STRUCTURE AND DIVISION OF PROTOCOCCUS 73 



called Protococcus vulgaris, which inhabits the surface 

 of the damp tree bark, and feeds by absorbing rain 

 water that falls on the tree and trickles down over 

 the surface. This rain water dissolves carbon dioxide 

 from the air and small quantities of mineral salts, and 

 thus it contains all the elements necessary for the 

 food of the green alga. 



Under the high power of the microscope (Fig. 4), 

 each cell of Protococcus is seen to be bounded by a 

 colourless cell wall of firm consistency. Inside this is 

 the essential living part of the organism — the protoplasm 

 of the cell. With the low power thi^s appears uniformly 

 green, but under the high power it can be observed 

 that the green protoplasm does not fill the whole of 

 the space within the cell wall, but takes the form of 

 a curved plate (chloroplast) in which can often be 

 observed a central shining body (pyrenoid),' the rest 

 of the cell being occupied by colourless cytoplasm. 

 By appropriate staining the cell nucleus can also be 

 distinguished, but it is generally hidden by the massive 

 chloroplast. 



Protococcus reproduces itself by cell division, the 

 nucleus dividing first and then the chloroplast ; a 

 delicate wall is afterwards formed across the cell, 

 separating it into two halves (Fig. 4, a, left-hand cell). 

 The dividing cell wall thickens and the cells may 

 separate, but very frequently they remain together 

 . in little groups. All stages of cell division and separa- 

 tion can frequently be seen. 



If we compare Amceba and Protococcus as a repre- 

 sentative simple animal and a representative simple 

 green plant, we note first that they resemble one another 



' A protein crystal-like body found in the chloroplasts of many 

 green aJgEe and acting as a centre of starch formation. 



