Genetic Factors 433 



differing in flowering time and found that a considerable number of 

 genes were involved in the determination of this character, probably 

 distributed among all nine chromosome pairs. 



Since developmental traits involve reaction of the genetic constitu- 

 tion of the organism to various other environmental factors— water, tem- 

 perature, light, mechanical factors, and others— it is obvious that genes 

 or their combinations must take part in these reactions, and although 

 there are not yet many cases in which the action of individual genes 

 has yet been analyzed as successfully here as it has in the biochemical 

 genetics of lower organisms, this will doubtless be accomplished and 

 will give information of value for the solution of the problems of de- 

 velopment. This knowledge will be second in significance only to that 

 which may be gained as to the mechanisms by which these innumerable 

 gene reactions are so organized in the growth and activity of the indi- 

 vidual that it becomes an organism. 



CYTOPLASM 



The fact that an entire and normal plant may be produced by regenera- 

 tion from a single cell (p. 253) or a group of similar cells in different 

 regions and from different tissues is evidence that all the cells of the plant 

 are genetically alike. This implies that all the nuclei are similar, a conclu- 

 sion supported by the fact that in all cells (save in cases of polysomaty) 

 the number and structure of the chromosomes are constant. 



If this conclusion is valid, the basis of differentiation would appear to 

 lie in the extranuclear portion of the cell, the cytoplasm. Much less is 

 known about the cytoplasm than about the nucleus, and events in it are 

 not as dramatic and easily observed. It is clear, however, that beneath 

 its relatively homogeneous superficial appearance there must be a high 

 degree of chemical and physical diversity, an understanding of which 

 is necessary before the problems of differentiation can be mastered. The 

 basis for cytoplasmic differentiation is doubtless at the submicroscopic 

 level. Weiss (1956), Schmitt (1956), and some others have shown that 

 the elements of the macromolecular pattern are markedly different from 

 one another in size and distribution (see also Tartar, p. 455). A wide field 

 of research at this level is now developing. 



There are a few cases where the cytoplasm can be shown to be im- 

 mediately concerned in determination of structural characters. This is 

 particularly true of the cell wall, which is directly produced by the 

 cytoplasm and is a primary element in many differences between cells. 

 Examples of this are seen in the regenerative conversion of thin-walled 

 parenchyma cells into xylem cells with reticulate lignified walls. The pat- 

 tern of these lignified thickenings is preceded in development by an 



