THE INFLUENCE OF CORRELATION AND 



EXTERNAL FORMATIVE STIMULI UPON 



THE CONFIGURATION OF PLANTS 



INTRODUCTION. 



I HAVE shown in the preceding Section that external circumstances 

 often exercise a profound influence upon the formation of organs ; thus 

 the formation of a shoot may be induced upon the leaves of Pteris 

 quadriaurita in a position where no such shoot normally appears through 

 the attack of a parasitic fungus, and where this occurs the structure 

 and configuration of the leaf of the fern become changed x . In this case, 

 a definite external factor, the parasite, acting upon the definite peculiarities 

 of the protoplasm of the host-plant has influenced the formation of organs 

 and their further development ; in like manner the normal formation 

 of organs is also determined and influenced by external factors, assuming 

 of course that, as is necessary for all life-processes, the general conditions 

 for life are present. The investigation of these factors however falls 

 within the province of experimental physiology, as does also the study 

 of those special cases in which the unfolding of organs from their primordial 

 condition, or their development generally, takes place only under definite 

 conditions acting as stimuli ; the consideration of these is not a part of 

 our task here. It is, for example, of no moment to organography that 

 the germination of the seeds of Orobanche takes place only when 

 they are brought into contact with the roots of a host-plant, or that 

 the spores of the liverworts will, according to Leitgeb, only germinate 

 in the light. If, on the other hand, it were shown that the configuration 

 of the seedling is different according to the greater or less intensity of 

 the light, this would be a fact of the highest importance for organo- 

 graphy because it would show a direct dependence of configuration 

 upon external conditions. There are of course no absolute limits to be 



1 See p. 193. 



