24 PROTOPLASM AND PLANT CELLS 



the cell into two parts. This membrane splits and be- 

 tween these two plasma membranes is secreted the first 

 layer of the cell wall (middle lamella). It is of interest 

 to note that mitotic nuclear division is essentially the 

 same in animals and plants. In the former, however, 

 centrosomes are usually present while they are lacking in 

 plants except in some of the lower groups. 



34. In internal cell formation the nucleus usually 

 divides several times before the cytoplasm separates. 

 Usually the new cells are formed almost simultaneously 



in this case. In many cases the cleavage of 

 the cytoplasm is such that all of it is used up 

 in forming the new cells, the spindle fibrillae 

 taking no part in the process. In other cases, 

 as in the formation of ascospores in the ascus, 

 the kinoplasmic fibrillae radiating from the 

 centrosome outline the new cell in the midst 

 of the mass of cytoplasm, leaving much of 

 the latter outside of the new cells, the so-called epiplasm. 



35. Cell formation by union is in the main the opposite 

 process to that by division. The union of the cytoplasm 

 of the uniting cells is usually followed by the union of the 

 nuclei to form one nucleus. If the cells are naked the 

 process is comparatively simple, but when enclosed in 

 walls the cells must either escape before uniting, or open- 

 ings must be made in the walls so that one cell can pass 

 into the other. By the union of the two nuclei the num- 

 ber of chromosomes is doubled and remains at this so- 

 called diploid number until by a peculiar modification of 

 the mitotic process (the reduction division or meiosis) the 

 number is reduced to the original (or haploid) number. 



Laboratory Studies, (a) Scrape off, after moistening with 

 alcohol, a little of the young white moldy growth on a lilac 

 leaf (powdery mildew) or of similar mildews on cherry shoots 



