Whether it would be constant, if fertile, is therefore
impossible to decide. _Berberis ilicifolia_ is considered as a hybrid
between the European barberry (_B. vulgaris_) and the cultivated shrub
_Mahonia aquifolia_. The latter has pinnate leaves, the former undivided
ones. The hybrid has undivided leaves which are more spiny than those of
the European parent, and which are not deciduous like them, but persist
during the winter, a peculiarity inherited from the _Mahonia_. As far as
I [271] have been able to ascertain, this hybrid never produces seed.
Another instance of an absolutely sterile hybrid is the often quoted
_Cytisus adami_. It is a cross between the common laburnum (_Cytisus
Laburnum_) and another species of the same genus, _C. purpureus_, and
has some traits of both. But since the number of differentiating marks
is very great in this case, most of the organs have become intermediate.
It is absolutely sterile. But it has the curious peculiarity of
splitting in a vegetative way. It has been multiplied on a large scale
by grafting and was widely found in the parks and gardens of Europe
during the last century. Nearly all these specimens reverted from time
to time to the presumable parents. Not rarely a bud of Adam's laburnum
assumed all the qualities of the common laburnum, its larger leaves,
richer flowered racemes, large and brightly yellow flowers and its
complete fertility.
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