STRUCTURE AND GROWTH OF PLANTS. 69 



live through the cold of winter without making progress, 

 antil the warm sim of spring gives a genial heat to the soil. 

 When seeds are too deeply covered, not only is the free 

 access of air prevented, but they arc not sufficiently wanned 

 by the sun. In undrained soils the seed immersed in stag- 

 nant water does not receive a proper supply of air and that 

 increase of temperature which we have seen facilitates the 

 transfonnation of starch into grape sugar and other com- 

 pounds (68) is prevented, so that the plant which springs 

 up is insufficiently nourished and seldom comes to full per- 

 fection. You will now be enabled to understand how in the 

 thorough-drained field not only earlier but more luxuriant 

 crops may be produced, and that by carefully pulverising the 

 soil so as to allow the air to pass freely through it, we will 

 materially hasten vegetation. 



97. But, though it is injurious to bury seeds too deep in 

 the soil, it will not do to leave them uncovered on its sur- 

 face — not only does a light coveriHg of porous earth protect 

 them from sudden changes of temperature which might destroy 

 them, but it prevents the action of light, which is found to 

 interfere with those chemical changes by which oxygen gas is 

 absorbed and a portion of the carbon of the seed converted 

 into carbonic acid (95). 



98. Growth of the plant Let us suppose that all the con- 

 ditions required for the germination of the seed have been 

 present, and that the machinery of the young plant thus set 

 in motion has produced a root, furnished with its spongelike 

 fibres stretching out into the surrounding soil, that a stem 

 rises up into the air, and that waving leaves, agitated by every 

 breeze, are hung around it with their pores or mouths pre- 

 pared for the reception of food, and that it is capable of deriving 

 the materials for its increase from all the sources described. 

 How are these materials made to contribute to its develop- 

 ment? 



99. No sooner is the first true leaf of the young plant 

 produced, than the mode of its growth undergoes a complete 

 change, and a new circle of chemical operations begins. It 

 no longer requires the matters contained in the seed, but seeks 

 its nourishment in the soil in which it is placed and hi th(^ 

 air that surrounds it. At this period the curious principle 

 diastase is found to haye disappeared, its assistance bemg no 

 more wanted. 



