When the sexes are separated, although the male and female flowers may be produced on the same tree, pollen must be regularly carried from flower to flower; and this will give a better chance of pollen being occasionally carried from tree to tree.

That trees belonging to all orders have their sexes more often separated than other plants, I find to be the case in this country; and at my request Dr.

Hooker tabulated the trees of New Zealand, and Dr.

Asa Gray those of the United States, and the result was as I anticipated.