156 Inheritance in the Stock (Matthiola incana) 



development is uniform for all members of a category (stem, leaves, etc.) 

 and is coincident with the coming into being of each member, so that 

 the characteristic full degree of hoJiriness is to be seen from the moment 

 that the plumule appears between the cotyledons. This is in marked 

 contrast to the progressive development of hairs seen in the following 

 class. 



2. Partially hoary. Here the characteristic leaf-surface appearance 

 is only assumed gradually as we pass from the first to the succeeding 

 leaves, and this delayed effect becomes more pronounced as we descend 

 the scale. 



{a) The haff-hoary variety semi-incana is chiefly distinguished by 

 having the stem almost devoid of hairs for the greater part of its length. 

 The pedicels, glabrous like the stem on the adaxial, may become some- 

 what hairy on the abaxial side. The upper surface of the leaves is 

 distinctly less hairy than in the type, so much so as to appear greenish 

 and almost glabrous when seen in contrast with the under surface, 

 which, as well as sepals and ovary, approaches very nearly to that of 

 incana. This condition in the case of the leaves is attained by degrees. 

 The first one or two formed by the developing plumule are glabrous 

 except for a few hairs on the apical margin. In each succeeding leaf 

 the hairs become more numerous, extending in those next formed along 

 the whole margin : then, as others follow, appearing on the under surface, 

 thinly at first, more abundantly as we proceed up the stem : finally, as 

 the rosette begins to take shape, becoming numerous on the upper 

 surface also. The stage at which there is a well-developed rosette but 

 as yet no flowering axis is the best for identification. During the 

 flowering period the much-branched stem lessens the general mass effect 

 of the leaves, and later, when axillary shoots begin to develop, their 

 earlier leaves often show a return to the glabrous condition, to be fol- 

 lowed only in the ensuing season by others of the characteristic appear- 

 ance pf semi-incana. It is as though the forces (factors) governing 

 hair-production either gathered intensity as development proceeded, or 

 as if some inhibitory condition were more pronounced at the beginning 

 of any growth period. (Pi. VII, fig. 1 (middle pod).) 



(6) The mid-hoary grade, the next in the series and an entirely 

 new form, shows a similar progressive development of hairs, but they 

 are both less abundant and later in making their appearance, and the 

 stationary point is reached at a lower stage. The stem is glabrous, 

 showing few if any scattered hairs below the region of the inflorescence. 



