THE BIOLOGY OF PLANT GROWTH 449 



bud of our plant receives the floriterous signal from the leaf, what hap- 

 pens is that the signal says to these genes which contain the flowering 

 information, "Please get busy and make the materials concerning which 

 you have information. Make the ribonucleic acid and thence the en- 

 zymes which are required for floral differentiation." And in the induced 

 plant these genes, once turned on, stay turned on for evermore. Al- 

 though we can and should continue to try to find out something about 

 the nature of the signal that travels from leaf to bud, still, the basic 

 problem of floral induction resides again in the mysterious realm that 

 has to do with the control of genie activity itself. 



And so we must come to the conclusion that in the fourth and 

 final sense the determining facts concerning plant growth are those 

 written in the nucleus in every plant cell, written in its DNA and in the 

 language of A, T, G, and C. I have said that plant growth is controlled 

 by the efficiency with which plants convert solar energy to plant ma- 

 terial, but the very facts of photosynthesis are of course written in the 

 genetic book. That the size of the photosynthetic unit itself is geneti- 

 cally controlled is indicated by work with mutants of barley obtained 

 by Highkin ( 1959 ) and of blue-green algae which behave as though 

 possessed of photosynthetic units smaller than the usual. I have spoken 

 of the control of plant growth by climate, by which I mean that some 

 climates are bad for some plants. But here again the climate that is bad 

 for a plant is determined by the genetic information that our plant 

 possesses. 



To take a simple instance, the ecotypes of Arabidopsis studied by 

 Langridge and Griffing ( 1959 ) , which are intolerant of high tempera- 

 ture because they cannot make biotin or cytidine at high temperature, 

 all behave as though they differ in one gene from Arabidopsis ecotypes 

 tolerant to high temperature. We may suppose, therefore, that intol- 

 erance to high temperature resides in some gene which produces an 

 enzyme that is suitable for synthesis of the metabolite in question at 

 ordinary temperatures but is unstable to or unsuitable for operation in 

 high temperature. Or take the biennial plants— for example, Hyoscy- 

 amus, so favored by my colleague Anton Lang ( 1956 ) . Hyoscyamus 

 will not send up a flower stalk and flower until its bud has been sub- 

 jected for a suitable time to low-temperature treatment or, alterna- 

 tively, until an application of the plant growth substance gibberellin is 

 made to the non-cold-treated bud. Applications of gibberellin can, as it 

 were, cure Hyoscyamus of the requirement for low-temperature treat- 

 ment. Low-temperature treatment, we believe, causes the bud to be 

 able to make its o\\n gibberellin. But the biennial strain of Hyoscyamus 

 differs by one gene from the annual strain, a strain in which the gene 

 for making gibberellin need not be subjected to low-temperature treat- 



