14(j Half-ltoariness in 3Iatthiola 



shoot is far less profoundly modified than is the case with the single. 

 In the presence of corolline hairs, which are not found in the single, 

 even when hoary, we see the retention of yet another vegetative 

 character. 



The half-hoary type as well as the fully hoary was employed in the 

 earliest stages of these investigations'. The strain then used was an 

 annual white-flowered form which had been obtained together with 

 some true incana from the Isle of Wight. It was ascertained that this 

 half-hoary strain bred true and exhibited certain definite relations to 

 the glabrous forms with which it was crossed. Unfortunately before 

 these experiments were complete the half-hoary form was lost, and 

 further investigation was held up for several years until a new half- 

 hoary strain with pale purple (azure of connnerce) Howei's was by chance 

 obtained. Being of slower growth, rather intermediate than annual in 

 habit, this new strain was not so well suited for experiment as the one 

 originally employed. In respect of surface character however the two 

 types were precisely similar. 



The distinctive feature of the half-hoary type is a certain definite 

 localised inequality in the distribution of the hairs. The under surface 

 of the leaves, the sepals and the pods are fully hoary. On the upper 

 surface of the leaves the number of hairs is far fewer ; the effect 

 by contrast is to give a deceptive appearance of smoothness, although in 

 fact this surface is thinly covered all over with hairs. The stem is for 

 the most part actually glabrous, but as we pass up the shoot a certain 

 overflow as it were of hairs from the leaf bases may give rise to 

 decurrent hairy streaks which may become gradually more emphasised, 

 until, in the shorter internodes of the flowering region, the glabrous 

 area, in an extreme case, may be limited to the opposing (axillary) faces 

 of the main axis (peduncle) and the pedicels. The partially glabrous 

 habit readily distinguishes this from the fully hoary type. 



In order to make clear the relations of the half- hoary form to the 

 fully hoary and the glabrous types respectively it will be advantageous 

 to restate briefly the position as to the relation in which these two 

 latter types stand to one another. As we conclude, from the fact that 

 some true-breeding ■white Stocks, when crossed, give rise to coloured 

 offspring, that the combination of two factors (indicated by C and R) is 

 required to produce colour, so the fact that it is possible to obtain a 

 hoary F^ from two coloured glabrous strains, or even from two indivi- 

 duals of the same coloured strain, leads us to assume the existence of 



' See " Keports to the Evolution Committee of tbe Koyal Society," Efjiorl I, p. 33. 



