INFLUENCE OF EXTERNAL STIMULI. LIGHT 245 



of production of flower takes place is preceded by one in which whilst the 

 buds are laid down they die off at an early period of youth. The intensity 

 of illumination which marks this lower limit is again very different for 

 the different species.' The influence of diminution in the degree of 

 illumination shows itself in the first instance in the corolla. In some 

 species like Melandrium album, M. rubrum, and Silene noctiflora, it 

 remains in its early bud-condition, whilst the sepals, stamens, and carpels 

 attain their normal size ; in others, for example Mimulus Tilingii, all 

 the parts of the flower diminish in size, but the stamens and carpels 

 show themselves less dependent upon light than the corolla. In some 

 species when the illumination is deficient the flowers are always open, 

 even although there be a reduction in size of the corolla or of the whole 

 flower, whilst in others the flowers remain closed l ; this especially occurs 

 in forms which like Stellaria media have a tendency to cleistogamy, or 

 which produce special cleistogamic flowers like Linaria spuria. In such 

 cases it is possible to produce by regulation of the illumination either 

 flowers which open, that is to say are chasmogamic, or closed flowers, that 

 is to say cleistogamic ones ; this is not however the case in Viola. It 

 cannot well be doubted, and I may cite as ground for this a fact which 

 I have published elsewhere 2 , that in uniform conditions of light other 

 factors may evoke the production of cleistogamic flowers in plants which 

 usually produce them. We may also assume that the minimum of any 

 factor, for instance of temperature, for the formation of flower does not 

 usually coincide with that for the vegetative organs. We already know 

 from what has been said on page 212 that plants in which formation 

 of flower is hindered by insufficient intensity of light very often exhibit 

 luxuriant formation of vegetative shoots. 



Further investigation is required regarding the influence of light 

 upon the formation of sporangia in the Pteridophyta. I am disposed 

 to think that relationships analogous with those observed in connexion 

 with formation of flower exist there :! . 



(6} FLATTENING AND INCREASE OF SURFACE OF ORGANS IN RELATION TO LIGHT. 



It is characteristic of the condition about which we have now to 

 speak that it seems to occur only in organs containing chlorophyll. When 

 we observe a marked development of the surface in such organs, most 

 conspicuously in leaves, its advantage in assimilation which is dependent 

 upon light requires no demonstration. I have already briefly mentioned 

 on page 241 a case in point the germ-tubes of the prothalli of ferns only 



1 See what is said about Tropaeolum on p. 244. 



2 Goebel, Pflanzenbiologische Schilderungen, ii. p. 363. 



3 See what I have said on this subject in Flora, 1895, p. 116. 



