EVOLUTION OF PLANTS 



cr - 



B 



of the higher forms, such as the long cells in the stems 



of a good many flowering plants. 



Much more commonly the division of the nucleus is 



preceded by a number of complicated changes, result- 

 ing in the breaking 



A c up of the linin-thread 



into separate pieces 

 or segments (chro- 

 mosomes) and a fur- 

 ther splitting of these 

 segments into halves. 

 Two groups of 

 segments are thus 

 formed, which sep- 

 arate and rearrange 

 themselves to form 

 the daughter-nuclei. 

 This indirect division 

 (Mitosis, Karyokine- 

 sis) is the only form 

 found in the actively 

 dividing cells of the 

 higher plants. 



Besides the nucleus 

 there are found 



mm 



i> VAx^i- 



.. cr 



FIG. 3. Four cells from the growing tip of 

 the root of an onion, showing different 

 stages in the division of the cell-nucleus. 

 In B the nuclear membrane has disap- 

 peared and the nuclear segments or chro- 

 mosomes (cr) are arranged in a plate at 

 the equator of the nuclear spindle, which 

 is composed of the "spindle-fibres," /. 

 In C the two groups of chromosomes 

 have moved to the poles of the nuclear 

 spindle. In D the young division-wall, 

 p, has been formed. 



Ill 



most plant cells certain bodies known as u plastids." 

 (Fig. 1, B, pi.) These are similar to the cytoplasm in 

 composition, and are very important in the nutrition of 

 the cell. Among them are the green corpuscles 

 " chloroplastids ' or chromatophores - - in which are 

 contained the green pigment, chlorophyll, which plays 

 so important a role in the green plants. The red and 



