24 The Life Cycle of fhe Single Cell 



plete adult at least once. However, to do so again and again requires a 

 continual supply of energy and materials, and for this to occur, the nucleus 

 must be present. 



One can also ask whether the ability of a segment to form a new cap 

 depends on the size or position of the segment. It turns out that long 

 anucleate pieces of stalk can construct caps better, faster, and with greater 

 frequency than short pieces, and pieces of stalk removed from near the 

 cap have greater competence than pieces of equal length taken from posi- 

 tions near the rhizoid. These data have been given several interpretations. 

 As an exercise, see if you can explain these phenomena and can design an 

 experiment that would test the validity of your explanation. 



The various species of Acetabularia display wide differences in the 

 form of the cap ( number, shape, and size of the leaves, etc. ) . This specific- 

 ity, i.e., the power to control cap design, appears to reside in the nucleus. 

 Grafts have been made between two species, Acetabularia mejditerranea 

 ( med ) and A. crenulata ( cren ) . The following results have been obtained 

 and are summarized in Fig. 11: 



Fig. II. Regeneration in Acefabularia. Experiment I shows that a piece of 

 one species, containing a nucleus, when joined to a piece of another species, 

 without a nucleus, at first forms an intermediate cap, but, in subsequent 

 regenerations, forms a cap characteristic of the species that contributed the 

 nucleus. Experiment 2 shows that two nucleated pieces not only form an 

 intermediate cap at first but also in subsequent regenerations. Experiment 3 

 shows that when two pieces of one species are joined to a piece of the 

 other species, all containing nuclei, the cap is intermediate but resembles 

 more closely the species that contributed the two nuclei. 



Type of cap 



First: cren-med intermediate 

 Later: only WCT» 



Type of cap 



First: more med than cren 

 Later: more med than ere 



Type of cap 



First; more cren than med 

 Later: more cren than med 



