160 Inheritance in the Stock (Matthiola incana) 



a rule beneath each leaf insertion ; in less vigorous individuals, e.g. pot- 

 grown plants, it is often hairless throughout. Of the pods some few, 

 mostly the lower ones on the earlier flowering axes, may develop a 

 sprinkling of hairs along the lines of the sutures (Text-figure 2). Such 

 pods when seen in valve view show the hairs projecting on either side, 

 but when turned so that one suture is seen in face view they appear 

 hairless. The enormous majority are however completely glabrous up to 

 the ridge defining the stigma lobes. On the stigma lobes themselves, on 

 the other hand, in all or almost all of the pods a few hairs are to be 

 seen standing out in sharp contrast with the smooth outline of the pod, 

 affording a useful means of identification at a stage when the leaves may 

 fail to give a clue (PI. VIII, fig 9). 



{d) This form represents in this direction, like its corresponding 

 grade (6) on the other line, the final stage before the complete dis- 

 appearance of the hairs. On stem and pod they are probably entirely 

 wanting ; and whereas on the preceding grade they are characteristic of 

 stigma lobe and leaf tip at certain stages, here they are but rarely found 

 in either position, so rarely indeed that discrimination in mixed families 

 between this grade and the wholly glabrous condition was not attempted. 



2. Wholly glabrous. The individuals included under this category 

 appear to be entirely destitute of hairs and occupy the zero position in 

 the scale (PI. VII, fig. 1, right-hand pod). 



The serial relation of the whole group of forms here described is 

 made clear in the scheme shown on p. 161. 



It will be seen from the descriptions given above that all grades 

 except the two extremes exhibit a range as regards hair production 

 having some relation certainly to the developmental cycle, and possibly 

 also to external conditions. This is most apparent in those forms where 

 the hairy character is on the verge of disappearance. For example in 

 the sub-glabrous grade (c) we can clearly see a regular succession of 

 stages as development proceeds, but the even course of this general 

 sequence may be broken by an occasional irregularity, e.g. when a 

 leaf lacking a terminal hair is interposed in a trichome-bearing series : 

 or again — a case perhaps more distinctly traceable to environmental 

 effect — when the later stem internodes of a pot-grown individual remain 

 hairless. Hence for a proper classification of the individual a mere 

 catalogue of the morphological characters without regard to age does 

 not suffice. We must also consider the organism from the physiological 

 and the developmental standpoint, Hardly any one. organ, whether leaf, 



