630 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



again, so that four daughter individuals instead of two are formed. Eventually the 

 daughters escape from the membrane of the mother, leaving it quite empty. 



The process of reproduction by simple division of all the protoplasm of an indi- 

 vidual's body into parts, each of which forms the body of a daughter individual, is 

 an example of almost the simplest type of reproduction known. It is true that in 

 the lower Protozoa, which have no rigid cellulose membrane, we find an even simpler 

 type. Since the entire organism consists of protoplasm, there is nothing left of the 

 parent individual after division has taken place. The body of the parent simply 

 becomes the body of the offspring. In the type of Chlamydomonas we have the 

 dead cellulose membrane representing all that is left of the body of the parent. In 

 many of the higher Algas, and in all plants above the level of Thallophytes, only a 

 part of the protoplasm of the plant-body is used in the formation of the reproductive 

 cells. The rest must then eventually die. But in these lower forms, where all the 

 protoplasm of the body is used in the production of new individuals, death, as a 

 necessary event, can hardly be said to occur. 



Gametes are formed in Chlamydomonas in exactly the same way as daughter 

 individuals. They are, however, smaller and have no cell- wall. In one species at 

 least the gametes are of two sizes. Of the smaller (microgametes) eight are produced 

 from a parent individual, while only two of the larger size (megagametes) are formed 

 from the parent cell. In the process of conjugation a microgamete and megagamete 

 come into contact at their anterior colourless ends, the flagella are drawn in, and a 

 thick cellulose membrane is secreted round the bodies of both. The protoplasm of 

 the microgamete then passes over into the space inclosed by the part of the mem- 

 brane belonging to the megagamete, and completely fuses with the protoplasm of 

 the latter. A wall is then formed, cutting off the empty shell of the microgamete. 

 The contents of the zygote eventually divides to form two or four new individuals 

 which escape from its membrane. In the conjugation of most species where there 

 is no distinction in size between the gametes, a cell- wall is only acquired after the 

 foundation of the zygote. This is the regular course of events in the conjugation of 

 the motile gametes of Green Algse. 



The genus Sphcerella resembles Chlamydomonas in the fundamental points of its 

 structure. The main distinction is the existence of a considerable space separating 

 the membrane from the main body of the protoplasm of Sphoerella. This space is 

 bridged by fine strands of protoplasm, which radiate from the central mass and end 

 in fine branches under the membrane. The anterior colourless protoplasm is drawn 

 out into a beak, and to the extremity of this the two flagella are attached. The 

 flagella often pass through two very delicate cellulose tubes, which in the common 

 species, S. pluvialis, diverge from the extremity of the beak, and end on either side 

 at the membrane. In another species {S. Bwtschlii) the beak runs right up to the 

 membrane, and the flagella-tubes, which are short and slightly curved, lie on the 

 outer surface of the membrane. 



Sphcerella pluvialis is a very well-known microscopic object, being extremely 

 common in pools of rain-water. Its protoplasm often contaiiis a good deal of the red 



