564 THE GENESIS OF NEW FOKMS AS A RESULT OF CROSSING. 



under the name of investments (indumentum), are very constant characters in most 

 species of plants. The occurrence of stellate hairs, in particular, is looked upon by 

 systematic Botanists as an important point in assisting them to distinguish between 

 similar species, and so also is the presence of glandular hairs composed of simple 

 rows of cells, and terminating in globular bladders full of ethereal oils. Hybrids 

 exhibit the most varied combinations of the indumenta of their parents. In tie- 

 majority of cases the characteristics of the two stocks in this respect are mixed, but 

 less frequently are they united, and in the latter case the shape, size, and number of 

 hairs, bristles, scales, and glands are intermediate between those of the same appen- 

 dages in the two parent-species. The Lungwort genus (Ptdmonaria), which has a 

 special tendency to hybridization, includes only a few sjaecies, but each one may be 

 recognized by the nature of its indumentum. Thus, Pulmonaria officinalis is dis- 

 tinguished by the thousands of short unicellular prickly hairs, scarcely perceptible 

 to the naked eye, which are interspersed amongst the long scattered bristles on the 

 upper surfaces of the leaves. In Puhnonaria angustifolia the leaves are destitute 

 of these minute prickles, but bear on their upper surface a more abundant quantity 

 of straight appressed bristles of ecpual length. The leaves of the hybrid derived 

 from the two preceding species, viz. Pulmonaria hybrida, are richly supplied with 

 long bristles, and interspersed amongst these may be seen a large number of shorter 

 bristles which are about two or three times as long as the prickly hairs of PvX 

 monaria officinalis. A very instructive example is also afforded by the hybrid 

 Rhododendron intermedium, which is easily produced by crossing the two Alpine- 

 Roses (Rhododendron ferrugineum and Rhododendron hirsutum). The upper 

 faces of the leaves of R. ferrugineum are dai-k-green, smooth, and shining, whilst 

 their backs are rusty and dull owing to the presence of a dense crowd of tiny scales. 

 The margins are not ciliate. The leaves of R. hirsutum are lierht-jjreen and beset 

 with scattered whitish glands (see vol. i. p. 232, figs. 54 5 and 54 6 ), and their margins 

 are fringed with long hairs. In Rhododendron intermedium both kinds of epider- 

 mal appendage are displayed side by side. The under surface of the leaf is furnished 

 with brown scales, though not so profusely as in Rhododendron ferrugineum , and 

 its edge is fringed with hairs, but not so thickly as in R. hirsutum. The same seit 

 of thing occurs in Roses, Cinquefoils, Blackberries, Drabas, Hawkweeds, and many 

 other plants. Where one parent Rose bears only non-glandular and the other only 

 glandular hairs the hybrid is sure to be clothed with a mixture of the two kinds 

 of hairs. Several species of Cinquefoil (Potentilla) have stellate or tufted hairs, 

 whilst others are entirely free from them and bear none but simple hairs on their 

 leaves. In hybrids derived from two of these species — one with compound and the 

 other with simple hairs — stellate or fasciculated hairs are invariably intermixed with 

 a large number of simple hairs. A few species of the perennial Whitlow-grasses 

 (Draba), which are indigenous to mountainous districts in Central Europe, have 

 rectilinear anvil-shaped hairs, whilst others have three- or four-rayed stellate hairs. 

 In the hybrids which spring from these different species rectilinear and stellate 

 hairs grow together on the same leaf. If the hairs of two parent-species are of the 



