126 PLANT PHYSIOLOGY 



set on one side the discussion of such adaptations in dealing with the physio- 

 logy of formation, and refer to DETTO'S (1904) exposition of the subject. We 

 will note only that we have not to deal here with such purposeful adaptation 

 in structure and function as every organism constantly exhibits, but such 

 reaction and regulatory responses as result from the operation of external 

 factors. The changes which appear in the application of external stimuli are 

 doubtless as a rule to be regarded as individual variations, and hence are not 

 inherited ; but if, perchance, mutation or inherited variation also makes its 

 appearance as a result of environmental causes, it is not obvious in the face 

 of it why adaptations should not be included under this head. Physiology 

 cannot explain causally why the plant often replies in a purposeful manner to 

 such stimuli ; the most it can do is to suggest how this capacity has been 

 developed historically. 



In this lecture we have not been able to give more than the briefest sketch 

 of the theory of the origin of species, and have been compelled to omit con- 

 sideration of very many most important observations and views. We must 

 therefore refer to the special literature on the subject, and especially the 

 writings of the following authors : CORRENS (1904) ; DETTO (1904) ; ERRERA 

 (1904) ; GOEBEL (1904) ; KLEBAHN (1904) ; KLEBS (1903 and 1906) ; LOTSY 

 (1906) ; PLATE (1903) ; REINKE (1901) ; SOLMS-LAUBACH (1905) ; DE VRIES 

 (1901-3 and 1906) ; WALLACE (1891) ; WEISMANN (1902) ; WETTSTEIN (1903). 

 The Archiv fur Rassenbiologie should also be alluded to, a journal which is 

 specially devoted to the subject of species formation. 



By way of summary we must say that mutations, doubtless, do not form 

 the only or even the chief mode of origin of species, and hence further in- 

 vestigations are urgently needed both as to mutations themselves, and especially 

 as to their causes. 



397. Lecture XXXI is XIX in 2nd German Edition. 



397, 11. 1-42, for Side by side ... of such cases ; read We have now 

 obtained a general idea of the metabolism of a green plant and have seen that, 

 notwithstanding its endless variability in detail, two chief processes make 

 themselves evident, viz. assimilation and dissimilation. The significance of 

 assimilation is obvious on the face of it ; the plant goes on growing, and its 

 continued growing involves a continuous absorption of materials from without 

 and a transformation of these into the constituent units of its body. The 

 meaning of dissimilation is not quite so apparent, for it appears to undo what 

 assimilation has done. We have, however, learnt that destructive metabolism 

 results in a gain in energy ; hence we must naturally discuss the gain in energy 

 in connexion with the problems of metabolism. 



Any considerations of this sort must obviously take into account the law 

 of conservation of energy. The plant can as little create matter as it can 

 create energy ; it must obtain both from without, and can only alter them 

 within itself. 



The sources of energy available are, in the first instance, kinetic, e. g. 

 electric energy, heat, light ; there is also the energy potential in substances 

 containing it. It is improbable that the plant can make any use of the electric 

 energy presented to it in the free state ; at least, we do not know of any instance 

 of it. 



398, 11. 12-17, for Probably . . . within it. read As to the third form of 

 kinetic energy, the answer is entirely different. We know that the green plant 

 requires luminous energy in order that it may grow, that it develops large leaf 

 surfaces in order to intercept light and absorb it. Part of the light energy 

 absorbed is transformed into chemical energy which reappears in the assimilata, 



