64 THE FLOWERING PROCESS 



2. The Hypothesis of Phasic Development 



In Chapter 6, we shall discuss "ripeness to flower". The concept 

 states that a plant must attain a certain stage before it is capable of 

 becoming reproductive. It was formulated by Klebs in 1918, and 

 following work on vernalization, it was used as a point of departure 

 by Lysenko for the theory of phasic or stadial development. In its 

 simplest form, the theory states that plants must progress in order 

 through a series of developmental stages, each subjected to environ- 

 mental control. In addition, the theory now includes the restrictions 

 that the stages are firmly set, the same in all species, absolutely 

 irreversible, and due to physical changes of the protoplasm itself. 

 Such rigid requirements should, of course, make the hypothesis quite 

 simple to test, and most evidence seems negative. First, the many 

 response types described in Chapter 2 make us sceptical of the theory's 

 universal application. Second, devernalization, reversion to vegetative 

 conditions under low intensity light, or after defoHation (see Chapter 

 10), and the perennial habit itself make us highly sceptical of the part 

 of the theory that says that the stages are irreversible. 



Thus in the western world, the evidence has been examined and 

 the theory rejected. In the Iron Curtain countries, however, Lysenko 

 and others went on to deduce from the theory that our present 

 concepts of genetics were inadequate. They returned to the dis- 

 credited idea of modification of the genetic material by environment. 

 I have so far been unable to follow the reasoning, but apparently it 

 arose from the vernalization of winter cereals — the "conversion" 

 of these varieties to the spring form. At any rate, Lysenko has since 

 had varying degrees of influence on Russian biology (particularly 

 genetics), and the science has never quite recovered (see 53). 



To summarize, we would readily admit that plants go through a 

 series of steps in their life cycle, but we see no reason to restrict this 

 within the limits of a formal theory as narrow as the one under 

 discussion. Certainly the steps in a life cycle may be determined by 

 vernalisation or photoperiodism, but we see no simple theory which 

 relates this to the grov^h and development of all plants. We need 

 many more data before such a theory can be formulated — and all 

 indications are that it will not even then be simple. 



3. The Hypothesis of Flower Producing Substances 

 Experience in biological research teaches that it is fairly safe to 



