_ Influence of Environment on Plant-organs 233 
ii, Influence of the environment on the form of single organs’. 
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? that tubers may be developed 
on the aerial stem in place of foliage shoots. These observations 
were considerably extended by Véchting®. 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‘. 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 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. 
7 1 A considerable number of observations bearing on this question are given by Goebel 
in his Experimentelle Morphologie der Pflanzen, Leipzig, 1908. It is not possible to deal 
here with the alteration in anatomical structure; cf. Kister, Pathologische Pflanzen- 
anatomie, Jena, 1903. 
: Knight, Selection from the Physiological and Horticultural Papers, London, 1841, 
Voehting, Ueber die Bildung der Knollen, Cassel, 1887 ; see also Bot. Zeit. 1902, 87. 
“ Reference may be made to the full summa i in hi i 
ry of results given by Goebel in his Experi. 
mentelle Morphologie, Leipzig and Berlin, 1908, Section rv. 
