THE FACTORS OF DISTRIBUTION 29. 



to the convergence of its few rays upon the chloroplasts. 

 Spheroidal transparent cells in the protonema of a moss 

 (Schizostega) growing in caves, and the projecting 

 epidermal papillae causing the velvet-like " pile " on many 

 leaves in dense tropical forests have been thus explained. 



Too little light is often fatal to buds and lateral shoots, 

 as when the lower branches of trees are killed off under 

 dense forest canopy. Under such weak illumination 

 flower-buds will often not form; or " cleistogene '* 

 flowers, those which do not open, may, as under the 

 leaves of violets in summer, replace those that do. 



Light of moderate intensity is favourable to the- 

 growth of leaves in area, and is essential for (i.) the 

 formation of chlorophyll and some other pigments; 

 (ii.) photolysis, that decomposition of carbon-dioxide 

 which precedes the formation of formaldehyde; 

 (iii.) some movements; and (iv.) the assimilation of 

 nitrates. For the formation of chlorophyll, yellow or 

 orange light is most effective; and for photolysis the 

 red end of the spectrum is the " optimum," or most 

 favourable, condition; but for the assimilation of 

 nitrates the violet end is the optimum. Some free- 

 swimming Algae move towards the light, others away 

 from it: most roots curve away from light, or are 

 "negatively helio tropic "; whilst most shoots, on the 

 contrary, are " positively heliotropic," i.e. bend towards 

 the light. The chloroplasts near the surfaces of leaves 

 arrange themselves parallel to the leaf-surface, which 

 thereby appears darker green, in diffused light; and at 

 right angles to the surface in direct light, when the 

 leaves appear paler. The former movement is termed 

 epistrophe ; the latter apostrophe. 



Light, even when weak, and especially violet light, 

 checks the growth of axial structures in length. The 

 dwarfed, tufted habit of Alpine plants may certainly be 

 correlated with this condition: they are "sun-plants," 

 specially adapted to the intense direct sunlight of 

 cloudless skies and shadeless situations. At the same 

 time, such plants often avoid direct sunlight to some 

 extent by turning their leaves edgewise. They have, as 

 a rule, colourless epidermal cells, the chlorophyll being 

 in the well-developed palisade-tissue or hypoderm. 

 Sun-plants are also characterised by the intense colouring 

 of then: numerous flowers. The House-leek, a sun-loving,. 



