REGENERATION IN PLANTS 77 



studied. This analysis that Vochting has made of the problem of 

 regeneration is in the highest degree instructive, since it shows how 

 several factors, some internal, others external, take a hand in the 

 result ; and it is only possible to unravel the problem by combining 

 different experiments carried out in such a manner that one by one 

 the different factors at work are separated. 



If a piece of a young stem of Salix iriminalis is suspended ver- 

 tically in a moist atmosphere, with the lower end in water (for | of a 

 centimetre), and the piece kept in the dark, the result is, in the main, 

 the same as when similar pieces are suspended in moist air without 

 coming into contact with water. Roots arise near the base, and 

 shoots near the apex, without regard to which end is in the water. . 



If the same experiment is repeated in ordinary air, i.e. air not 

 saturated with water, the result is somewhat different. If the twig 

 is suspended vertically with its apex upivard, roots soon appear on 

 the basal end that is in the water, but no roots develop above the 

 water. Small protuberances may appear above the water in the 

 places at which roots would develop if the piece were surrounded by 

 a moist atmosphere, but they do not break through the bark. If the 

 piece is then covered by a jar containing air saturated with moisture, 

 these protuberances may become roots. It is clear, therefore, that 

 the dryness of the air has prevented their development. 



If a similar twig is suspended (in the air) with its apex dowmvard, 

 and the lower end in water, root protuberances appear, at first, only 

 around the base, i.e. at the upper end. Under the water, at the 

 apical end, small and weak roots may develop, or may even not 

 appear at all. 



These results agree, in the main, with those in which the piece is 

 surrounded by moist air, and give evidence of an inner polarity that 

 is an important factor in the regeneration. The results show that in 

 a piece with the basal end in water and the rest of the piece in the 

 air the tendency to produce roots above the water is suppressed by 

 the dryness of the air. In an inverted piece, however, with the apex 

 in water, the innate tendency to produce roots at the basal end is 

 strong enough to overcome the effect of the dryness of the air to 

 suppress their development. The abundance of water absorbed 

 by the apex of the piece makes the development of the roots possible 

 under these conditions despite the dryness of the air. 1 



There is another factor connected with the submergence of the 

 end of the stem in water that can be shown by putting a longer part 

 of the end under the water. Neither roots, if it is a basal end, nor 

 leaf-buds, if it is an apical end, appear on the deeper parts of the 

 submerged end. This is due, in all probability, to the insufficiency 



1 A piece suspended in ordinary air dries up without producing any new structures. 



