April, 1928 
The Queensland Naturalist. 
65 
The ordinary poinsettia, which is at its best with ns 
in midwinter, depends for its flowering period not so 
much, on the temperature at that period as on the length 
of the day. By shortening the day to ten hours, this 
plant may he made to flower at any time of the year. 
Potatoes illuminated till midnight grew to about six 
feet and produce few tubers. The plants, developing 
rapidly under these conditions, produce luxuriant fol- 
iage, but tubers, being resting organs, are not formed. 
Garner and Allard formulate the principle of the action 
of the length of day in these words: 4 4 Sexual reproduc- 
tion can be attained by the plant only when it is exposed* 
to a specifically favourable length of day (the require- 
ments in this particular varying widely with the species 
and the variety) and exposure to a length of day un- 
favourable to reproduction but favourable to growth 
tends to produce gigantism or indefinite continuation of 
vegetative development, while exposure to a length of 
day favourable alike to sexual reproduction and to vege- 
tative development extends the period of sexual repro- 
duction and tends to induce the “everbearing type of 
fruiting. ’ ’ 
Wanser’s work on wheat has an important bearing 
on the acclimatization of some varieties. In America 
winter Wheat when planted in spring produces a well- 
developed rosette and flowering is delayed. This was 
formerly thought to be a matter of temperature, hut 
Manser has shown that a shortening of the day to cor- 
respond with the length of the winter day induces nor- 
mal behaviour. 
An important commercial application of the principle 
is made in the raising of a variety of tobacco known as 
Maryland Mammoth. In Southern Maryland this var- 
iety is valuable because of its luxuriant vegetative growth. 
It gives a high yield of leaf, but does not flower except 
in the greenhouse in winter. The days are short in 
winter, but the temperature outdoors is unfavourable. 
In the greenhouse, however, the greenhouse trouble is 
overcome. Seed is now obtained from plants raised in 
the open in 'winter in Southern Florida, where a suitable 
temperature is accompanied by a suitable length of day. 
The rapid development of flowers by lettuce and by 
cabbage in Brisbane in summer may be explained as a 
photoperiodic response, and experiments with these and 
other plants behaving in a similar way should be valu- 
able from an* economic point of view. By shortening 
* 
