216 STRUCTURE AND VITAL PROCESSES OF PLANTS 



power of thus feeding on the carbonic acid gas in the air. It 

 is only the green plants (name plants that are not green!), and 

 only the green parts of these that can feed on the carbonic acid 

 gas in the air. If we repeat the above experiment with a potato 

 tuber, no oxygen will be given off. The green leaves are, there- 

 fore, the most important organs for nourishing the plants. Plants 

 which are repeatedly robbed of their leaves, as for instance, by 

 the ravages of caterpillars, become sickly and die. 



(c) The Presence of Sunshine is required.— The green parts of 

 plants can absorb carbon only under certain circumstances. They 

 require sunshine for their action. If we place the apparatus of 

 the experiment, described above, in a dark place, there will be 

 no formation of bubbles, and there can, therefore, be no absorption 

 of carbon. At night also this process cannot take place. 



The fact that plants require light for their life, explains nu- 

 merous features of the structure of plants: The green part of the 

 plants are placed in the light; stems and branches, the supports 

 of the green leaves, rise above the ground ; climbers bring their 

 leaves from the shade below to the life-giving light above; many 

 jungle plants that would not get sufficient light on the dark- 

 shaded ground have assumed the habit of perching on the branches 

 of trees where they have a chance of getting more light; the 

 leaves themselves are generally dark-green on their upper surface 

 and whitish on the lower one; the insertion of the leaves in the 

 stem is always such that all of them get their due share of light; 

 those placed at the base of a stem are in many cases larger, 

 long-petioled and Hatly exposed to the light, those above, small 

 and pressed towards the stem (^lustard, Ladies' linger); if the 

 stem is weak and straggling (Cucumber, page 74), the petioles 

 by twisting and bending themselves, assume such a position as 

 to place every leaf in the light; and large leaves are often 

 divided into smaller parts so as to let the light pass through 

 tlieir holes to any leaves tbnt grow below them. 



{d) The Inner Structure of the Leaf: Chlorophyll and Photosyn- 

 thesis. — In order to understand better the manner in wliich leaves 

 absorb tlieir food, we must examine the inner structure of them. 

 The illustration on p. 217 shows the vertical section of a leaf as 



