MINUTE STRUCTURE OF HYBRIDS, öiiö 



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

 which corresponds approximately to tin- mean between tin- length- in t lie parent- 

 species. Tims the length of tin- liairs mi the hacks of the leaves is 0"3 mm. in 

 Salix a/writa, 1*2 mm. in Salix repens, and Ol! mm. in their hybrid Soils ^llroto. 

 The hairs in Salix Capren measure 0*8 mm., in Salix viminalis 0*3 mm., and in 

 Salix aewminata, their offspring, 0'5 mm. Whenever one stock is glabrous and the 

 oiler hairy, one may lie quite sure thai the corresponding pails of their hybrid will 

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

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

 Stwrii, the hybrid produced by crossing the glabrous Primula minima with Pri- 

 mvla vülosa, which has glandular hairs. The leaves of the latter are thickly 

 O .v. rod with these hairs, which vary from nun. to 1 mm. in length, and Primula 

 Sturii has scattered glandular hairs which measure 03 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 elatagnifolia — 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 

 tleir under surfaces, parallel to the lateral nerves, and measuring 0:'. nun There 

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

 elazagnifolia are of the same length as those of S. 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 (Juniperus) 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 Juni 

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

 in the corresponding cellular structures in the leaves. It has also been shown by 

 Hildebrand that in the Wood-Sorrel (Oxalis) hybrids also the anatomical eh uaefc ra 

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



