72 THE ELEMENTS OF VEGETABLE HISTOLOGY 



those responsible for the life of the cell, and the non- 

 protoplasmic contents or inclusions, substances resulting 

 from the activity of the protoplasmic contents. The 

 protoplasmic contents include the nucleus and chrom- 

 atophores or plastids. The nucleus is separated from 

 the protoplasm by a thin membrane and contains a 

 granular substance termed chromatin, together with 

 delicate thread-like structures termed linin threads 

 and one or more spherical bodies termed nucleoli. 

 The chromatophores are small bodies embedded in 

 the protoplasm and are of importance in the manu- 

 facture of the non-protoplasmic contents. According 

 to location and specific function, the chromatophores 

 are colored (chromoplastids) or colorless (leucoplastids) . 

 The green color of leaves is due to the presence of 

 green chromatophores or chloroplastids and, as will 

 be noted in a subsequent section, these are directly 

 concerned in the production of starch by the plant. 

 The position occupied by the nucleus in plant cells, 

 and the appearance of the protoplasm depend upon 

 the age of the cell. In newly formed cells, or those 

 in parts of the plant where growth is active, the nucleus 

 is in the center of the cell and the latter is entirely 

 filled with protoplasm. In mature plant cells, or 

 those located in parts of the plant not active in growth, 

 the protoplasm undergoes shrinkage and cavities or 

 vacuoles are formed. As the cell grows older and less 

 active, the vacuoles increase in size until the remain- 

 ing protoplasm is merely a narrow strip in contact 

 with the cell wall. The vacuoles are rapidly filled 

 with a liquid termed cell sap, which is of importance 

 in the distribution of nutrients to the growing portions 

 of the plant. The nucleus always remains embedded 



