MINUTE STRUCTURE OF HYBRIDS. 565 



same form but of unequal length, those of their hybrid offspring have a length 

 which corresponds approximately to the mean between the lengths in the parent- 

 species. Thus the length of the hairs on the backs of the leaves is 0-3 mm. in 

 Salix aurita, 1-2 mm. in Salix repens, and 0-6 mm. in their hybrid Salix plicata. 

 The hairs in Sahx Caprea measure 0-8 mm., in Salix viminalis 0-3 mm., and in 

 SaUx acuminata, their offspring, 0'5 mm. Whenever one stock is glabrous and the 

 other hairy, one may be quite sure that the corresponding parts of their hybrid will 

 be furnished with hairs, but less profusely than the parent-species from which that 

 particular characteristic is derived. This is the case, for instance, with Primula 

 Sturii, the hybrid produced by crossing the glabrous Primula minivia with Pri- 

 mula villosa, which has glandular hairs. The leaves of the latter are thickly 

 covered with these hairs, which vary from 0'7 mm. to 1 mm. in length, and Primula 

 Sturii has scattered glandular hairs which measure 0'3 mm. The hybrids obtained 

 by crossing the Purple Willow (Salix purpurea) with the Common .Osier (Salix 

 viminalis) are distinguished by Botanists into two sections, one of which — Salix 

 rubra — -approximates to the Purple Willow and the other — Salix elceagnifolia — to 

 the Common Osier. The leaves of the Purple Willow when mature are glabrous at 

 the back, those of the Common Osier have small glistening hairs lying appressed to 

 their under surfaces, parallel to the lateral nerves, and measuring 03 mm. There 

 are about 1800 of these hairs on a square millimetre. The hairs of the hybrid 

 Salix elceagnifolia are of the same length as those of ;Si. viminalis, but there are 

 only about 800 of them to the square millimetre, whilst the hairs of the hybrid 

 Salix rubra are somewhat shorter, and there are only 400 to the square millimetre. 

 Recently the discovery has been made by Wettstein that the form and dis- 

 position of the cells and tissues in hybrids is also a combination of the corresponding 

 characteristics in the parent-species. The various species of the Pine genus 

 (Pinus) may be distinguished with certainty by the anatomical structure of their 

 needle-shaped leaves, in particular by the thickness of the epidermal cells, the 

 number of the stone-cells lying beneath the epidermis, and the number of the resin- 

 ducts. In the hybrids the anatomical characters of the parents in these respects 

 are united, and the result is indeed often an exact arithmetic mean between the 

 two. Thus a needle of the Scotch Pine (Pinus sylvestris) contains from 6 to 10 

 resin-ducts, that of the Mountain Pine, Pinus Mughus (montana), contains from 

 3 to 5, and that of the hybrid offspring of the two from 5 to 7 such ducts. The 

 Junipers (Juni/perus) afford a similar instance. In their case the leaves are dis- 

 tinguished by the various thickness and length of the layer of sclerotic-cells which 

 covers the back of each leaf, by the width of the resin-duct running through the 

 middle of the leaf, and by the number of the cells encasing that duct. In the 

 hybrids, such as Juniperus Kanitzii, which is produced by crossing Juniperus 

 communis and J. sabinoides, there is evidently a union of the parental attributes 

 in the corresponding cellular structui'es in the leaves. It has also been shown by 

 Hildebrand that in the Wood-Sorrel (Oxalis) hybrids also the anatomical characters 

 of the parents are united, but by far the most comprehensive study which has 



