EVOLUTION OF PLANTS 



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 

 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 in 

 most plant cells certain bodies known as "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 



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. 



