LIGHT 119 



The effect of gravity on the lower i)lants is eoin})arahle to that 

 on seed i)huits. For example, the orientation of the fruit-bodies of 

 fungi is due, for the most part, to gravity. In the ease of the toad- 

 stool type of mushrooms, with gills on the under side of the cap, 

 the spores cannot be shed unless the cap is horizontal and the gills 

 vertical. When such a mushroom grows from the side of a stump or 

 log, the stem, through a response to gravity, always curves in such 

 a way as to bring the cap into a horizontal position (Fig. 56). 



72. Light.— Light is not, like gravity, a universal factor. The 

 amount of light in different places and at different times varies 

 sreatlv. The source of all light is the sun, and the amount of light 



' -111 



received by any place on the surface of the earth depends upon the 

 length of the day and the angle at which the sun shines. At the time 

 of the spring equinox in ISIarch and again at the fall equinox in Sep- 

 tember the day is twelve hours long everywhere on the surface of 

 the earth. But at all other times the days are twelve hours long only 

 at the equator and become either longer or shorter as one goes to- 

 ward either pole. The intensity of light also varies greatly as the 

 season advances because of the changing angle at which the sun 

 shines, being most intense where the incident rays of light are most 

 nearly vertical. 



The intensity of light is also greatly affected by the condition of 

 the weather as everyone who has done photographic work well 

 knows. The merest film of cloud in front of the sun brings about an 

 appreciable reduction in the light intensity and the effect of thicker 

 clouds is of course much greater. There are thus three variables 

 that change the amount and intensity of light in any given place; 

 namely, the time of year, the time of day, and the condition of the 

 weather. 



The quality of light is also an important factor. A beam of white 

 light consists of many rays which differ in length and which, when 

 separated, form the series of colors that are characteristic of a rain- 

 bow or a spectrum. The colors run in order through shades of red, 

 orange, yellow, green, blue, indigo, and violet. Beyond the violet 

 are the ultra-violet rays which are invisible to the human eye but 

 which produce certain effects upon plants. These ultra-violet rays 

 have very short wave lengths while the red rays have the longest 

 wave lengths. The rays with long wave lengths, that is those at the 

 red end of the spectrum, are the ones most used by the plant in the 

 manufacture of carbohydrates. On the other hand the shorter wave 

 lengths, violet and ultra-violet, are most important in checking 



