MITOSIS 27 



us take that found in the root-tip cells of a hyacinth. The cells are 

 six-sided, nearly cubical bodies, fitted to form concentric layers around 

 a central row of slightly larger cells. The cells are of an average 

 size for plant cells. They divide constantly in a plane usually at right 

 angles to the length and direction of growth of the root. This division 

 and the subsequent elongating of the newly formed cells results in the 

 lengthening of the root. Examples taken about 10-45 ce ^ s back from 

 the apex of the root will form 

 the basis of our description and 

 are represented by Figures 25 to 

 36. 



A cell while resting between 

 divisions (Fig. 25) possesses a 

 dense cytoplasm, of apparently 

 alveolar formation, which at first 

 sight is apt to give one the im- 

 pression of a reticulum. Several 

 vacuoles of large size are present 

 in the cytoplasm, and these in- 

 crease in size with the age of 

 the cell. No especial organs are FlG ' 25 '~ 

 to be seen in the cytoplasm except 

 an occasional vacuole containing spindle-like crystals. The nucleus is 

 large, of a diameter over two thirds the average diameter of the cell, 

 and round or slightly compressed in outline in the narrower cells. It is 

 surrounded by a well-developed nuclear membrane. 



The karyoplasm is dense and is composed of two visible substances, a 

 deeply staining chromatin and a semitransparent linin. The chromatin 

 is plainly seen to be divided into many small portions equally spaced 

 through the karyoplasm and connected with one another by strands of 

 the linin. Resting in the mesh-like karyoplasm are to be seen one or 

 more nucleoli. If a single body, the nucleolus is found nearly in the 

 center of the nucleus; if two or more exist in the same nucleus, they 

 occupy the centers of portions of the karyoplasm proportionate in size 

 to themselves. Each nucleolus always lies in the center of a vacuole. 

 It always lies in the center of this space and never rests against the side, 

 thus leaving a clear zone around its body and between its own substance 

 and that of the chromatin-linin network. This indicates that the vacu- 

 ole is occupied during life by a more or less solid substance of sufficient 

 density to prevent the nucleolus from moving about. Besides the vacu- 

 oles which contain nucleoli, other spaces apparently without nucleoli are 

 found ; these are always smaller than the others and at first sight con- 

 tain only the clear substance. A closer examination, however, reveals 



