68 BRITISH PLANTS 



in summer they revolve in the fuU light of the sun, and 

 there is no night. The long duration of the summer 

 days in Northern regions compensates, in some measure, 

 for the shortness of the summer and the low intensity 

 of solar radiation. In Norway, for example, cereals 

 ripen in a much shorter time than in England. In the 

 Tropics there is no alternation of summer and winter, 

 because the sun is never far from the zenith at noon ; 

 the vegetation is therefore always green, and, provided 

 sufficient water is present, does not display the seasonal 

 changes of activity and rest so famihar to us. 



Heliotropism. — The growing parts of plants are affected 

 by certain physical forces in the environment, with the 

 result that the different organs tend to turn towards a 

 position of advantage and away from a position of dis- 

 advantage. Curvatures of this kind are known as 

 tropisms. Perhaps the most obvious of these tropisms is 

 that which is brought about by oblique iUumination. 

 Stems grow towards the light, and the leaves bend so as 

 to receive as much of it as possible. The heliotropism 

 (Gr. helios, sun ; tropos, turning) of stems and leaves is 

 easily observed in plants grown before a window in a 

 , dwelling - house. The same thing happens outside. 

 Plants growing in shady situations bend towards the 

 direction in which they can catch the greatest amount 

 of light. The stems curve towards a position of advan- 

 tage, and here the position of advantage is that best 

 suited for assimilation. 



Leaf - Mosaic. — Closely associated with heliotropism is 

 the phenomenon of leaf -mosaic. If the leaves of a horse- 

 chestnut be viewed from above, it will be found that they 

 are so arranged in regard to size and position that each 

 leaf gets its full share of the light without let or hindrance 

 from its fellows. On looking down upon the leaf -surf ace 

 of a shoot there is little or no overlapping, and each 

 portion of its area is occupied by a leaf. This arrangement 

 is brought about by the bending of the leaf-stalks under 

 heliotropic influences, and by variation in the length of 

 the petioles and the size of the leaves — e.g., climbing plants 

 like ivy (Figs. 20 and 21), prostrate stems like grormd-ivy, 

 rosette-plants like dandelion, and bushes with obhque 

 or horizontal stems like the box, yew, and maple (Figs. 22 

 and 23). 



