64 



BOTANY 



Internal Cell-division 



Internal cell-division differs from the ordinary form of fission only 

 in having the division confined to the protoplast, a new cell-wall 



being formed about the new cells, 

 either while still contained within 

 the mother-cell or after their escape. 

 Where the protoplast divides after 

 each nuclear division, it is hardly 

 distinguishable from typical fission; 

 but often there is repeated nuclear 

 division and a simultaneous division 

 of the protoplast into as many parts as 

 there are nuclei. Internal division 

 is especially common in the formation 

 of the reproductive cells of many 

 plants, such as the zoospores and 

 sperm atozoids of many Algae, the 

 pollen-spores of Flowering Plants, etc. 



Free Cell-formation 



Free cell-formation is a form of 

 internal cell-division, where a cell-wall 

 is formed about the nuclei in the proto- 

 plasm, leaving a certain amount of the 

 cytoplasm unused. The commonest 

 example of this is found in the forma- 

 tion of the so-called " Ascospores " of 

 many Fungi and Lichens. Free cell- 

 formation has also been observed in the 

 development of the embryo in Ephedra 

 and some other Gyrnnosperins. 



FIG. 46. A, sporogenous cell of 

 Azollafiliculoides, dividing into 

 four by internal division 

 (X800). B, an older stage, 

 with the four spores completely 

 divided ; only two of the spores 

 are completely shown in the 

 section. C, ascus, or spore- 

 sac, of a Cup-fungus (Peziza), 

 containing eight spores formed 

 by free cell-formation ( X 250) . 



Conjugation 



In most plants there arise, at certain times, new cells, formed by 

 the union of the protoplasm of two independent cells. These unit- 

 ing cells are the sexual cells, or Gametes, and the cell produced from 

 their union is a Zygote. In their simplest form the gametes are 

 entirely similar, either free-swimming ciliated cells, e.g. Pandorina, 

 or non-motile, as in Spirogyra, where the protoplast of one cell flows 

 through a tube into a neighboring one. 



In most plants there is a marked difference in the character of the 

 two gametes. One is much larger than the other, and is passive 

 this is the female cell (Egg or Ovum). The other, the male or sperm- 



