Vernalization and Photoperiodism — 78 — A Symposium 



Still greater interest attaches to the action spectrum for leaf elonga- 

 tion and internode shortening in etiolated pea seedlings. Results of 

 Went (Am. Jour. Bot. 28:83-95, 1941) on effects of light on stem and 

 leaf growth of etiolated pea seedlings indicated that the action spectrum 

 might resemble that of photoperiodism. This is borne out by a 

 detailed determination made in association with Dr. Went of the action 

 curve for leaf elongation in etiolated pea seedlings, use being made of the 

 spectrograph designed for the work in floral initiation. In this work 

 peas (variety Little Marvel) were grown in sand culture in complete 

 darkness for three days and then illuminated with radiation of the various 

 wave lengths at definite energies for four minutes on each of the succeed- 

 ing four days. The peas were harvested on the ninth day and the lengths 

 of the first and second leaves and the various internodes were measured. 

 From these data energies required to give a definite leaf length at the 

 different wave lengths were determined. The response is similar in detail 

 to that of floral initiation. 



Pea seedlings grown in complete darkness produce no chlorophyll. 

 The leaves are yellowish in color and the internode at the time it begins 

 to elongate also has a slight yellow coloration. The effective pigment, 

 however, must absorb in the red and thus is not the apparent one, but 

 rather must be minor in its contribution to the color of the slightly pig- 

 mented seedlings. Nevertheless this plant material offers the greatest 

 hope for identification of the effective pigment since the dominating 

 chlorophylls and carotenoids of normal leaves are absent. 



A fundamental similarity in controlling mechanism is thus established 

 between floral initiation of long- and short-day plants, of stem elongation 

 in long-day plants, of leaf elongation, and of internode shortening in 

 etiolated pea seedlings. Naturally one wonders what other phenomena 

 of growth are controlled through this same mechanism and one looks 

 more deeply for the nature of the mechanism itself than merely effect upon 

 a meristem. 



We might hazard the speculation that a property of the protoplasm 

 in the leaf, subject to change by a photochemical process, operates as the 

 controlling mechanism. 



