19．
It happens every spring．Flowers come out everywhere on the same day．But how do plants"know"when to flower？
For years，it has been a difficult question for scientists．But a US research group may have finally answered it-the secret lies in a protein（蛋白质）called FKF1．This helps plants to know when the days become long and when they get short．Then they can tell the seasons are changing．
Researchers found the FKF1protein when they were studying a plant called Arabidopsis（拟南芥）．It is sensitive（敏感的）to sunlight，and can be made active by sunlight．
Plants produce this protein every day in the late afternoon all year．If there is no light at this time，for example in winter when the sun goes down early，the protein won't be active．But when spring comes and the days get longer，the FKF1protein can be made active by day light and the plants"know"it is time to flower．"If there is light in the late afternoon，plants will know that the days are getting longer and that it is the best time for flowers to come out，"explained Takato Imaizumi，leader of study．But when conditions are poor for growing，such as during autumn or winter when the weather is cold and days are short，plants won't flower．
Although researchers have only studied how the FKF1protein works in the Arabidopsis，they believe that it is the same with many other plants，including crops such as rice and wheat．This could be useful to the agricultural industry．
"If we can control when flowers come out，we might be able to increase crop production，"Imaizumi said．
38．The passage is mainly aboutA．
A．how plants"know"when to flower
B．what people can do with plants
C．how seasons are changing
D．what kind of plant Arabidopsis is
39．Scientists found the FKF1protein when they wereD．
A．answering a question
B．controlling the clours of flowers
C．planting flowers in the garden
D．studying a plant called Arabidopsis
40．The best time for flowers to come out is whenB．
A．it rains in spring
B．there is light in the late afternoon
C．it is getting dark
D．the sun goes down early
41．Scientists believe thatA．
A．other plants also know when to flowers
B．flowers are more valuable than fruits and vegetables
C．people have known how to control when flowers come out
D．if the weather is cold and days are short，plants will flower．
