188 THE FLOWERING PROCESS 



even if they are left in darkness continually until the buds are 

 examined one or two weeks later.^^ Furthermore, an effect of high 

 intensity light following an inductive dark period cannot be demon- 

 strated at all in the Japanese morning glory, nor in Perilla. Thus the 

 so-called Second High Intensity Light Process in cocklebur is not 

 typical of plants in general. It seems to be a situation which some- 

 times complicates the events which lead to flowering of cocklebur. 



Recently, Norman E. Searle (70) of E. I. Du Pont de Nemours and 

 Company has performed a number of experiments on translocation 

 of the stimulus out of the leaf. Cockleburs were grown under 

 completely artificial conditions, watered with nutrient solutions, etc. 

 Plants that grew poorly under continuous light and constant tem- 

 peratures were discarded. Induction was performed by darkening a 

 single leaf of plants with the tip removed (the axillary bud grew out 

 to provide the measure of flowering), while two other leaves left 

 below the darkened one remained in continuous light. Under these 

 conditions Searle found no inhibitory effect of darkness or low 

 intensity light following induction. Furthermore, translocation rates 

 of the hormone out of the leaf were essentially the same under high 

 intensity light (2000 ft-c fluorescent plus 3.5% incandescent), low 

 intensity light (0.3 ft-c through small holes in the box used to cover 

 the leaf), and darkness. 



These experiments seem to support the findings of Carr. The two 

 leaves left in the light might be expected to supply the darkened leaf 

 with sugars. Thus Searle's experiments seem to be a rather compli- 

 cated but very efficient way of arriving at the situation which Carr 

 tried to provide : a leaf kept in the dark is supplied with a continuous 

 ample amount of sugar, and the requirement of that leaf for high 

 intensity light following induction is thus obviated. 



The effects upon translocation are especially interesting. It has 

 clearly been shown that a darkened (or non-photosynthesizing) leaf 

 acts as a sink for sugars produced by photosynthesizing leaves. Thus 

 sugar must be flowing into Searle's darkened leaf. Yet most experi- 

 ments up to now have shown that flowering hormone moves only 



^^ Of course, there is always a limit to how long plants can be left in the dark ; 

 eventually as all their nutrients become depleted they die. Actually, many species 

 will flower in continuous darkness. These include short-day plants, long-day 

 plants, and day-neutral plants. The topic is itself an interesting one, although it 

 will not be discussed here (see 32, 37, 38). 



