Cell-Pedigrees 83 



with the development of the heteroplastids from the/ 

 homoplastids. The undifferentiated cellular pedigrees of 

 the latter do not afford us any clue for judging the phe- 

 nomena of heredity. Hence we leave them aside, and 

 turn our attention entirely to the heteroplastids. 



j. The Cellular Pedigree of Eqidsctum 



Before we begin describing, at least in their main 

 lines, the extremely complex cellular pedigrees of the 

 higher plants, we will elucidate the whole method with a 

 rather simple example. I choose for the purpose the genus 

 of the horsetails (Equisetum). Their cellular pedigree 

 belongs, in spite of their alternation of generations, to the 

 simplest that are to be found among the leaf-forming 

 plants, or Cormophytes. There are two ways of arriving 

 at a conception of the main lines of the picture. One of 

 them is the progressive, the other the retrogressive. The 

 first one follows up the track of ontogeny, the second 

 one descends in the opposite direction. If one is inter- 

 ested in deciphering the combination for all the cells of 

 one plant, then the first method is obviously the simplest 

 and the safest. But, in choosing it, the relative value of 

 the two new twigs, into which the stem divides, can only 

 be judged when the ends of both twigs are constantly and 

 simultaneously kept in view. But, in tracing only the 

 main lines of the picture, it is, in most cases, much more 

 convenient to choose the opposite direction. For, in the 

 retrogressive direction, all paths evidently lead back to 

 the egg-cell, so that in this direction no erring is ever to 

 be feared. 



I assume that through a combination of both methods 

 the picture of the cellular pedigree of an Equisetum- 

 species, e. g. of E. palustre has been developed and lies 



