Difficult words in the third chapter

While reading the third chapter I came across a long and complicated paragraph with many words I do not know. This is the passage:

“One grave in every graveyard belongs to the ghouls. Wander any graveyard long enough and you will find it – water-stained and bulging, with cracked or broken stone, scraggly grass or rank weeds about it, and a feeling, when you reach it, of abandonment. It may be colder than the other gravestones, too, and the name on the stone is all too often impossible to read. If there is a statue on the grave, it will be headless or so scabbed with fungus and lichens as to look like a fungus itself. If one grave in a graveyard looks like a target for petty vandals, that is the ghoul-gate. If the grave makes you want to be somewhere else, that is the ghoul-gate.”

“Wander any graveyard” – I think it means something like “walking over the graveyard”.

“Webster’s Comprehensive Dictionary” gives as definition “to move or travel about without destination or purpose”.

“water-stained and bulging” – “Water-stained” could mean that when rain flows over the tombstone it leaves grooves, but I have no idea what “bulging” might mean.

In “Webster’s Comprehensive Dictionary” you can’t find “water-stained”, but “stained” describes “a discolouration from foreign matter”. “Bulging” means “to swell out”.

“petty vandals” – Maybe “vandals” are persons who break things even though they aren’t allowed to. I cannot deduce the meaning of “petty” but in this context it could mean something like “illegal” or “not allowed”.

“Webster’s Comprehensive Dictionary” defines “petty” as “having little worth, importance, position or rank” and “vandal” as a “ruthless plunderer”.

“scabbed” – I think it could mean to be overgrown by plants.

According to “Webster’s Comprehensive Dictionary” it is ”to form or become covered with a scab”, “scab” meaning “a crust formed on the surface of a wound or sore”.

I also didn't know the words "weeds", "fungus" and "lichen". Here are some pictures that are better than an explanation:
"weeds":
 http://c0006023.cdn2.cloudfiles.rackspacecloud.com/debate/wp-content/weeds.jpg
"fungus":
http://www.fwallpapers.net/pics/nature/fungus/fungus_15.jpg

"lichen": 
http://schmidling.com/lichen2.jpg

(339 words)