Influence of Environment on Plant-organs 233 



ii. Influence of the environment on the form of single organs 1 . 



If we look closely into the development of a flowering plant, we 

 notice that in a given species differently formed organs occur in 

 definite positions. In a potato plant colourless runners are formed 

 from the base of the main stem which grow underground and pro- 

 duce tubers at their tips: from a higher level foliage shoots arise 

 nearer the apex. External appearances suggest that both the place 

 of origin and the form of these organs were predetermined in the 

 egg-cell or in the tuber. But it was shown experimentally by the 

 well-known investigator Knight 2 that tubers may be developed 

 on the aerial stem in place of foliage shoots. These observations 

 were considerably extended by Vochting 3 . In one kind of potato, 

 germinating tubers were induced to form foliage shoots under the 

 influence of a higher temperature ; at a lower temperature they formed 

 tuber-bearing shoots. Many other examples of the conversion of 

 foliage-shoots into runners and rhizomes, or vice versa, have been 

 described by Goebel and others. As in the asexual reproduction 

 of algae quantitative alteration in the amount of moisture, light, 

 temperature, etc. determines whether this or that form of shoot is 

 produced. If the primordia of these organs are exposed to altered 

 conditions of nutrition at a sufficiently early stage a complete sub- 

 stitution of one organ for another is effected If the rudiment has 

 reached a certain stage in development before it is exposed to these 

 influences, extraordinary intermediate forms are obtained, bearing 

 the characters of both organs. 



The study of regeneration following injury is of greater import- 

 ance as regards the problem of the development and place of origin 

 of organs 4 . Only in relatively very rare cases is there a complete 

 re-formation of the injured organ itself, as e.g. in the growing-apex. 

 Much more commonly injury leads to the development of comple- 

 mentary formations, it may be the rejuvenescence of a hitherto 

 dormant rudiment, or it may be the formation of such ab initio. In 

 all organs, stems, roots, leaves, as well as inflorescences, this kind 

 of regeneration, which occurs in a great variety of ways according 

 to the species, may be observed on detached pieces of the plant. 

 Cases are also known, such, for example, as the leaves of many plants 

 which readily form roots but not shoots, where a complete regeneration 

 does not occur. 



1 A considerable number of observations bearing on this question are given by Goebel 

 in his Experimented Morphologic der Pflanzen, Leipzig, 1908. It is not possible to deal 

 here with the alteration in anatomical structure ; of. Kiister, Pathologische Pflanzen- 

 anatomie, Jena, 1903. 



2 Knight, Selection from the Physiological and Horticultural Papers, London, 1841. 



3 Vochting, Veber die Bildung der Knollen, Cassel, 1887 ; see also Bot. Zeit. 1902, 87. 



4 Reference may be made to the full summary of results given by Goebel in his Experi~ 

 mentelle Morphologie, Leipzig and Berlin, 1908, Section iv. 



