80 



HOW GREEN PLANTS MAKE FOOD 



one end of a cigar box and barriers were erected in the interior of 

 the box so that the seeds planted in the sawdust received their hght 

 by an indirect course. The young seedUng in this case responded 

 to the influence of the stimuhis of hght so that it grew out finall}^ 

 through the hole in the box into the open air. This growth of the 

 stem to the light is of very great importance to a growing plant, 

 because food making depends largely on the amount of sunlight 

 the leaves receive. 



Effect of Light on Leaf Arrangement. — It is a matter of common 

 knowledge that green leaves turn toward the light. Place growing 

 pea seedlings, oxalis, or any other plants of rapid growth near 

 a window which receives full sunlight. Within a short time the 

 leaves will be found in positions to receive the most sunlight 

 possible. Careful observation of any plants growing outdoors 

 shows us that in almost every case the leaves are so arranged as 

 to get much sunlight. The ivy climbing up a wall, the morning- 

 glory, the dandelion, and the bur- 



*}^ 



I 



dock, all show different arrange- 

 ments of leaves, each presenting 

 a large surface to the light. 

 Leaves are often definitely ar- 

 ranged, each fitting in between 

 ■^^ 'I others so as to present their 



H^^ J upper surface to the sun. Such 



an arrangement is known as a 

 leaf mosaic. In the case of the 

 dandelion, a rosette or whorled 

 cluster of leaves is found. In 

 the horse-chestnut, where the 

 leaves come out opposite each 

 other, the older leaves have 

 longer petioles than the younger 

 ones. In the mullein the entire plant forms a cone. The old 

 leaves near the bottom have long stalks, and the little ones near 

 the apex come out close to the main stalk. In every case each leaf 

 receives a large amount of light. Other modifications of these 

 forms may easily be found on a field trip or in a study of house 

 plants. 



A dandelion, showing a whorled arrar.ge 

 ment of long irregular leaves. 



