题目解析：
本题为事实信息题。由于整段文字都在描述苍耳实验，所以题干中的“cockleburs”对我们定位的意义不大。因此这里我们用选项中的关键词来定位，用排除法来做题。
A选项中，根据关键词“multiple”和“hormone”我们定位到这句话“it is likely, however, that interactions among multiple and yet
unidentified plant hormones stimulate or inhibit flowering”说明多种未知的植物激素之间的相互作用刺激或抑制开花。但A选项说只有bud花芽中的一种hormone可以促进开花。另外，文中还说“Clearly, a signal that induces flowering transmitted from the leave
to the flowering bud.”说明刺激开花的信号分子其实是从叶片转移到花芽中的，不是花芽自己产生的。故A选项错误。
B选项中说“the entire plant”整株植物都要处在长夜环境下才能开花，但是文中提到“while the rest
of the plant
continues to experience short
nights.”除了叶片外，苍耳的其它部分其实是在短夜环境下的。故B选项错误。
C选项中，根据“siginal”定位到这一句“Clearly,
a signal that induces flowering transmitted from the leave to the flowering
bud.”说明信号分子是在叶片中产生的，符合原文，故C正确。
D选项：植物开花并不总是由黑夜的长短来决定的。但是这一点并没有在苍耳实验中提到。故排除。
Plant scientists can induce flowering. Plant scientists can induce flowering in the cocklebur by exposing leaves to long nights (longer than its 8.5 hour critical period) in a special chamber, while the rest of the plant continues to experience
short nights. Clearly, a signal
that induces flowering transmitted from the leave to the flowering bud. Plant physiologists have been attempting for decades to isolate these elusive signaling molecule often called florigen
(literally, flowering maker). Some
researchers believe they are close to demonstrating a flower's stimulating substance for specific type of plant. Using genetic manipulation, it is likely, however, that interactions among multiple and yet unidentified planthormones stimulate or inhibit flowering, and that these chemicals may differ among plant species. Researchers have had more success in determining how plants measure the length of
uninterrupted darkness, which is a crucial stimulus for producing whatever
substance control flowering.
