lO THE PLANT CELL 



In Chapter IX the chloroplasts will be more fully discussed in 

 connection with the photosynthetic system. The chromoplasts 

 impart red, orange, and yellow colors to flowers and fruits, 

 where, within the chromoplast body, the reds occur as crystal- 

 line carotin and the yellows as amorphous xanthin. These 

 may be found separately or together in the same chromoplast. 

 The presence of both produces an orange color (Fig. 2). The 

 reddish and bluish pigments occurring in solution in the cell- 

 sap are known as anthocyanin, and it does not appear that the 

 chromoplasts are necessary to their production. 



Cell Division. — In the growing apices of root and shoot 

 and in the cambium cell division may go on indefinitely. The 

 process of cell division begins in the nucleus and terminates 

 by the formation of a dividing cell-wall, as will be seen in Fig. 

 3, where the different stages may be followed. The nuclear 

 reticulum (in a) becomes transformed into a thick winding 

 thread (in h), which in successfully stained sections is seen 

 to consist of colored discs or granules termed chromatin, im- 

 bedded in a colorless matrix called linin. The thread splits 

 longitudinally throughout its length (in c), and then breaks 

 into rod-shaped pieces, each of which consists of two longi- 

 tudinal halves arising from the longitudinal division of the 

 thread (in d). These rods are known as chromosomes, and 

 their number varies with the species. Next the nuclear wall 

 disappears and threads arise in the cytoplasm and converge to 

 a point at two opposite poles, forming what is known as the 

 nuclear spindle (in e). Some of the threads extend uninter- 

 ruptedly from pole to pole, while others become fastened to the 

 chromosomes. The chromosomes in some unknown way line 

 up in an equatorial plane half-way between the poles, and then 

 one-half of each chromosome is drawn to one pole and the re- 

 maining half to the other when they form at each pole a nuclear 

 thread {e,f, and g). This spins itself out into a nuclear reticulum 

 around which a nuclear membrane is soon organized (in h and i). 

 The connecting fibers extending between the poles bulge out at 

 the equator more and more and new ones evidently are formed 



