HOW PLANTS LIVE AND WORK. 323 



regularly, side by side, like three or four rows of bricks, and with 

 their ends pointing towards the middle of the leaf. In the lower 

 half the arrangement is much less regular, and spaces are left 

 between the cells. Each of these cells contains several green 

 bodies called chlorophyll granules. As the life of all green plants, 

 and indirectly even of ourselves, would be impossible without 

 these chlorophyll granules, the introduction of the word chloro- 

 phyll, however unfamiliar it may appear, is perhaps excusable. 

 These chlorophyll-containing cells, then, form the packing between 

 the veins of the leaf. Remember that the chlorophyll cells of the 

 lower half of the leaf are loosely set, leaving irregular air-spaces. 

 The whole of this central part of the leaf, chlorophyll-bearing 

 cells and veins, is shut in by a sort of external skin. The air- 

 spaces of the lower part of the leaf communicate with the external 

 air, however, by means of an enormous number of little pores, 

 called stomata. This word is simply Greek for mouths, and is, 

 therefore, very appropriate. Each mouth or stoma has two lips 

 or guard-cells. 



This part is perhaps rather uninteresting, but it is necessary 

 for the intelligent consideration of one of the most wonderful and 

 fascinating sides of plant life. As most people know, the air 

 contains a minute proportion of a gas called carbon dioxide. This 

 gas, which is always present, even in the purest air, is made up of 

 carbon (or charcoal) and oxygen. To separate the carbon from 

 the oxygen without employing intense heat would tax the utmost 

 resources of the cleverest of chemists, and yet this is done daily 

 by every healthy green plant in creation^ at the ordinary tempera- 

 ture of the air. In the day-time, the air gains access through the 

 mouths or stomata of the leaves to the internal air-chambers I 

 have mentioned above. The chlorophyll-containing cells of the 

 leaf lay hold of the traces of carbon dioxide present in this air, 

 and, by means of energy supplied to them by the sunshine, 

 separate the carbon from the oxygen. They keep the carbon and 

 return the oxygen to the air. 



Plants thus purify the air from the poisonous carbon dioxide, 

 and replace the latter by life-giving oxygen. This process can 

 only go on in the presence of sunshine and by means of chloro- 

 phyll. Without sunshine chlorophyll cannot put the carbon of 



