THE ROLE OF STUDY OF ALGAE IN DEVELOPMENT OF BOTANY 475 



the same process, even though the two fusing protoplasts are identical in 

 size and form. Within a few years there was discovery that certain algae 

 have a fusion of two flagellated cells, cells which were given the name gamete 

 by De Bary and Strasburger (1877). Fusion of two motile gametes was first 

 observed by Pringsheim (1870), who found it in Pandorina. He recognized 

 that this is a more primitive type of sexual reproduction than a fusion of 

 antherozoid and egg. In Pandorina the two of a fusing pair are unequal in 

 size. What is more frequently the case, a fusion of two motile gametes of 

 identical size was first reported in Ulothrix (Cramer, 1871). 



We now know that the nuclear fusion following a union of gametes is a 

 feature of fundamental significance. This was discovered by Hertwig (1876) 

 when studying fertilization in the sea urchin Toxopneustes. The first record 

 of this for the plant kingdom is that by Schmitz (1879) for Spirogyra. 



The great diversity of types of gametic union recorded for algae has con- 

 tributed greatly to our ideas concerning what is often called evolution of 

 sex. A more accurate phrase than "evolution of sex" would be progressive 

 differentiation of gametes. Certain species of Chlamydomonas and Dimaliella 

 show that there may be gametic union without any specific differentiation of 

 gametes. In these species every cell is capable of functioning as a gamete 

 under proper conditions. Gametic union in the great majority of algae is in 

 advance of this in that the protoplasts of all or certain cells of a thallus give 

 rise to one or more gametes. Among them the most primitive type is a union 

 of two gametes of like size and structure, a type known as isogamy. Among 

 isogamous species with both gametes motile, one of a uniting pair is male 

 and the other female, but it is impossible to determine microscopically which 

 is which. This is not the case when one of a uniting pair of motile gametes is 

 regularly larger than the other, a type of gametic union known as anisogamy. 

 Here there is good reason for believing that the larger of the two is the 

 female gamete. An evolutionary loss of flagella by the female gamete of an 

 anisogamous form, coupled with retention of flagella by the small male 

 gamete, leads to those algae in which a small flagellated male gamete (an- 

 therozoid) unites with a large non-flagellated female gamete (egg), a type of 

 gametic union known as oogamy. The evolutionary series frequently cited as 

 exemplifying an evolution from isogamy to oogamy is the family Volvocaceae 

 of the Chlorophyceae. Here Goniuni is isogamous, Pandorina is anisogamous, 

 Eudorina is transitional between anisogamy and oogamy, and Volvox is 

 oogamous. In this series with a progressive differentiation of gametes there 

 is also a progressive increase in number of cells in a colony together with a 

 progressive differentiation into somatic (vegetative) cells and gametic cells. 

 The assumption is frequently made that there is a correlation between progres- 

 sive differentiation in tj^De of gametic union and progressive specialization 

 in structure of the colony. The incorrectness of this assumption is shown by 

 certain unicellular Volvocales, including Chlamydomonas, Chlorogonium, and 



