MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 



205 



The length of the main axis and its branches, and the superficial 

 extent of foliar organs have been found to depend upon the intensity 

 of the illumination in a large number of species. Such variations in 

 stature are coupled with corresponding alterations in internal struct- 

 ure. Adaptations of this character may be generally attributed 

 to responses to the transpiratory conditions set up and to various 

 mechanical factors. The marked features of alpine types consisting 

 chiefly of dwarfing of the shoot and additional checks upon transpira- 

 tion may be ascribed in part to the increased intensity of the illumi- 

 nation at higher levels due chiefly to the lessened absorption by the 

 atmosphere of the blue-violet end of the spectrum, and to the altered 

 moisture-relations by which the danger of drying out is much greater 

 than upon plants at lower levels. 



The change of the light-conditions entailed when aerial members 

 are converted functionally into underground organs is over-balanced 

 by the altered mechanical conditions, 

 so that only a part of the differences 

 maybe ascribed to altered illumination. 

 The character of the stresses to be 

 borne are so altered that the develop- 

 ment and arrangement of the me- 

 chanical tissues are necessarily dif- 

 ferent from those of homologous aerial 

 stems. Then again, the humidity of 

 the medium is so much greater, and 

 the transpiratory conditions so dif- 

 ferent that the epidermal surfaces 

 and subepidermal tissues are widely 

 variant from those of aerial stems and 

 branches. 



The terms "light" and "illumin- 

 ation" have been used in the preceding 

 discussions to allude to the ordinar}'^ 

 exposures of plants to daylight of an 

 intensity varying with the local en- 

 vironment and longitude, and alterna- 

 ting with the nocturnal periods of dark- 

 ness. A review of the records of investigations cited in the opening 

 section of this book brings out most conclusively that the greater major- 



FiG. 157. Ranunculus Astaticus. 

 16D, leaf developed in discontinuous 

 normal illumination ; 16C, leaf de- 

 veloped in continuous electrical il- 

 lumination. 16N, etiolated leaf. 

 After Bonnier. 



