Vernalization and Photoperiodism — ISO — 



A Symposium 



be administered during storage. In this way performance in practically any 

 climate can be guaranteed. This is actually put into practice now so that 

 shipments of bulbs can be treated individually to insure best flowering at the 

 point of destination {see e.g. van Slogteren 1935, 1936). 



Many more important results were obtained by Blaauw and co-workers 

 on development and temperature, but the previous review summarizes the 

 results most important for interpretation of thermoperiodicity. 



Another set of phenomena, which are closely related to vernalization, are 

 the chilling requirements for development of buds of deciduous trees. In 

 most of these plants the buds are dormant for a considerable part of the win- 

 ter, and can be forced into growth only after having been subjected to freez- 

 ing temperatures. In some cases the low temperatures may have no other 



1.5° 



13° 17° 20° ^3° ^S.5° 26° 31° 



35" 



Fig. 4. — Length of the growing point and flower cluster (ordinate) in the hyacinth (var. 

 Queen of the Blues), after they have been kept for various lengths of time at U different tempera- 

 tures (abscissa, in degrees centigrade). Stippled line: original length; dots and broken line: 

 after 3 weeks; triangles and solid line: after 5 weeks; squares and dash-dot line: after 8 weeks; 

 crosses and broken line: after 12"^ weeks (jrom Blaauw 1924, p. 35). 



effect than supplying a stimulus, so that a definite time after being subjected 

 to a sudden drop in temperature, irrespective of the duration of this lower 

 temperature, development occurs. The flower buds of the orchid Dendro- 

 bium crumenatum offer a clear-cut example (Coster 1926, Kuijper 1933). 

 Nine days after a sufficiently rapid drop in temperature (usually associated 

 with a heavy rainfall) the flowers of this orchid open, causing a sudden 

 burst of flowering over a wide area. Some other orchids seem to behave in 

 the same way, and probably other plants as well (gregarious flowering of 

 Co§ea liberica). In these cases the flower buds develop gradually up to a 

 certain point, beyond which no growth is possible under the prevailing tem- 

 perature conditions. The longer the temperature drop is delayed, the more 

 flower buds will have reached the critical size, and the more abundant the 

 flowering is after the temperature drop. 



An intermediate case between the orchids and the deciduous trees is 

 found in the lily of the valley, Convallaria majalis (Hartsema and Luyten, 

 1933). Their rootstocks become dormant in summer, when the current 

 year's flowers and leaves have withered. Only after a one-week period of 

 0.5 to -2°C., or three weeks at 5° do the buds on these rootstocks start to 

 grow. The optimal temperature for this effect seems to lie so close to f reez- 



