22 The Life Cycle of the Single Cell 



2. The rhythm is persistent and is repeated by the organism even 

 under constant environmental conditions. 



3. The rhythm can be temporarily wiped out or changed to a differ- 

 ent time schedule by altering the environment. The change is only tem- 

 porary, however, for the organism reverts to its original rhythmic habit 

 once it returns to a normal environment. 



4. The rhythm can be reset (i.e., pushed forward or backward as a 

 clock is reset by appropriate signals in the environment ) . 



Biologists interested in these problems are presently inquiring into 

 the causes of rhythms. Some are proceeding on the assumption that organ- 

 isms possess an internal "clock" which serves as the master control of all 

 rhythmic action and are busily at work trying to find out the properties of 

 the alleged clock. Others assume that there is no internal clock, that some- 

 thing in the physical environment (perhaps the fall of cosmic rays upon 

 the earth) varies in a rhythmic manner which directly affects each me- 

 tabolic reaction and so serves as the master control of rhythmic processes. 

 A number of crucial experiments indicate that the first explanation is 

 probably correct. 



The Life Cycle of Acefabularia: 

 Morphogenesis in a Microorganism 



Acetahularia illustrates the capacity of some unicellular organisms 

 to change their form drastically during their life cycle and to create very 

 elaborate structures in doing so. Because of its relatively enormous size, 

 it can reach a higher degree of complexity than most microorganisms. 



Acetahulariae are green algae that live in tropical and subtropical 

 marine waters. As shown in Fig. 10, the cell consists of ( a ) a large cap with 

 leaf -like members arranged like the petals of a daisy, (b) a long stalk, 

 (c) a root-like process at the bottom called the rhizoid. The stalk can be 

 4-6 cm long and the cap up to 1 cm in diameter in a mature individual. 

 The growing cell contains a single, enormous nucleus usually situated in 

 the rhizoid but sometimes found in the lower stalk. 



The complete life cycle of Acetahularia mediterranea takes about 3 

 years. After one year of growth the cell consists of a rhizoid, which has 

 sent out rootlets that hold the alga fast to the substratum, and a cylindrical 

 stalk without a cap. In the autumn, the stalk dries up and falls off leaving 

 the rhizoid which subsists on stored food reserves through the winter. The 

 following spring a stalk grows out and forms a rudimentary cap before 

 regressing once more. In the third year a fully-formed stalk and cap 

 appear. At this time the single, huge, diploid nucleus undergoes meiosis 



