K. — BOTANY. 189 



largest and most dii-ect supply. In this same strain a leaf may now 

 and again be found lacking this hydathode trichome in an otherwise 

 continuous hair-forming series, an occurrence which may well result 

 from a slight fluctuation in physiological equilibrium such as is inherent 

 in all vital processes — a fluctuation wlaich, when the genetic indicator 

 is set so near to the zero point, may well send it off the scale altogether. 

 If, as is not improbable in this and similar cases, we are concerned with 

 a complex chain of physiological processes, investigation of the nature 

 of the differences for which the allelomorphs stand may present a more 

 difficult problem than where the production of a particular chemical 

 compound appears to be involved. In such a physiological conception 

 we have probably the explanation of the non-appearance of the recessive 

 character in certain dominant cross-breds. 



Up to this point we have treated of the organism from the aspect 

 of its being a wholly self -controlled, independent system. As regards 

 some characteristics, this may be regarded as substantially the case. 

 That is to say, the soma reflects under all observed conditions the 

 genetic constitution expressed in the Mendelian foi'mula. Corre- 

 spondence is precise between genotypic potentiality and phenotypic 

 reality, and we have so far solved our problem that we can predict cer- 

 tainly and accurately the appearance of offspring, knowing the consti- 

 tution of the parents. In such cases we may say that the efficiency 

 of the genetic machine works out at 100 per cent., the influence of 

 external environment at (I. Our equation somatic appearance=factorial 

 constitution requii'es no correction for effect of conditions of tem- 

 perature, humidity, illumination, and the like. But most somatic 

 characters show some degree of variability. Phenotypic appearance 

 is the outcome jirimarily of genotypic constitution, but upon this are 

 superposed fluctuations, slight or more pronounced, arising as the 

 result of reaction to environmental conditions. In the extreme case 

 the genetic machinery may, so to speak, be put out of action; geno- 

 typic potentiahty no longer becomes actual. We say that the character 

 is not inherited. We meet with such an example in Ranunculus 

 aquatilifi. According to Mer,-*^ the terrestrial form of this plant has no 

 hairs on the ends of the leaf segments, but in the aquatic individual the 

 segments end in needle-shaped hairs. That is to say, hairs of a definite 

 form are produced in a definite region. Again, Massart "^ finds that 

 in Polygonum amphib'mm the shoot produces characteristic multi- 

 cellular hairs when exposed to the air, but if submerged it ceases to form 

 them on the new growth. Every individual, however bred, behaves 

 in the same manner, and must therefore have the same genetic con- 

 stitution. In an atmospheric environment genotypic expression is 

 achieved, in water it becomes physiologically impossible. A limitation 

 to genotypic expression may in like manner be brought about by the 

 internal environment, for the relation of the soma to the germ elements 

 may be looked upon in this light. Thus in the case of a long-pollened 

 and round-pollened Sweet Pea Bateson and Punnett -* found that the 



=" Bull. Soc. Hot. (le France, i. 27. 1880. 



" B^lll. Jard. Bot. BruxelUs, i. 2, 1902. 



=s Report to the Erohition Committee, Roy. Soc, ii., 190.5. 



