Abstract
Plants grown in long photoperiods (18 hrs. of light daily) contain more vitamin B1 than plants grown under otherwise similar conditions but under short (9 hrs. daily) photoperiod. This was true for Xanthium, 2 spp. of Brassica, Cosmos, and tomato. In similar expts. the growth of Xanthiurn and mustard in sand culture was increased by adding small amts. of vitamin B1 t;o the nutrient soln., and the growth-promoting effect was greater in short photoperiods. The effect of photoperiod on the vegetative growth of plants may be in part mediated by the effect of photoperiod on the production of vitamin B1. Other growth substances may be similarly effected by varying photoperiod.