158 Inheritance in the Stock (Matthiola incana) 



(a) Here the hairs are restricted to two regions. They are found 

 scattered along the basal margins of some of the larger leaves (Text- 

 figure 8) and distributed very thinly and haphazard over the surface of 

 a few pods, generally one or two of the lowest on some of the earlier 

 flowering axes (PL VII, fig. 5). Thus we get here a very consider- 

 able drop in the scale. For in the partially hoary grades every ^^od on 

 the individual produces hairs in such abundance as to give a hoary 

 effect, whereas in the present case all but a few are absolutely glabrous, 

 and in these few the hairs are too scanty to produce any mass effect on 

 the eye, and might indeed be counted without much difficulty. Hence 

 the general impression is one of glabrousness throughout. 



(6) In this grade the plant is all but hairless ; only a few solitary 

 hairs are formed on the edges, in the basal region, of a few of the largest 

 leaves. We have the last stage traceable in this direction before final 

 disappearance of the hairy character. 



(c) This is a particularly interesting form. At the outset of the whole 

 inve'stigation it was noticed that in certain individuals included in the 

 glabrous category either a solitary hair or a small tuft was developed 

 over the region of the watergland at the apex of the leaf ^ (Text-figure 1). 

 Where a lateral vein ends in a hydathode (see p. 155) it also bears a 

 trichome. The occurrence of these hairs appeared at the time to be 

 quite uncertain and without significance, all that could be affirmed was 

 that they occurred in some individuals and not in others, and in the^ 

 former case on some leaves but not all. We can now give a more pre- 

 cise account of the distribution of these hydathode trichomes and of their 

 value as a diagnostic character. In the various hoary grades where the 

 leaf margins are hairy throughout their length these hairs over the 

 hydathode merge with their neighbours and are not specially noticeable, 

 but in the sub-glabrous individual with the whole leaf otherwise hair- 

 less they stand out conspicuously (PI. VIII, figs. 7 and 8). In plants of 

 this grade the first leaves which succeed the cotyledons as a rule show 

 no hair. Then follow a succession in which these hydathode hairs are 

 a characteristic feature — though occasionally a leaf may be missed, to 

 be followed in their turn as the plant reaches full vigour by many in 

 which they are wanting. Later the hairs again make their appearance 

 on the young leaves of the axillary shoots. The stem, glabrous below, 

 may in well-grown individuals become slightly hairy in the upper inter- 

 nodes (PI. VII, fig. 6), though in such case still remaining bare as 



^ Report to the Evolution Committee, Report I, footnote to p. 33. 



