In this thesis I present a study on floral determination in the short day plant Pharbitis nil. Shoot apical meristems are determined if, following induction, they form floral organs even if isolated in non-inductive conditions. Five day old P. nil is fully induced by a 48 h dark period. P. nil apices are determined with respect to carpels 24 h after the 48 h inductive treatment if cultured in glucose medium but not until 5 d after induction if cultured in sucrose. I found that similar differential effects existed in the determination times of the other floral organs by excising shoot apices periodically through a 48 h dark treatment and the following 24 h of continuous light, culturing them in glucose or sucrose medium and measuring the degree of floral development. I cultured apices with glucose analogues instead of glucose. The analogue 3 oxymethylglucose, which is transported into the cell but not phosphorylated, mimicked the effect of glucose so that glucose entry into the cell probably acts as a signal for floral development. Glucose was as prevalent as sucrose in the sap extracted from seedlings, regardless of induction. Structural homologues of the Arabidopsis thaliana genes LEAFY, AGAMOUS and CRABS CLAW were cloned in P. nil. The homologues PnAG1, PnAG2, PnCRC2 and PnLFY1 were found to be expressed more strongly in induced than in non-induced apices in vivo and more strongly in induced apices cultured in glucose medium than in sucrose medium using semi- quantitative RT-PCR. I conclude that PnAG1, PnAG2, PnCRC2 and PnLFY1 are floral homeotic genes and that glucose is involved in signalling for floral development and signals for the increased expression of these genes. Finally a model of floral determination, based on these conclusions, is proposed.