436 Morphogenetic Factors 



siderable degree by the cytoplasm with which they are associated. 

 Whether the specificity of the cytoplasm results from self-perpetuating 

 bodies such as the often postulated plasmagenes or from persisting 

 effects of genes on the cytoplasm (Mather, 1948) has not been de- 

 termined. This question is primarily of genetic rather than of morpho- 

 genetic importance. 



When a knowledge of the cytoplasm is more complete, that part of the 

 cell, somewhat neglected by genetic investigations in the past, will doubt- 

 less contribute much more significantly to our understanding of develop- 

 ment and differentiation. What has been called protoplasmatic plant 

 anatomy is concerned with some of these problems. Its contributions have 

 been summarized by Reuter ( 1955 ) . 



CHROMOSOMES 



The control of development and form lies chiefly with the genes and 

 their reaction to the environment, but it must also be recognized that 

 differences in the number and character of the chromosomes, apart from 

 the genes they contain, may be of considerable morphogenetic signifi- 

 cance. 



Polyploidy. Most plants in the sporophyte generation are diploid, the 

 cells containing two sets of chromosomes, one coming from the male 

 parent and one from the female, each chromosome belonging to a pair 

 of homologous ones. In some plants, however, the number of sets has 

 been multiplied so that every chromosome is represented by more than 

 two homologs. Such plants are polyploids. There are many cases where 

 the number of sets is doubled, to form tetraploids. Hexaploids, octoploids, 

 and many other polyploid types are known, though polyploidy cannot 

 be increased indefinitely throughout the plant because of loss of vigor in 

 higher members of the series. Individual cells, however, or groups of 

 cells may become very highly polyploid. 



Polyploids are often found in nature, many species belonging to so- 

 called polyploid series where each species has a particular multiple of a 

 basic number of chromosomes. 



Various ways of producing tetraploids artificially are known, and many 

 polyploids used in experimental work have arisen in this way. One 

 effective means is treatment with colchicine or certain other chemicals 

 which check mitosis after chromosome division but before the new 

 nuclear membrane is formed, so that the two daughter cells have the 

 double chromosome number. Colchicine may be applied to seeds or to 

 the whole plant. Growth after the latter treatment, as compared with 

 normal development, has been described for cranberry by Dermen 

 (1944). Many large cells in normal plants are polyploid, and a bud 



