Answer:
Because of the present of GFP, and GFP activities.
Explanation:
GFP plays an important role in which there is fluorescence or not.
W1 does not have fluorescence because it might not be inherited from its parents, or there is a mutation which inactivates or deletes or malfunctions the GFP.
W2-W4 manifest fluorescence because of GFP undoubtedly, actually functioned GFP. With the shortage information you have provided in the question, I could only say that GFP in W2-W4 is inherited from their parents and it works.
