The Sex Expression of Flowers • 131 



expression might best begin with a study by Nitsch et al. (1952) 

 on a monoecious plant, the acorn squash (a variety of Cucurbita 

 pepo). This plant produces one flower primordium at each node, 

 and the primordia develop differently depending on their position 

 in the sequence of nodes. The earliest give rise to underdeveloped 

 staminate ("male") flowers; these are followed by normal staminate 

 flowers that are followed in turn by normal pistillate ("female") 

 flowers; interspersed among the nodes bearing the latter are nodes 

 with inhibited staminate flowers. Still later, giant pistillate flowers 

 occur, again interspersed with inhibited staminates; finally even 

 larger pistillate flowers are produced that are parthenocarpic, pro- 

 ducing fruits (but not seeds) without pollination. This trend of 

 "feminization" occurs under all conditions, but the duration of 

 each phase in terms of node number is easily modified. High 

 temperatures and long days delay it, favoring the continued pro- 

 duction of staminate flowers, whereas low temperatures and short 

 days speed feminization greatly. Either daylength or temperature 

 can be made the dominating factor depending on the values used. 

 The control exerted is striking: for example, female flowers can 

 be made to appear as early as the ninth, or as late as the hundredth 

 node. 



It is not clear whether the effects referred to daylength are 

 photoperiodic in the strict sense. Supplementary light of 1000 foot 

 candles was used, and no low-intensity interruptions attempted. 

 One observation in the paper suggests that lower intensities might 

 not be as effective. In addition, some conclusions on the greater 

 effectiveness of "night" than of "day" temperatures are weakened 

 by the fact that the former were always given for 16 hours daily, 

 the latter for only the 8 hours of daily sunlight employed, in each 

 treatment, irrespective of supplementary light schedules. These 

 points do not detract from the dramatic climatic effects reported, 

 but the paper is chief among those usually cited as indicating 

 control of sex expression by "photoperiod" and "thermoperiodicity," 

 interpretations that may be overstated. 



Most other investigations with temperature, on both monoe- 

 cious and dioecious plants, agree with the results described in 

 showing low temperatures favoring pistillate development and 

 high favoring staminate. The effects ol daylength, whether strictly 

 photoperiodic or not, are more complex. Apparently the general- 



