276 THE EVOLUTION THEORY 



■of flowers in phanerogams. These showed that the common balsam 

 {Impatiens noil me tangere) produces its familiar open flowers in 

 a strong light, but in weak light only bears small, closed, so-called 

 ' cleistogamous ' flowers. But it would be utterly erroneous to suppose 

 that the strong or weak Jight is the real cause, the causa materialis, 

 of these two forms of flowers; the degree of illumination is merely the 

 stimulus which provokes one or other of the primary constituents 

 to development, both kinds being present in the constitution of the 

 plant. As has long been known, the balsam normally possesses two 

 kinds of flowers, and the slumbering primary constituents of these 

 are so arranged that the open flowers develop where there is a prospect 



Fig. i2g. A, an aberration of Arctia caja, produced by low temperature. 

 B, the most divergent member of its progeny. After E. Fischer. 



of insect visits and cross-fertilization, that is, in sunny weather or 

 in a strong light, while closed and inconspicuous flowers adapted for 

 self-fertilization develop in weak light, that is, in shady places and 

 in concealed parts of the plant, where insect visits are not to be 

 expected. 



Among plants we find thousands of instances of such reactions 

 of the organipm to external stimulus — reactions which are not of 

 a primary nature, that is, are not the inevitable consequences of the 

 plant's constitution, but which depend upon adaptations of the special 

 constitution of a species or group of species to the specific conditions 

 of its life. To this category belong all the phenomena of heliotropism, 

 geotropism, and chemotropism, which have been discovered by the 



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