78 PLANT CELLS 



sharply with the regular curved surface of the chloroplasts. In some species, 

 however, chloroplasts may develop red or yellow pigments and completely lose 

 their green color. The red and yellow colors of some fruits, notably members 

 of the Solanaceae, and some flowers are caused by chromoplasts. 



The colorless leucoplasts are usually present in the cells of meristematic 

 tissues in which they often represent juvenile stages in the development of 

 chloroplasts and chromoplasts. In tissues not exposed to light they remain 

 as colorless plastids and are the structures in which starch grains are formed. 



Specialized plastids known as elaioplasts have been described as occurring 

 in the cells of some species. These plastids appear to be centers of oil 

 formation. 



The Nucleus. — The nucleus is a conspicuous spheroidal body which is 

 imbedded in the cytoplasm. In most plant cells the nucleus has a diameter 

 which falls within a range of 5 ju to 25 /x. In the vascular plants there is 

 usually only one nucleus to a cell, although in certain types of cells 

 several may be present. The sieve tube elements are the only well known 

 example of living cells in the higher plants in which no organized nucleus is 

 present. 



The nucleus is surrounded by a definite membrane and possesses a com- 

 plicated internal structural organization. The larger portion of the volume 

 of the nucleus is composed of a transparent, optically homogeneous sol or gel, 

 the nuclear sap, which surrounds a sj'stem of delicate, anastomosing threads, 

 the reticulum. The reticulum is not structurally homogeneous but contains 

 irregular granules of m.aterial known as chromatin. Most nuclei also contain 

 one or more nucleoli. Typically a nucleolus is a small spherical droplet of 

 deeply staining protein and lipoidal material that is attached to the reticulum. 



The hereditary factors which influence the development of the organism 

 are known to reside in the nucleus of the cells. It is clear therefore that the 

 nucleus must exert a controlling influence over the physiological activities of 

 the cell. There is some evidence that the nucleus is concerned with the pro- 

 duction of the enzymes which catalyze many, if not most, physiological proc- 

 esses. Very little is known, however, regarding the exact role of the nucleus 

 in protoplasmic activities. 



The Vacuole. — One of the most characteristic features of a mature plant 

 cell is the presence of a large central vacuole filled with cell sap and entirely 

 surrounded by the cytoplasm. 



Merismatic cells in the tips of stems and roots usually contain numerous 

 small vacuoles scattered throughout the cytoplasm. The shape of these 

 vacuoles varies greatly and seems to be determined by the activity of the cyto- 

 plasm. In quiescent cytoplasm the small vacuoles are usually spherical but 



