, at a number of totally distinct times and in connection with as many independent lines of ascent. We can see no reason why zoospores might not at any time take on the attributes of sex, for the latter conditions are controlled and probably in large measure developed by the chemical and physical environment of the plant. Although sex has arisen a great many times in the plant kingdom and in groups independent of one another, the steps in the process and the structure of the primitive gametes are essentially the same in all cases. There are good reasons for believing that even now certain groups of the algae are developing sexuality, and that this process may be expected to continue wherever the lower algae have the habit of reproducing by zoospores.
With the origin of sex in any group of plants there is immediately presented the possibility of such further evolution as will give the differentiated and highly specialized sexual cells, the eggs and sperms. This evolutionary history is briefly expressed as the development of heterogamy from isogamy or the oosporic type of reproduction from the zygosporic.
Isogamy is the term applied to conditions in which the sexual cells are similar in form, morphologically identical. Heterogamy is the condition in which the female gamete is a motionless cell without cilia and the male gamete generally a ciliated sperm frequently highly specialized in form. Our problem is to understand the steps in the evolution of heterogamy from isogamy and, as far as possible, the factors influential in the process.
Isogamy is a condition very generally distributed among the various groups of algae. We may find it in almost all lines of ascent above the unicellular forms and it is not uncommon among these. Heterogamy always appears at higher levels of development than isogamy and in connection with general advances in vegetative complexity.
It customary to speak of the forms expressing the highest development of evolutionary lines as climax types. Climax types among the algae are almost all heterogamous. There are a few lines in which this level of sexual evolution has not been attained, as for example, the pond scums (Conjugates), the Hydrodictyaceae, Ulvaceae and several smaller groups of the lower algae. On the other hand, there are several heterogamous types standing quite by themselves as the final expression of certain lines of evolution that we can only conjecture because the lower representatives have become extinct. Notable illustrations of this character are the stoneworts (Charales) and the Oedogoniaceae.