6 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. 
Fic. 3.— Four cells from the growing tip of This indirect division 
the root of an onion, showing different 4 ; ine- 
stages in the division of the cell-nucleus. (Mitosis, Kary okine 
In B the nuclear membrane has disap- sis) is the only form 
peared and the nuclear segments or chro- . 
mosomes (cr) are arranged in a plate at found in the actively 
the equator of the nuclear spindle, which we eye 
. canned of the ‘‘spindle-fibres,”’ f. dividing cells of the 
the two groups of chromosomes : 
have moved to the poles of the nuclear higher plants. 
Cae ae ee es division-wall, Besides the nucleus 
there are found in 
most plant cells certain bodies known as “ plastids.” 
Cig. 1, B, pl.) 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 réle in the green plants. The red and 
