BUD^ AND STEMS 



99 



modified leaves because they come out on the stem in the position 

 taken by leaves. The stem of the plant grown in the darkness is 

 much longer and thinner than the one grown in the light. Can 

 you think of any reasons for these differences ? 



Effect of Light on Plants. — We can explain the changed condi- 

 tion of the seedling grown in the dark only by assuming that light 

 has some effect on the protoplasm of the seedling and induces 

 the growth of the green part of the plant. Numerous instances 

 could be given in which plants grown in sunlight are healthier 

 and better developed as to their green parts than those in the 

 shady parts of a garden or field. On the other hand, some plants 

 thrive in the shade. Such plants are the mosses and ferns. Still 

 other plants, minute organisms hardly visible to the eye, do not 

 thrive in the light, and may be killed by its influence. Such are 

 molds, mildews, and some bacteria. It is to be noticed that 

 such plants as do not need light are not green. As a matter of 

 fact, the stem, which has but little chlorophyll, develops some- 

 what more rapidly under conditions where it receives no light. 



Heliotropism. — We saw 

 that the stems of the 

 plants kept in the dark- 

 ness did not always lift 

 themselves erect as in the 

 case of stems in the light. 

 If your seedlings have 

 been growing on a win- 

 dow sill, or where the light 

 comes in from one side, 

 you have doubtless noticed 

 that the stem and leaves 

 of the seedlings incline in the direction from which the light 

 comes. The tendency of young stems and leaves to grow toward 

 sunlight is called 'positive heliotropism. 



The experiment pictured on the following page shows this effect 

 of light very plainly. A hole was cut in one end of a cigar box 

 and barriers erected in the interior of the box so that the seeds 

 planted in the sawdust received their light by an indirect course. 



The growth of young stems and leaves of oxalis 

 toward the light. 



