A POND SCUM 



47 



contact, and the wall of each projection is dissolved at the point of 

 contact ; the two cells, one in each plant, are now joined by a con- 

 jugation lube. The two plants are thus connected by a series of tubes. 

 Occasionally, of course, since the cells are not all of the same length, 

 a cell in one plant fails to find a mate in the other ; but the majority 

 of the cells are connected in pairs. While the conjugation tubes are 

 being formed, the contents of each cell shrinks away from the wall 

 and rounds up ; and of the two cells of a pair (one in each plant), one 

 begins to round up a little earlier than the other. We shall speak of 

 the one that rounds up first as the male cell, or male gamete, and of 

 the other as the female cell or female gamete. After the conjugation 

 tube has been formed, the male cell (of course without its wall) moves 

 through the tube into the cavity occupied by the female cell. The 

 two cells now lie close together inside the same wall. They gradually 

 unite to form a single rounded cell, the zygote (Fig. 17, O, which builds 

 a new thick wall about itself. In the union of the male and female 

 cells, the cytoplasms of the two flow together, and their nuclei unite 

 to form a single nucleus (Fig. 17, A, B) ; the chloroplast of the male 

 cell disappears, and that of the female cell becomes the chloroplast 

 of the zygote. When conjugation takes place by the method just 

 described, all the gametes produced in one plant are usually of the 

 same sex ; id such a case we may speak of male plants and female plants. 



71. Lateral Conjugation (Fig. 16, B) . — In some species of Spirogyra, 

 conjugation takes place between two cells which lie next each other in 

 the same plant — that is, one plant bears both male and female gametes. 

 Each gamete puts out a short 



projection close to the cross 

 wall that separates them, and 

 these projections unite to form 

 a short conjugation tube. In 

 other respects, this method of 

 conjugation is the same as that 

 already described. 



72. Germination of the 

 Zygote. — The zygote is a 

 resting cell which can endure 

 unfavorable conditions (as the 

 spores of bacteria and yeasts 

 can) and can remain dormant 

 for a long time. Ordinarily 

 the zygote is formed by con- 

 jugation in the late spring or 



Fig. 17. — .4, a section of a zygote of 

 Spirogyra, showing the nuclei from the 

 two gametes. B, the union of the nuclei 

 completed. C, a mature zygote. D, a 

 germinating zygote. A and B after 

 Trondle ; C and D after Strasburger. 



