8. Combination of Methods in One Parent. There are 

 some organisms that use several methods of reproduction. 

 As we have seen, one of the molds produces many small 

 spores, and at the same time, or a little later, it sends out 

 a stolon or two and thus starts a new plant as vigorous 

 as the first. By the first method it gets the advantage of 

 numbers and distribution; by the second it gets more 

 quickly matured offspring, but not so many of them. This 

 combination is a most successful one to insure the 

 adjustment of the plant to its conditions of living. It is 

 found in many plants and animals. 



CHAPTER NINE. 

 CONJUGATION. 



1. Resume. Let us bring together the kinds of 

 reproduction we have studied thus far. In some forms we 

 have seen the whole parent divide into two offspring, each 

 one-half as large as the original parent. In others we 

 have seen the whole parent divide into many, small 

 spore-like offspring. Yet others put forth smaller bodies 

 which grow into new individuals without exhausting the 

 mother organism. Still others have special organs in 

 which many minute, single-celled spores are produced 

 and from which the spores escape without the destruction 

 of the parent. 



In many of these cases the offspring begin at once to 

 grow into the adult. Some of them, however, may rest 

 days or months before they begin to develop. All of 

 them have the power to develop directly without any 

 special stimulation other than from heat, moisture, food, 

 and so forth. 



2. Offspring That May Unite. We have purposely 

 omitted to describe an interesting thing that sometimes 

 takes place even in many of the plants and animals 

 mentioned above, side by side with what we have already 

 described. In some of the fresh water algae (as Ulothrix, 

 a simple filamentous plant) the contents of the cells under 

 certain conditions divide into many motile spore-like 

 bodies. Some of these may swim around for a while and 

 then settle down and germinate into the filamentous form 

 which becomes adult. Some of them, smaller ones, 

 behave with a surprising difference. Instead of 

 germinating, these latter swim a while and then unite in 

 pairs, completely fusing the two small offspring into one, 

 which is twice as large as either. After uniting, this 

 resulting cell germinates and produces a new plant very 

 much as the swimming spores do. Here we find two 

 methods, somewhat similar and yet with a most important 

 difference. 



It is so important and plays such a part in the 

 reproduction of all higher forms that we must examine 

 it more carefully. The new point is that offspring, instead of 

 developing, unite by twos. This union is not reproduction, 

 although it starts the development of a new plant. Instead 

 it is just the opposite of reproduction. The plant 

 reproduced when the divisions occurred that formed the 

 swimming spores. When these unite in pairs there are 

 only half as many individuals as before the unions. There 

 has been decrease instead of increase. 



The individuals resulting from the union of the two 

 cells are more vigorous and more capable of development 



than either of the cells was before the union. The union 

 has a value in connection with reproduction; but it is not 

 reproduction. 



3. Gametes. Single-celled offspring thus may have 

 either of two fates: (1) they may develop without union 

 of any sort, in which case we call them spores; or (2) 

 they may unite by twos to form a body that develops with 

 especial vigor. In this case we call them gametes, which 

 means mates or uniting bodies. You will see that this 

 new united body is formed very differently from the way 

 in which spores are formed. The union probably means 

 something in strength and vitality for the next generation. 



In the illustration above, all the gametes are of the 

 same size and form. No one could in any way tell them 

 apart. Such union of similar cells we call conjugation. 



4. Spirogyra. Of the thread-like green algae that we 

 find in our ponds this is one of the most beautiful. The 

 pupil should see it under thg microscope, if possible. If 

 is a chain of elongated cells joined end to end. The green 

 matter (chlorophyll) is arranged in one or more strikingly 

 regular spiral bands. This is a perfect cell with nucleus, 

 protoplasm, and all the cell organs. As in cells generally, 

 these have been formed by the division of mother cells. 

 The whole filament of cells came by division from one 

 cell. When the right time comes, two filaments that are 

 lying side by side send out from their various cells short 

 branches. Each of these branches grows and unites with 

 a branch from the nearest cell in the other filament. 

 When they meet, the partition dissolves at the point of 

 union, and the cavities of the two cells are thus put into 

 communication. A little later the protoplasm of one of 

 these cells flows through this little tunnel into the other 

 cell. In this cell the two masses fuse as completely as 

 the gametes of Ulothrix did. The new body, made up of 

 the protoplasm and nuclei of the cells, forms a wall 

 about itself inside the wall of the mother cell. 



In the meantime the other cells of the two filaments 

 have been doing the same thing. Thus a row of bodies 

 may be found in one of the mating filaments, while the 

 other filament will be just a row of empty cells. This is 

 conjugation also. Later each of these united bodies 

 germinates and begins a new filament, which again 

 increases by division. 



5. Paramecium. This extremely active little animal is so 

 abundant and so transparent that it is very satisfactory 

 for students to study with the microscope. It reproduces 

 by fission, much as the bacteria do. But it frequently 

 varies this by another performance no less interesting. 

 Ordinarily paraxnecia swim about without paying much 

 attention to each other. At certain times, however, owing 

 to causes we know very little about, they begin to unite 

 in pairs. They do not fuse into one, as other cells we 

 have studied in this chapter. They bring their mouths 

 together, and for a considerable time swim round side by 

 side as one animal. While they are together thus the 

 nucleus in each divides, and a part of the nucleus of each 

 Paramecium passes into the other by way of the mouth. 

 This incoming nucleus in each case unites with the part 

 of the nucleus of the other which it finds remaining. Each 

 animal now has part of its own nucleus and part of that 

 of another animal. After this the animals separate. This 

 differs in several respects from the other conjugations 

 described above. The union of the animals is temporary. 

 Each, however, contributes a portion of its nucleus 

 permanently to the other. The number of individuals is 

 not diminished as in other cases we have described. There 

 has just been a change in the inner substance of each 

 animal. After a union of this sort the parafnecium has a 

 period of reproduction by fission. 



