224 LIFE HISTORIES AND EVOLUTION OF THE ALG^E 



has been noted in species of Chlamydomonas (Fig. 177), and some 

 other green algse (as the siphon alga Bryopsis) show it, while 

 among the brown algse certain species of Ectocarpus and Cut- 

 leria (Fig. 205) furnish especially good illustrations. The larger 

 gamete is female and often has a relatively short motile period, 

 being fertilized when at rest by the smaller male gamete, thus 

 resembling an egg (Fig. 205, B). These are transitional condi- 

 tions leading towards the highest types of gametes, the egg 

 and sperm. The term isogamy (similar gametes) is applied to 

 sexual conditions when the gametes are similar in form, 

 that is, have the same structure, or morphology, even though 

 they may be very different in size. The sexually formed cell is 

 called a zygospore. 



Heterogamy (dissimilar gametes) is the sexual condition in 

 which the gametes are unlike in form, that is, have a dif- 

 ferent structure, or morphology, one being the larger nonmo- 

 tile egg, and the other the small, specialized, motile sperm. The 

 sexually formed cell is called an oospore, or the egg is said to 

 be fertilized after the union with the sperm. Several eggs may 

 be formed in the mother cell, or obgonium, as hi Fucus, but 

 they are generally developed singly. This latter condition is the 

 result of evolutionary processes by which all of the protoplasm 

 and nutritive material in the ob'gonium is preserved for a single 

 egg, thus giving it all the energy and power of growth possible. 

 The sperms, on the contrary, are frequently developed in very 

 great numbers in their parent cells, and are consequently small, 

 and must die quickly if they are unable to fertilize eggs. 



It is very important to note that the principles affecting the 

 evolution of sex are always at work and have undoubtedly 

 operated separately in various groups of plants. Thus heter- 

 ogamy has developed independently in the lines of the green 

 algae, ending in Volvox, CEdogonium, Coleochcete, Vaucheria, and 

 Chara, and in the rockweeds (Fucales) as well. Heterogamy 

 is the highest point of sexual evolution, but plants above the 

 thallophytes show some important advances hi their methods 



