io8 



BIOLOGY AND HUMAN LIFE 



Palisade layer 



-Fibrovascular 

 bundle 



Air space in 

 ongy tissue 



levers or other moving parts these machines are chemical engines, 

 each consisting of a lump of protein with some of the chlorophyl 

 {chloros, ''green"; phyllum, "leaf") that gives familiar plants 

 their distinctive color. Chlorophyl is the tool, or transformer of 



energy, in the food-making 

 process (see Fig. 6i). 



3. The energy for doing 

 this work is the light from 

 the sun. Although the work 

 cannot go on at too low a 

 temperature, it is the light 

 that is used in the process, 

 and not the heat. 



83. Oxygen a by-product. 

 The starch or sugar formed 

 by the action of sunlight 

 upon chlorophyl contains 

 the elements present in the 

 raw materials, namely, car- 

 bon, hydrogen, and oxygen. 

 In starch, hydrogen and 

 oxygen occur in the same 

 proportions as they do in 

 water (H2O). The raw 

 materials taken in by the 

 plant contain an excess of 

 oxygen, since the carbon 

 dioxid (CO2) also furnishes 

 oxygen. This element is 

 given off in a free, or 

 uncombined, state during the process of starch-making. 



84. Sunlight and life. Some green plants never form starch ; 

 but they produce some kind of carbohydrate (usually some kind 

 of sugar) by the sunlight acting on chlorophyl. This process of 

 carbohydrate formation is called photosynthesis, from Greek 

 words meaning "light" (compare photogrdc^h.) and "put to- 



Fig. 61. 



Stomate 

 Structure of leaf 



The cells containing the chlorophyl (the pal- 

 isade cells and the spongy tissue) get their 

 income from the surrounding cells or from 

 the surrounding air spaces. The water is 

 brought up through the vessels of the wood 

 {fibrovascular bundle) and it soaks through 

 the cell walls. The carbon dioxid is ab- 

 sorbed from the air inside the leaf, and this 

 air is in direct communication with the outer 

 air by wayof the breathingholes,or stomates 

 (from the Greek stoma, "mouth") 



