350 A TEXT-BOOK OF BOTANY. 



Functions of the Leaf. — When we speak of the leaves of 

 the plant we usually have in mind the foliage leaves or green 

 chlorophyll leaves. 



Under the influence of sunlight the chloroplasts are able to 

 rearrange the elements in carbon dioxide and water, which are 

 looked upon as inorganic substances, into starch or related com- 

 pounds which are of an organic nature. This process is known 

 as carbon dioxide assimilation, or photosynthesis, which latter 

 term means the building up of a compound under the influence of 

 light. In this process, which is sometimes expressed by the fol- 

 lowing formula, oxygen is given off : 



6CO, + sH,0 =. QH.oO, + 60, 



Carbon dioxide Water Starch Oxygen 



The importance of this function can be best appreciated by 

 bearing in mind that all of the organic products built up by the 

 plant are derived almost entirely from the carbon dioxide of the 

 air which is taken in through the leaves. (Consult also pages 



157-159-) 



Transpiration and respiration are also functions of the leaf. 

 Transpiration is the giving off of water (through water-pores), 

 or watery vapor (through the stomata), which has been absorbed 

 by the root-hairs and transported through the tissues of the root, 

 stem and leaf; the process of breathing, or respiration, consists 

 in the taking in of oxygen and giving off of carbon dioxide, the 

 exchange being just the reverse of what it is in photosynthesis. 

 These several functions are, however, not confined to the leaf 

 alone, but are carried on by all the green parts of the plant. 



Physiological Experiments. — The leaf is undoubtedly the 

 most active part of the plant from the physiological point of view. 

 Some of its activities can be demonstrated by comparatively sim- 

 ple means. * 



For instance, it can readily be shown, that leaves or rather 

 chloroplastids form starch when exposed to sunlight but that no 

 starch is formed when the light is not admitted, by a simple con- 

 trivance called a leaf shield. This is a device by which a thin 

 piece of glass can be clamped against the leaf. Over a portion of 

 this glass a piece of tinfoil may be pasted, thereby shutting off 

 the light from the underlying area. The procedure consists as 



