432 



Minnesota Plant Life. 



While light has a retarding influence upon the growth in 

 length of stems, it has a strong directive influence upon organs, 

 so that they tend to place themselves parallel with the rays, or 

 transverse to them, as their nature may be. It is well known 

 how the leaves of geraniums growing in the window turn to- 

 ward the light. Nasturtium vines turn very quickly and if 

 one of these plants be put in the window, it will in a short time 

 stretch out its leaves toward the light and place them vertically 

 to the rays, thus securing a maximum illumination for the 

 starch-making apparatus. Some plants are not thus sensitive 



FIG. 216. Reaves of the sensitive fern, a shade-loving variety. After photograph by Hibbard. 



to light. For example, such climbing plants as the ivy or 

 the woodbine do not instinctively bend toward the light, because 

 to do this would tear them from their supports ; therefore, they 

 remain either insensible to the directive influence of light, or 

 they actually turn from it, as do most roots. 



Where the light is strong and abundant, there are often de- 

 veloped purple layers on the under sides of leaves to utilize the 

 surplus light by converting it into heat and employing it for 

 the growth-energy of the plant. This is true of such large 



