KINETIC ANALYSIS OF PHOTOPERIODISM 451 



all plants, anthocyanin appears in certain portions of the stem and 

 leaf. In certain varieties of bean and corn, Withrow et al. (1953) and 

 Klein et al. (1957) showed that the rate of anthocyanin development 

 is accelerated by very small amounts of red energy. However, syn- 

 thesis does not appear to be a direct photochemical reaction in that 

 the anthocyanin does not immediately appear after the exposure to 

 red energy; there is a lag period which coincides with the growth lag 

 phase. We concluded that it was in some way coupled to the growth 

 reactions which are the end products of the photochemical stimula- 

 tion. Siegelman and Hendricks (1957) have recently reported another 

 anthocyanin-inducing photochemical reaction that requires relatively 

 high energies and appears to be a direct photochemical conversion of 

 a precursor. This process seems quite unrelated to the photomor- 

 phogenic red induction since the peak is in the far red and it is not 

 reversible. 



There is developing a considerable body of evidence that the 

 change of proplastids into functioning chloroplasts involves more than 

 simply the conversion of protochlorophyll to chlorophyll. At the 

 Smithsonian we have been investigating this problem with the view 

 that chloroplast development is, in part at least, controlled by the red, 

 far-red photomorphogenic reactions (Withrow et al., 1956). 



Recent work by Hillman (1957) has demonstrated that the 

 aquatic floating plant Lemna minor is unable to maintain continued 

 growth in the presence of an adequate supply of sugar in complete 

 darkness. A photochemical step is necessary for heterotrophic growth, 

 which can be supplied only by very low intensities of red energy, and 

 its red induction is reversible by the far red. 



Photoperiodism is the other side of the red, far-red photoreaction 

 coin. As a plant approaches maturity, many species exhibit marked 

 sensitivity to day length in that their flower bud initiation processes 

 can be controlled by the length of the photoperiod as long as the light 

 intensity is above a very low threshold value. It was very early shown 

 with light filters that it was the red end of the visible spectrum which 

 induced the day-length responses, but the paradox of the phenomenon 

 is that the same photochemical stimulus produces diametrically op- 

 posed responses in different types of plants. The existence of two 

 major classes of plants, those which flower in days longer than a 



