E. R. Saunders 171 



number of co-ordinated physiological processes. Again such a cause 

 might well account for the absence of hairs from the higher stem inter- 

 nodes frequently observed in pot-grown individuals of the same grade. 

 Although under these conditions the plant often shows a marked modi- 

 fication in habit, producing a much-branched fibrous root-system in 

 place of the few strongly developed roots having scarcely any fibrils 

 usually found in plants grown in the ground, it may well be that perfect 

 compensation in regard to water content is not attained. We can 

 also understand why a particular character phase should recur at a 

 stage in development when we may suppose that a particular growth 

 phase also recurs, as is seen when the hydathode trichomes, occurring 

 on the early leaves of the main axis but not later, reappear on the 

 developing axillary shoots in autumn ; or again, when, in those grades 

 in which the bulk of the pods are hairless, the sprinkling of hairs found 

 on the lower fruits on the main axis is sometimes seen again on those 

 occupying a corresponding position on the lateral axes. The idea that 

 the development of hairs is related to varying environmental conditions 

 such as dryness or humidity of the soil and atmosphere is not new. It 

 has been advanced in varying forms by many writers^ But the conclu- 

 sions drawn by different observers from observations of different plants 

 are by no means altogether in accord, and it does not appear that any 

 broad generalisations can be deduced from them apart from the well- 

 known fact that species growing in dry habitats are often characterised 

 by a high degree of hairiness. It is more than doubtful if the explana- 

 tion of the appearance of hairs which holds good in one species will be 

 found to apply in precisely the same form to another; for the causes 

 may be as diverse in different cases as the causes which lead to 

 doubling in flowers. Most of the observations hitherto recorded deal 

 only with the question of the direct effect of the environment on the 

 production of hairs. The genetic analysis of these forms has yet to be 

 carried out. An interesting case described by Mer^ which seems to 

 present a certain similarity to that of the sub-glabrous grade of Stock 

 referred to above is that of Ranunculus aquatilis. Mer states that he 

 finds that when growing in water this plant has hairs terminating all 

 the leaf segments, even the last, whereas in the terrestrial form the 

 segments are without hairs. The suggestion put forward in explanation 



^ A full acconnt of the literature on this subject up to 1895 is given by Henslow in his 

 "Origin of Plant Structures by Self -Adaptation to the Environment," International 

 Scientific Series. 



« BuU. de la Soc. Bot. de France, T. 27, p. 50, 1880. 



