78 



THE VEGETATIVE ORGANS 



that iron is essential. The shoots grown in solutions from 

 which iron is excluded are a sickly yellow colour (Fig. 25, c). 

 Further, a supply of oxygen and a suitable temperature are 

 also requisite. If 3^011 examine the young shoots of plants as 

 they emerge from the soil in the cold early spring, you will 

 find they are often very pale, and contain little chlorophyll. 



24- 

 22 

 '20 

 16 

 16 

 1+ 

 12 

 10 

 8 

 6 



4- 



s 



JC2 



Days 



10 12 14- 16 18 



Fig. 48. Growth Curve of Bean Seedlings. 

 a, in the light ; b, in the dark. 



The conditions necessary for the formation of chlorophyll 

 are light, warmth, and a supply of oxygen and iron. 



We are now in a position to realize some of the most 

 general points of difference between roots and shoots. 

 These are, that roots grow downwards into the soil, avoid 

 light, are not green, and do not produce leaves, while shoots 

 grow upwards, bear thin flat leaves, and expose a large 

 green surface to air and sunlight. We will now endeavour 

 to find out the importance of these peculiarities of a shoot. 



