194 TEXTBOOK OF BOTANY 



portant to the growth of roots? It is not in the ash (inorganic salts) of 

 the yeast. It is organic. Is it one substance or more than one? Can any- 

 thing else be substituted for it? Are some substances necessary for the 

 starting of roots, and others for root growth? Are the same substances 

 necessary for the roots of all plants? Are the formation and growth of 

 tubers also dependent upon such substances? These are some of the 

 questions being investigated at the present time. It has been found that 

 vitamin Bi ( thiamin ) is at least a partial substitute for yeast extract. 



Other types of experiments indicate that the formation and growth 

 of roots and tubers are dependent upon the migration of hormones and 

 vitamin Bi in addition to sugar from the tops of the plant. These dis- 

 coveries are the basis of the attempts being made to discover what sub- 

 stances may be used to hasten root formation in numerous plant cut- 

 tings. Certain substances are now being advertised for this purpose. 



Summary. The organs of a plant are mutually dependent because of 

 substances made in, or absorbed by, one part of the plant tliat are neces- 

 sary in the growth of other parts. Many common interrelations in plant 

 organs are dependent upon the manufacture and transfer of sugar and 

 of the substances used in protein synthesis. As a result of these interrela- 

 tions environmental factors may influence the development of one part 

 of a plant through their influence upon the processes in another part of 

 it. Recent discoveries indicate that minute amounts of hormones and 

 vitamins are involved in the way one plant organ affects. another. Other 

 interrelations between the parts of plants will be noted in later chapters. 



