662 THE GENESIS OF NEW FORMS AS A RESULT OF CROSSING. 



hybrid — i.e. by fusion, by union, or by mixture — one alone sometimes prevails in all 

 the parts; but usually, on the contrary, there is an incalculable degree of variation 

 in this connection. There are, for example. Rose hybrids in which the outline of 

 the leaves exhibits a union, the colour of the flowers a fusion, and the hairs a mix- 

 ture of the corresponding characters in the parents. 



A brief account will now be given of a few examples to illustrate the manner in 

 which the combination of parental characters in a hjd^rid is manifested throughout 

 the structure of the plant and particularly in the forms of the stems, leaves, and 

 flowers, and in those of thorns, bristles, hairs, and other epidermal appendages. The 

 species of Willow known as Salix Caprea grows in the form of a little tree, with 

 thick, straight, erect branches, each of which bears about 25 leaves; Salix repens, 

 on the other hand, is a low shrub with a procumbent stem and slender, rod-like 

 branches ascending in curves from it, and each bearing about 40 leaves. The 

 hybrid of these two is a small tree with a bent stem and ascending branches, which 

 in lencrth, thickness, curvature, and direction are intermediate between the Salix 

 Caprea and Salix repens, and wdiich are furnished wäth some 30 leaves apiece. 

 Again, the foliage-leaves of Prunella vulgaris have their margins entire, whilst those 

 of P. laciniata are deeply cut, and their hybrid Prunella intermedia has lobed 

 leaves. The leaves of Potentilla sterilis (or P. Fragariastrum) possess three 

 leaflets, each leaflet being furnished on either side with from four to five serrate 

 indentations. The leaves of Potentilla micrantha likewise possess three leaflets, 

 but each leaflet has from seven to ten serrate teeth on each side. In the hybrid 

 of these two species the leaflets have from six to eight indentations on each side. 

 The shape of the leaf is, as is well known, intimately connected with the course, 

 ramification, and disposition of the bundles called nerves. Now, if the net- work of 

 strands in the leaves of the parent-species is compared with that in the leaves of 

 the hybrid, it is astonishing to find how the union of the two systems may be 

 traced in the minutest details. No group of plants lends itself better to this sort 

 of investigation than the Willows. Even if a single leaf of the hybrid offspring 

 of two species of Willow is submitted for inspection, it is possible in most cases to 

 determine, from the number and distribution of the nerves, the identities of both 

 its parents. Salix purpurea produces one hybrid when crossed with Salix grandi- 

 folia, and a second when crossed with Salix Caprea. Salix grandifolia has twice 

 as many lateral nerves in each leaf as Salix Caprea, and this difference is repro- 

 duced in the corresponding hybrids, whose leaves in all other respects resemble one 

 another closely. 



The involucral leaves of Composites are well known to be extremely diverse in 

 shape, and systematic Botanists have always attributed great importance in the 

 discrimination of species to the size, shape, and margination of these leaves and to 

 the peculiar appendages at their apices. Now, the hybrids of Composites not 

 infrequently have involucral leaves which differ widely from the forms character- 

 istic of the parent-stocks. Thus, for instance, each leaf of the involucre in a 

 capitulum of Centaurea rupestris terminates in a long yellow prickle, whilst the 



