8. Ikkno 39 



densely covertxl iin<lemeat.h with hmg Imiix «»f HJIvory lustro. The >', 

 hybrids made in either of the two reciprocal wiiyH have leavcH which are 

 always hairy undenieath but much Icms densely so than in S. viminaliH. 

 In the F^ j(eneniti«>n fi-oin N. multinervvi x S. viminnJis I wjih able t-o ^vi 

 6nly 7G planU in all. Of these 'M have leaves <|uite glabrous as in the 

 one parent, whilst the ivmaining 45 were hairy in various degrees, and 

 of the latter one plant was found to |)os8i«ss l«»aves which were jw densely 

 hair)' as in N. viminalis. 



The segn'giition of the character "hairiness" is acconlingly (piite 

 evident in both cases above described, but the projxirtion of dominants 

 »uid recessives in F,j is very ditferent from the usual Mendeli.in ratio, and 

 reminds us of the occurrence of a complex segregation. 



Heiv u»ay we be allowed to make a little digression. As stated 

 above, the leaves of F, hybrids between »S. niultiiiervis and f/radiistt/ld 

 are wholly glabrous, whilst in hybrids between the former and S. viitiinalis 

 they are hairy underneath though le.ss densely .so than in the latter, so 

 that the.se hybrids may be regarded in this respect as intermediate 

 between the two j^iirents. The hairy condition is apparently recessive 

 in the former case, and dominant (or strictly s[)eaki!ig intermediate) in 

 the latter. How such different conditions may occur in spite of the 

 fjvct that we have u.sed the same species S. multinervis — one and the 

 same tree in both cases — must of course remain a matter of conjecture 

 80 long as no extensive culture of several later generations of such 

 hybrids has been made, but the following may be perhaps one of the 

 probable explanations based on the presence-and-absence hypothesis. 

 Let H represent the ftictor (or the factor-complex) for the hairy condition 

 in S. gracilistyla and let I represent the inhibitory factor contained in 

 S. multinervis, then we have S. gracilistyla = HHW and S. midtinervis 

 = hh 1 1, therefore Fi=Hhli, and since the factor I is able to suppress 

 wholly the hair- producing action of H we have in the hybrid Hhli 

 leaves which are entirely glabrous. Since in the hybrids between 

 S. multinervis and viminalis leaves are hairy in contrast to tho.se 

 between the former and S. gracilistyla we are led to think that the 

 factor (or the factor-complex) for the hairy condition in S. vimiiialis is 

 different from that in S. gracilistyla. If we represent that factor by 

 H\ then we have S. viminalis = WH^W, S. multinervis =h^h^\\, and 

 Fi = H'hHi. The si\me inhibitory factor I which was responsible for the 

 entire suppression of the hair-producing action of the factor H in 

 S. gracilistyla may be regarded as being less i)otent against H' than 

 against H and able to prevent the action of H' (►nly partially, so that in 



