618 TEXTBOOK OF BOTANY 



of the alga. Reduction division occurs during the first nuclear division in 

 the zygote. Hence each motile spore of Ulothrix and each cell of the fila- 

 ment has the monoploid number of chromosomes. The zygote alone has 

 the diploid number of chromosomes. 



Oedogonium. In fresh water throughout the world, attached to vari- 

 ous objects during at least a part of their lifetime are several hundred 

 species of Oedogonium. All of them are filamentous and unbranched, 

 and each cell has a hollow cylindrical meshwork chloroplast. The method 

 of reproduction is in some respects similar to, but in others quite unlike, 

 that of Ulothrix. 



Motile spores develop singly in the vegetative cells ( Fig. 281 ) . Each 

 spore has a ring of flagella at one end and escapes from the cell through 

 a circular break in the wall. Upon coming to rest, it germinates directlv, 

 and the new plant elongates by subsequent division of certain cells 

 scattered at intervals in the filament. The first cell is the holdfast cell. 



The two kinds of gametes are quite distinct in appearance. The 

 male gametes, or sperms, formed singly or in pairs from the protoplast 

 in special short cells known as antheridia (sing, antheridium, a sperm 

 case), are quite similar in appearance to small motile spores. The female 

 gamete, or egg, develops singly within a swollen vegetative cell, the 

 oogonium (pi. oogonia, egg cases). The egg is much larger than the 

 sperm, never escapes from the egg case, is incapable of locomotion, and 

 contains much food, chiefly oils. 



The sperms and eggs may develop in cells of the same filament or of 

 separate filaments. That is, some species are bisexual, or monoecious; 

 others consist of two kinds of unisexual filaments, one of which is a male 

 filament, the other female. They may or may not differ in size. In some 

 species the male filaments are very small and grow as epiphytes on the 

 female filaments near the oogonium. These dwarf males develop from 

 motile spores which germinate only when they become attached to cells 

 adjacent to an oogonium. They usually consist of a holdfast cell and one 

 or more antheridia. 



The sperm after liberation from the antheridium may swim to and 

 enter an oogonium through a pore or slit, and fuse with the egg. This 

 fusion appears to occur mostly late at night or early morning. The re- 

 sulting oospore^ is thick-walled, remains dormant for some months or 



^ The term oospore is often used to indicate the cell resulting from a union of two 

 gametes that may be distinctly recognized as an egg and a sperm; zygote is used when 

 the two gametes are similar in appearance. The term zygote is often used also as a general 

 term for any cell formed by the union of gametes, regardless of their appearance. 



