Then if we went still further north, even these would begin to grow more scanty and stunted, until the low pines in which the Grosbeak nests would be the only trees seen.

Then beyond this parallel of latitude comes the 'tree limit'-- " "Oh, I know what a 'parallel of latitude' is, because I learned it in my geography," said Dodo, who had been pouting since Nat teased her about the cracked ice; "it's a make-believe line that runs all round the world like the equator.

But what is a 'tree limit' ?" "Don't you remember, little girl," answered the Doctor, "what I told you about the timber-line on a mountain--the height beyond which no trees grow, because it gets too cold for them up there?
It is just the same if you go northward on flat ground like Orchard Farm; for when you have gone far enough there are no more trees to be seen.

In that northern country the winter is so long and cold, and summer is so short, that only scrubby bushes can grow there.

Next beyond these we should find merely the rough, curling grass of the Barren Grounds, which would tell us we were approaching the arctic circle, and already near the place where wise men think it is best to turn homeward; for it is close to the Land of the Polar Bear and the Northern Lights--the region of perpetual snow.