124 PHYSICAL FACTORS: LIGHT 



in light, and it cannot function for carbohydrate synthesis without 

 Hght. Many saprophytic and parasitic fungi which can flourish 

 vegetatively in the dark require light at the time of fruiting. 



Most plants actually use only a small part of the light that they 

 receive. Many of them could probably get along with less than 1 

 per cent of the light that is normally available for them. Yet there 

 is for each species an optimum amount as well as an upper zero point 

 or maximum and a lower zero point or minimum. In general, low 

 intensities of light favor the growth of vegetative structures. The 

 largest leaves and the fullest development of many kinds of stems are 

 found in partial shade. Many vegetative crops, such as potatoes, 

 beets, carrots and turnips, yield best in regions where there is a high 

 percentage of cloudy days. Ginseng, when grown commercially, 

 is covered with slat frames so that it will not be fully exposed to the 

 light, and tea is usually planted in alternate rows with taller trees, 

 the shade causing the tea leaves to become large and of good quality. 



On the other hand, intense light favors the development of flowers, 

 fruits, and seeds. The best regions in which to grow fruits and grains, 

 therefore, are those in which there is a high percentage of bright, 

 sunny days, provided there is also a sufficient amount of moisture. 

 Furthermore, the brightest colors and the greatest profusion of 

 flowers are found in Arctic meadows where the light is very intense, 

 while in the shade of a dense forest flowers are scarce. 



75. Tolerant and Intolerant Plants.— Plants that are able to sur- 

 vive, grow and develop in the shade of other plants are said to be 

 tolerant while light-demanding plants are intolerant, that is, in- 

 tolerant of shade. Light is not the only factor involved in tolerance 

 but it is an extremely important one. Tolerance is particularly 

 important in a forest. Some trees, such as sugar maple, beech, and 

 hemlock, are very tolerant. The seedlings of such trees can develop 

 and grow very slowly in the shade of other trees for many years and 

 then make a very rapid growth later if the shade is by some means 

 removed. Such trees as soft maple, bur oak, poplars, ponderosa 

 pine, and lodgepole pine, on the other hand, are very intolerant and 

 cannot develop at all in the shade of other trees. This difference 

 between species becomes very important when tolerant and intoler- 

 ant trees are growing together. The seeds of both may germinate 

 but only the seedlings of the tolerant species can continue to grow. 

 Whenever any of the old trees die or fall, therefore, their places are 

 certain to be taken by tolerant trees. The obvious result of this is 

 that finally the forest comes to be composed of tolerant trees only. 



