Complex sentence: The exact purpose of this is not fully explained in "The Day of the Triffids"; tt is originally assumed that they are used to attract mates, but Bill Masen's colleague, Walter Lucknor, believes that they are employed for communication.
Split into simpler sentences: The exact purpose of this is not fully explained in "The Day of the Triffids". It is originally assumed that they are used to attract mates, although Bill Masen's colleague, Walter Lucknor, believes that they are really employed for communication.