48 TEXTBOOK OF BOTANY 



summer, or in some species of Spirogyra in the fall ; it sinks to the bottom 

 of the body of water in which the plants live and remains there until the 

 next spring, when it germinates. If the plants live in a small pond 

 that dries up during the summer, the zygote remains alive in the dried 

 mud and germinates when the pond again fills with water the next 

 spring. The old cell walls, within which the zygotes were formed, 

 decay and largely or entirely disappear. In germination, the zygote 

 simply grows in one direction (Fig. 17, D), becoming a long cell and 

 breaking through its outer wall at one side or end. Then it divides and 

 a cross wall is formed ; the zygote has now become a two-celled plant. 

 One of the two cells (the one that is not surrounded by the outer wall 

 of the zygote) divides, its daughter cells divide, and by a series of re- 

 peated divisions a long, thread-like plant is again formed. 



73. Nature of Sexual Reproduction. — A union of two sexual cells 

 (gametes) to form a zygote is commonly called sexual reproduction. 

 Conjugation — that is, the union of two gametes that are nearly or 

 quite alike, as those of Spirogyra are — is one form of sexual reproduc- 

 tion. Strictly speaking, a union of gametes is not reproduction, be- 

 cause reproduction means an increase in numbers, and when two cells 

 unite to form one there are fewer cells than there were before. How- 

 ever, each of the many zygotes formed by the gametes from two Spiro- 

 gyra plants may grow into a new plant ; in this way the number of 

 plants is increased, and so it is true that indirectly the union of gametes 

 results in a reproduction of the plant. A similar union of gametes 

 occurs at some stage in the life, not only of many other green algas 

 more or less lilte Spirogyra, but also' in that of most of the higher plants. 

 The same is true of the higher animals. The gametes of Spirogyra 

 are very much alike in size and appearance ; they differ in behavior 

 in that one (the male gamete) is more active than the other (the female). 

 In most of the plants that we are to study, we shall find a great differ- 

 ence in size and appearance as well as in behavior between the male 

 and the female gametes. 



74. Distribution and Relationships. — The various species of 

 Spirogyra occur in bodies of fresh water in all parts of the world. Spiro- 

 gyra is a representative of the higher green algae, others of which differ 

 from Spirogyra in many respects, but most of which resemble it 

 in being thread-like. Some of them are unbranched threads like 

 Spirogyra ; others are branched. A common branched form is Cladoph- 

 ora (Fig. 18, A), dense masses of which are found in the same sort 

 of places as those in which Spirogyra grows. Vaucheria (Fig. 18, B), 

 which forms a dark-green felt on damp soil and greenhouse benches 

 or is attached to rocks and other objects in streams, is a branched alga, 



