12 



THE PLANT CELL 



brane which organizes in its median plane a cell-wall dividing 

 the mother cell into two. The fibers disappear, the cytoplasm 

 becomes reticulate throughout, and the daughter nuclei increase 

 rapidly in size. 



About the time when the nuclear membrane disappears the 

 nucleolus, as a rule, also vanishes from sight and cannot again 



be found until the completion 

 of the daughter nuclei. This 

 has led to the conclusion that 

 the nucleolus is not a living 

 part of the protoplast, but is 

 simply a form of reserve food 

 needed at the time of nuclear 

 and cell division. It does not, 

 however, behave uniformly in 

 all subjects and its exact 

 nature is still a subject of 

 debate. 



In the nascent endosperm 

 of seeds the division wall be- 

 tween daughter cells may not 

 be formed until the n.uclei 

 have many times divided. 

 Finally, when the embryo-sac containing the endosperm has 

 completed its growth, new connecting fibers spring up in the 

 cytoplasm between the nuclei, and cell- walls are laid down in 

 the usual way (Fig. 4). 



In the formation of spores in the Ascomycetes nuclear divi- 

 sion takes place in the usual way, but the method of formation 

 of the wall about the spores is unique. Here nuclear division 

 continues within the mother cell until the number of nuclei 

 equals the number of spores to be produced, when, at the close 

 of the last nuclear division fibrillar radiations from the polar 

 ends bend back about the daughter nuclei, and finally by 

 their fusion form a complete plasma sac enclosing, together 

 with the nucleus, a part of the cytoplasm of the mother cell 



Fig. 4. — ^Formation of endosperm in the 

 embryo-sac of Agrimonia Eupatorium. Cell- 

 walls are being formed between the nuclei. 

 (After Strasburger.) 



