How is Friar Laurence's speech in Act 2, scene 3, which can be summarized as: Man like many plants, possesses the capacity of evil as well as good, applicable to the events and characters in Romeo and Juliet by Shakespeare?
One way this speech is applicable is as foreshadowing of what is about to come. Though Romeo is a hero and not a villain, his entrance immediately after Laurence's speech connects him to the content of that speech. Thus Romeo and his actions are cast in a duality since Laurence's speech is all about the opposition of dualities. Here are some instances of opposition in the speech:
The grey-eyed morn smiles on the frowning night,
Morning opposes night, and smiles oppose frowns.
Chequering the eastern clouds with streaks of light,
Checkered clouds oppose streaking beams of light.
drunkard reels / ... Titan's fiery wheels
A reeling drunkard opposes the fiery wheels of Titan's chariot (Titan: the Greek god of the sun, properly named Helios).
With baleful weeds and precious-juiced flowers.
Harmful weeds oppose medicinal flowers.
There are many more oppositions of duality spoken of by Laurence but these are enough to illustrate the point and to guide you to discovering the rest. Laurence's point is that within all of nature, there are oppositional dualities wherein one force opposes another, wherein harm opposes benefit.
Thus, through Laurence's speech, Romeo is cast as embodying oppositional dual forces within himself and, by extension, within his actions. The first duality exposed is Rosaline versus Juliet. The second exposed is enmity versus love:
I have been feasting with mine enemy,
... my heart's dear love is set
On the fair daughter of rich Capulet:
Laurence's last comment to Romeo,
Wisely and slow; they stumble that run fast.
foreshadows the ultimate results of not governing the oppositional forces within himself--Laurence earlier attributes this oppositional characteristic to all humans--as he warns that Romeo will fall if he does not tread wisely by acknowledging his oppositional duality and choosing his course with care, thereby governing the oppositions. As we know, the ultimate end for Romeo and Juliet is tragedy and needless death. Laurence's speech points out that the tragedy comes from not governing the oppositions within human nature and from running headlong with "Titan's fiery wheels," as it were, into dualistic oppositional impulses that comprise human nature.