238 THE INFLUENCE OF LIGHT ON LIFE. 



light and heat, so that at the equator it will come to maturity so 

 much the sooner. The more the light and heat the quicker they 

 obtain the sufficient quantity for their development. It is worthy 

 of note that, though, as is the case when fruit-trees are transported 

 from this country to another clime of greater light and heat, a 

 great acceleration takes place in the growth, the period of life is 

 much shortened. Prof. Stokes points out how rapidly vegetation 

 grows in the Arctic regions when summer has fairly set in, and 

 how that rapidity is the result of the long summer days, the sun, 

 indeed, remaining above the horizon for the greater part of the 

 twenty-four hours. Vegetation in those northern latitudes, how- 

 ever, gets a long rest during the winter, and thus is compensated 

 for the extra activity induced by the continuous sunlight of the 

 summer days. 



It would, perhaps, now be well to call attention to the beha- 

 viour of chlorophyll granules under varying quantities of light, and 

 when we consider how directly dependent upon light the chloro- 

 phyll is, and how entirely the chlorophyll depends upon that 

 influence in order to perform its work and supply the plant with 

 nutriment, we are not altogether surprised to find that the granules 

 endeavour to accommodate themselves to any changes which may 

 occur. Those who have closely examined their behaviour inform 

 us that the granules in some cases alter their shapes, while in other 

 cases they arrange themselves in the cells in positions which are 

 most suitable to receive the proper quantity of light. If the light 

 is just sufficient for their needs, the granules will (as in the Ivy- 

 leaved Duckweed) spread themselves over the cell- walls (Fig. i). 



If the light is altogether absent, then the granules crowd up 

 close under the epidermis, as though striving hard to get nearer to 

 the source of light which has been temporarily shut off from them, 

 and take up a position as exposes the largest surface possible 

 (see Fig. 2). If, however, the light is too strong, then the granules 

 station themselves along the upright walls — a position which, it is evi- 

 dent, will be most conducive to shelter, as thereby the least possible 

 surface of chlorophyll will be exposed (Fig. 3). These movernents 

 of the protoplasm or chlorophyll granules are most interesting, and 

 the matter becomes even more interesting when we remember that 

 the leaves, twigs, and even branches of plants also take up posi- 

 tions in accordance with the degree of illumination. 



