28 A TEXTBOOK OF THEORETICAL BOTANY 



all fluid films, which is in fact the initial state of the cell plate. They are 

 as follows : — 



1. Cells normally divide into two equal halves. 



2. The plane of the new wall is perpendicular to the direction of maximum 



growth. 



3. The new wall tends to occupy the position of minimal area. 



4. The new wall intersects the old walls at right angles, and the line of 



contact is marked by a minute zone of curvature which makes the 

 old and new surfaces continuous. (The " bourrelet " of Plateau's 

 soap-film experiments.) 



The time which elapses between successive divisions of a cell may be 



looked upon as the duration 

 of a cell generation. This 

 period varies roughly in 

 direct ratio to cell size, 

 though exceptions occur. In 

 Bacteria the period may be 

 only twenty minutes; in larger 

 unicellular organisms such as 

 Diatoms it may be four or 

 five days. The tissue cells 

 of higher plants show corre- 

 spondingly great variations. 

 The cells of the staminal 

 hairs of Tradescantia divide 

 about every eighty minutes at 

 summer temperatures. This 

 is one of the shortest cell 

 generations. At the other 

 extreme are the medullary 

 ray cells of old trees, 

 which can exist without 

 growth or division for 

 periods of up to 100 

 years. 

 It is not known what controls the occurrence of cell division. Various 

 explanations have been suggested, such as the decrease in the ratio of surface 

 to volume as the cell grows, or a corresponding reduction in the ratio of 

 nuclear to cell volume. Decreasing viscosity may also be a cause, but it is 

 certain that no single theory by itself is sufficient. 



Fig. 13. — Diospyros sp. Protoplasmic connections 

 between cells in the endosperm of the seed. The 

 connections appear as darkly stained lines in the 

 thick cell walls. 



Sexual and Asexual Reproduction 



One of the most important characteristics of living organisms is their 

 power of reproducing their kind. Among plants reproduction may take 



