172 Inheritance in the Stock (Matthiola incana) 



of the facts seems to me however somewhat strained. Mer's view is 

 that with feeble transpiration the condition of turgescence set up inside 

 the epidermal cells remains constant, growth is uniform, and conse- 

 quently hairs are not formed : whereas with increased transpiration there 

 may be an accumulation of food materials at certain points which 

 causes the formation of hairs. On this supposition transpiration must 

 be assumed to be more vigorous in the aquatic than in the terrestrial 

 form, a view which seems hardly tenable. In this case, though we have 

 no actual experimental evidence as to the behaviour of the individual 

 when subjected to changed environmental conditions, it may well 

 prove, on investigation, to be comparable with that of Polygonum am- 

 phibium which has been studied by Massart\ According to Massart 

 the hairs produced in this species apart from the flabelliform scales can 

 be classified under three heads : (1) setiform (elongated and tapering to 

 a point), (2) glandular (elongated and terminated by a multicellular 

 gland), (3) scutiform (flat with a base of four cells disposed in a square)^ 

 The flabelliform structures occur in every individual. The terrestrial 

 plant also develops hairs of the setiform and scutiform types and under 

 xerophytic conditions it may produce glandular hairs in addition. In 

 the aquatic form only scutiform hairs are founds He has further es- 

 tablished by experiment that the individual can be made to assume 

 either the terrestrial or the aquatic habit in direct response to a change 

 of environment. Submerged shoots produced no setiform hairs, but if 

 brought above the level of the water the new growth became hairy. 

 And conversely, subaerial shoots were found always to have setiform 

 hairs, but if submerged they ceased to form hairs on the part developed 

 under water. It is evident that conditions which result in the formation 

 of scutiform hairs are set up in the plant under any of the environ- 

 mental conditions described above. Consequently we regard this- type 

 of hair as being strictly inherited. In the case of the setiform hair the 

 requisite conditions are only established in an aerial medium, hence 

 inheritance is shown only within this limited range of environment. 



i Bull. Jard. Bat. Bruxelles, Vol. i. 2, p. 73, 1902. 



2 Massart's figures and description appear to me not to exclude the possibility that the 

 glandular hair is an elaboration of the setiform type and the scutiform hair an arrested 

 form of the flabelliform scale. But whether these epidermal structures are classified under 

 two categories or four is unimportant for the present purpose. The important point is 

 that the species is endowed with more than one kind of hereditary capacity in regard to 

 hair formation. . 



' The further and more profound differences characterising these forms are shown in 

 Massart's figure, p. 75. 



