r 



STUDIES ON THE PLANT CELL. III. 



BRADLEY MOORE DAVIS. 



SECTION III. HIGHLY SPECIALIZED PLANT CELLS AND THEIR 



PECULIARITIES. 



VERY much of our knowledge of the structure and behavior 

 of protoplasm in plants has been derived from the study of cer- 

 tain cells whose organization has reached an exceptionally ad- 

 vanced degree of differentiation. The peculiarities of these 

 cells are obvious and have proved of great interest but we have 

 as yet scarcely made a beginning in the study which must trace 

 and relate these characteristics of the most complex products of 

 cellular evolution in plants to their more simple progenitors. 



This section will describe in some detail the structure and 

 protoplasmic activities of the following six highly specialized 

 cells : . i, The Zoospore ; 2, The Sperm ; 3, The Egg ; 4, The 

 Spore Mother-Cell ; 5, The Coenocyte ; 6, The Coenogamete. 



i. The Zoospore. 



Zoospores are interesting not only for their own peculiarities 

 but also because they are well known to be the progenitors of 

 the sexual cells or gametes which become later differentiated 

 into the egg and sperm. Comparative studies upon three cells 

 so closely related and yet so diverse in their extremes of struc- 

 ture are sure to yield important results. 



The zoospore is generally an uninucleate cell, colorless in the 

 Fungi, but containing a chromatophore or plastids in all other 

 groups of thallophytes. There are usually two or four cilia 

 attached to the anterior pointed end which is free from coloring 

 matter and at this region one may expect to find a red pigment 

 spot. Some zoospores are exceptional for special peculiarities, 

 as those of Vaucheria which are multinucleate, each nucleus 



