A GENERALIZED 

 EXAMPLE 



Before entering the detailed discussion, the study of an example of the 

 general problems involved will enable us to visualize the over-all 

 problems of physiological genetics within the compass of a single cell. 

 We refer to Hammerling's experiments on Acetahularia (see 1947, 

 1953), This marine green alga ( Siphonocladiales ) is unicellular, 

 though superficially it looks like a multicellular plant. It is attached 

 by a whorl of threads, a rhizome, from which a long stalk rises at the 

 top of which an umbrella-Hke structure (the cap) is located so that 

 the whole looks like a toadstool. At the base of the stalk in the rhizome 

 lies the single nucleus. The experiments analyze the formation of the 

 umbrella or cap in relation to the nucleus. Figures 14-16 review dia- 

 grammatically the main results. 



The first group of experiments is illustrated in figure 14. When 

 the cell is separated into a piece of the rhizome containing the 

 nucleus and the rest, the nucleated part, however small, grows into 

 a whole. Also, the enucleated parts may live for a long time, up to 

 seven months; the nucleated ones, for years. The stalk of a nucleated 

 piece always regenerates an umbrella. An enucleated piece sometimes 

 succeeds in doing so, but regeneration is more frequently successful 

 when an upper sector of the stalk has been cut out than when a 

 lower one has been cut. The details show that the nucleus must have 

 produced formative substances controlHng the formation of the um- 

 brella; these substances are produced in quantity and accumulate in 

 the stalk in a gradient from top to base {d). The ability of an enucle- 

 ated stalk tc form an umbrella depends upon the amount of these 

 stuffs in the cut-out section, within which the migration of the sub- 

 stance toward the top continues to take place. 



The next group of experiments (figs. 15, 16) deals with trans- 

 plants between two species A. mediterranea and A. Wettsteinii, which 

 have completely differently constructed umbrellae (fig. 16). It is 

 possible to transplant a W(ettsteinii) umbrella on top of the stalk of 

 a med{iterranea) nucleated base and vice versa (fig. 15). When 

 afterward the umbrella is cut off, a new one regenerates; it is not 

 251 



