In
the ordinary yew-tree the upright stem bears its needles equally
distributed around its circumference, but on the branches the needles
are inserted in two rows, one to the left and one to the right. All the
needles turn their upper surfaces upwards, [137] and their lower
surfaces downwards, and all of them are by this means placed in a single
horizontal plane, and branching takes place in the same plane. Evidently
this general arrangement is another response to gravity, and it is the
failure of this reaction which induces the branches to grow upwards and
to behave like stems.
Both weeping and fastigiate characters are therefore to be regarded as
steps in a negative direction, and it is highly important that even such
marked departures occur without transitions or intermediate forms. If
these should occur, though ever so rarely, they would probably have been
brought to notice, on account of the great prospect the numerous
instances would offer. The fact that they are lacking, proves that the
steps, though apparently great, are in reality to be considered as
covering single units, that cannot be divided into smaller parts.
Unfortunately we are still in the dark as to the question of the
inheritance of these forms, since in most cases it is difficult to
obtain pure seed.
We now consider the cases of the loss of superficial organs, of which
the nectarines are example.
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