What do we call the cells in xylem
that are hollow and connected
end-to-end?

the second type of galaxy is called a barred spiral, which has a thing called a bar straddling the nucleus

all matter is made up of atoms

in any element all atoms are the same

elements in a compound are always in a constant ratio: apex

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.

a bird that nests only in a particular tree species.